UNIVERSITY Of CALIFORNIA PUBLICATIONS 
 
 COLLEGE OF AGRICULTURE 
 
 AGRICULTURAL EXPERIMENT STATION 
 
 BERKELEY, CALIFORNIA 
 
 THE CONTROL OF RED SPIDERS 
 IN DECIDUOUS ORCHARDS 
 
 BY 
 
 E. R. deONG 
 
 BULLETIN No. 347 
 
 August, 1922 
 
 UNIVERSITY OF CALIFORNIA PRESS 
 
 BERKELEY, CALIFORNIA 
 
 1922 
 
David P. Barrows, President of the University. 
 
 EXPERIMENT STATION STAFF 
 
 HEADS OF DIVISIONS 
 
 Thomas Forsyth Hunt, Dean. 
 
 Edward J. Wickson, Horticulture (Emeritus). 
 
 , Director of Resident Instruction. 
 
 C. M. Haring, Veterinary Science, Director of Agricultural Experiment Station. 
 
 B. H. Crocheron, Director of Agricultural Extension. 
 
 C. B. Hutchison, Director of the Branch of the College of Agriculture at Davis. 
 H. J. Webber, Subtropical Horticulture, Director of Citrus Experiment Station. 
 William A. Setchell, Botany. 
 
 Myer E. Jaffa, Nutrition. 
 Ralph E. Smith, Plant Pathology. 
 John W. Gilmore, Agronomy. 
 Charles F. Shaw, Soil Technology. 
 John W. Gregg, Landscape Gardening and Floriculture. 
 Frederic T. Bioletti, Viticulture and Fruit Products. 
 Warren T. Clarke, Agricultural Extension. 
 Ernest B. Babcock, Genetics. 
 Gordon H. True, Animal Husbandry. 
 James T. Barrett, Plant Pathology. 
 Walter Mulford, Forestry. 
 Fritz W. Woll, Animal Nutrition. 
 W. P. Kelley, Agricultural Chemistry. 
 H. J. Quayle, Entomology. 
 Elwood Mead, Rural Institutions. 
 H. S. Reed, Plant Physiology. 
 L. D. Batchelor, Orchard Management. 
 W. L. Howard, Pomology. 
 *Frank Adams, Irrigation Investigations. 
 
 C. L. Roadhouse, Dairy Industry. 
 R. L. Adams, Farm Management. 
 
 W. B. Herms, Entomology and Parasitology. 
 John E. Dougherty, Poultry Husbandry. 
 
 D. R. Hoagland, Plant Nutrition. 
 G. H. Hart, Veterinary Science. 
 
 L. J. Fletcher, Agricultural Engineering. 
 Edwin C. Voorhies, Assistant to the Dean. 
 
 DIVISION OF ENTOMOLOGY AND PARASITOLOGY 
 
 W. B. Herms H. H. Severin 
 
 C. W. Woodworth E. R. de Ong 
 
 E. C. Van Dyke G. H. Vansell 
 
 E. O. Essig J. F. Lamiman 
 S. B. Freeborn 
 
 * In cooperation with office of Public Roads and Rural Engineering, U. 8. Department of 
 Agriculture. 
 
THE CONTROL OF RED SPIDERS IN 
 DECIDUOUS ORCHARDS* 
 
 By E. E. de ONG 
 
 CONTENTS page 
 
 Introduction 40 
 
 Life history notes and habits of the three species of red spiders 40 
 
 I Common red spider (Tetranychus telarius) 41 
 
 Rate cf oviposition 41 
 
 Winter habits 43 
 
 Food plants ? 44 
 
 II Brown mite (Bryobia praetiosa) 45 
 
 Variation in hatching dates of winter egg 46 
 
 Food plants 47 
 
 III Citrus mite (Paratetranychus pilosus) 48 
 
 Food plants 48 
 
 Why red spiders should be controlled 48 
 
 Effect of defoliation on the prune 50 
 
 Relation between irrigation and red spider injury 54 
 
 Bud development 54 
 
 Dispersal of red spiders 56 
 
 Preventive and control measures 58 
 
 I Common red spider (Tetranychus telarius) 58 
 
 Preventive measures : Abundant soil moisture. Selected cover crops 
 
 Cultural practices. Intercrops 58 
 
 Control measures 59 
 
 Spraying versus dusting 59 
 
 Sulfur mixtures for spraying 60 
 
 Recommended practice 60 
 
 Preparation of^sulfur pastes 62 
 
 Experimental spraying 64 
 
 Sulfur dusting 66 
 
 Sulfur fillers 67 
 
 Intervals between dusting 69 
 
 Substitutes for sulfur 70 
 
 Nicotine dusts .' 71 
 
 Natural enemies 72 
 
 II Brown mite {Bryobia -praetiosa) 72 
 
 Winter spraying: Crude oil emulsion 73 
 
 III Citrus mite (Paratetranychus pilosus) 78 
 
 Spray program 78 
 
 Cost of spraying and dusting 79 
 
 Summary 80 
 
 * This work was done in cooperation with members of the Pomology Division 
 of the College of Agriculture, University of California, to whom I am indebted for 
 the interpretation of horticultural data. I also wish to express my thanks to 
 
 Mr. H. B. Stabler, County Horticultural Commissioner of Sutter County, to the 
 orchardists of Sacramento Valley who have cooperated with me, to the student 
 
 helpers in this work, and others too numerous to mention. The photographs are 
 largely the work of W. C. Matthews. 
 
40 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 INTRODUCTION 
 
 The so-called Red Spiders are among the most persistent and 
 injurious pests of California's horticulture, yet no other pests are 
 more frequently allowed to go unchecked. The loss which they cause 
 is frequently not noticed, for, unlike apples attacked by the codling 
 moth, the fruit of prune and peach trees injured by red spider is 
 marketable. The grower's loss, however, is just as real, but comes 
 in so subtle a way as to be overlooked by many. Studies of yields 
 and of bud development, in attacked and in normal orchards, have 
 shown an annual crop loss ranging from $100 to over $400 per acre. 
 Besides this immediate loss there is the injury to the buds that will 
 produce next year's crop. These are so weakened that they may 
 shatter off, or at least produce infertile bloom, while the sap is so 
 thinned that the tree may become more susceptible to frost. 14 
 
 LIFE HISTORY NOTES AND HABITS OF THE THREE SPECIES OF 
 
 RED SPIDERS 
 
 There are three species of plant mites, commonly called "red 
 spider, ' ' which have long been recognized as more or less serious pests 
 of deciduous orchards, viz : 
 
 (I) Tetranychus telarius Linn,* the species active through mid- 
 summer and fall, generally known as the two-spotted, summer, or 
 common red spider. 
 
 (II) Bryobia praetiosa Koch,f the brown or almond mite, pri- 
 marily of the almond and prune, attacks during spring and early 
 summer. 
 
 (III) Paratetranychus pilosus Can. & Fanz.,$ commonly found on 
 citrus and deciduous trees through spring and summer. 
 
 * Tetranychus telarius Linn, and T. bimaculatus Harvey are considered synony- 
 mous, while T. sexmaculatus Riley is considered a distinct species. The first two 
 are commonly found on deciduous trees and other plants throughout the state, and 
 T. sexmaculatus (yellow mite) is usually found on citrus trees. 
 
 t Bryobia praetiosa Koch, 4 synonym Bryobia pratensis Garman. 
 
 X Paratetranychus pilosus Can. & Fanz., synonyms Tetranychus mytilaspidis 
 Riley, T. citri McG. Garman recently called attention to the appearance of a new 
 mite in Connecticut.^ On comparison of this species with our citrus mite they 
 seemed so similar that both Ewing and Quayle have expressed the belief that the 
 two species may be considered identical. McGregor, however, considers the two 
 species distinct. 
 
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 
 
 41 
 
 I. COMMON BED SPIDER (Tetranychus telarius Linn.) 
 
 The common red spider is a web-spinning species, usually found 
 on the under side of the leaf, excepting on the almond tree, where the 
 mite feeds on either leaf surface. This mite may be found in the adult, 
 and even in the egg and nymphal stages, in almost any month of the 
 year in the southern part of Sacramento Valley. The spring and early 
 summer months are spent on weeds and hardy cultivated plants, such 
 as strawberries and violets. Migration to orchard trees occurs at the 
 death or maturity of the spring host plant. During the middle and 
 late summer the mites increase rapidly and cause severe and pro- 
 tracted injury. 
 
 TABLE I 
 
 Rate of Oviposition of Red Spider (T. telarius) 
 Number of eggs from different mites. 
 
 Date 
 
 Max. 
 Temp. 
 
 Mite 
 No. 1 
 
 Mite 
 No. 2 
 
 Mite 
 No. 3 
 
 Mite 
 No. 4 
 
 Mite 
 No. 5 
 
 June 30 
 
 83° F. 
 
 3 
 
 
 
 
 
 
 July 1 
 
 85 
 
 4 
 
 
 
 
 
 
 July 2 
 
 72 
 
 6 
 
 
 
 
 
 July 3 
 
 82 
 
 6 
 
 
 
 
 
 July 4 
 
 90 
 
 
 
 
 
 
 July 
 
 88 
 
 14 
 
 
 
 
 
 July 6 
 
 83 
 
 
 14 
 
 
 
 
 
 July 7 
 
 79 
 
 
 8 
 
 16 
 
 16 
 
 13 
 
 July 8 
 
 80 
 
 
 4 
 
 3 
 
 13 
 
 8 
 
 
 July 9 
 
 79 
 
 
 2 
 
 16 
 
 6 
 
 4 
 
 July 10 
 
 76 
 
 
 
 16 
 
 16 
 
 13 
 
 
 July 11 
 
 77 
 
 
 
 9 
 
 
 2 
 
 July 12 
 
 70 
 
 
 
 4 
 
 
 2 
 
 Total 
 
 
 
 33 
 
 28 
 
 64 
 
 51 
 
 42 
 
42 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 Egg. — The egg, barely visible to the naked eye, is generally found 
 on the under surface of the leaf or scattered through the web attached 
 to the leaf. It is round, and when first deposited almost colorless, but 
 becomes yellowish-red before hatching. The rate of deposition of eggs 
 is dependent upon food and temperature ; temperature also influences 
 the time required for hatching. Ovipositing is rare at maximum daily 
 temperatures of 40°-50° P., but beyond this the rate rises rapidly, 
 the highest being at maximums of 76° F. to about 100° F. At the 
 latter temperatures, 12 to 16 eggs per day were frequently deposited, 
 but at daily maximum temperatures of 70° to 75° F. the daily rate 
 was from 2 to 8. At a mean maximum temperature of 87° F. the 
 duration of the egg stage was 3 days, and a drop to a mean of 77° F. 
 lengthened the average of the egg stage to 6 days. 
 
 The totals do not indicate the entire number of eggs which a single 
 individual would normally deposit but are records of a few days only. 
 Other investigators 4 have found total egg-laying periods of 12 to 36 
 days and a total number of eggs per female mite ranging from 51 
 to 110. 8 
 
 The variations in rate associated with the above temperature 
 changes are in accordance with Ewing's results in Oregon during the 
 month of October. 4 Temperature records were not given in Ewing's 
 report but probably were not above 72° F. The maximum number 
 of eggs deposited daily under these conditions was nine and the mini- 
 mum one, the average being two and three twenty-seconds. 
 
 The duration of the egg stage is directly associated with the tem- 
 perature, as is shown in Table II. At a mean maximum temperature 
 of 87° F. the length of the egg stage is 3 days; a mean maximum 
 temperature of 77° F. lengthened the egg stage to 6 days. The num- 
 bers of eggs used are too small to give reliable data, but they agree 
 with McGregor's and Donough's results in South Carolina where 
 larger numbers were used. 8 
 
 Larva. — The newly hatched larva is almost transparent and color- 
 less but after it begins to feed it changes to a greenish color. It is 
 six-legged in this first stage and has a round body. Movement over 
 the leaf surface is very restricted. 
 
 Nymphal stages. — The mite has eight legs beginning with the first 
 molt. In the older stages the mites feed voraciously and wander about 
 freely. The time of development from the egg to adult is from five 
 to ten days at summer temperatures. 
 
 Adult. — The adult mite moves rapidly over the leaf surface and 
 the webbing. The female is larger, with a body more nearly rounded 
 
BULLETIN 347] CONTROL OF RED SPIDERS IX DECIDUOUS ORCHARDS 
 
 43 
 
 than the male's. Both are so small as to be distinguished only with 
 difficulty by the naked eye. The adult is yellowish-green, usually 
 with one large irregular dark spot or a cluster of small ones on each 
 side of the upper surface (see Fig. 1, opposite page 44). The colors 
 of the common red spider, as shown in the figure, were chosen as 
 typical for the adult while feeding. These may vary with different 
 host plants, 4 but the most striking change results from starvation. 
 Mites which are hibernating or insufficiently fed usually assume a 
 yellowish or red color. 
 
 TABLE II 
 Duration of the Egg Stage of the Common Red Spider (T. telarius) 
 
 Length of egg stage Mean max. temp. 
 
 July 3 
 July 4 
 Julv 5 
 
 Eggs deposited 
 Eggs hatched 
 
 3 davs 
 
 87° F. 
 
 July 9 
 
 Eggs deposited 
 
 
 
 July 10 
 
 
 
 
 Julv 11 
 
 
 
 
 July 12 
 
 
 6 days 
 
 77° F. 
 
 July 13 
 
 
 
 
 July 14 
 
 Eggs hatched 
 
 
 
 July 17 
 
 Eggs deposited 
 
 
 
 Julv 18 
 
 
 
 
 Julv 19 
 
 
 
 
 Julv 20 
 
 
 6 davs 
 
 77° F. 
 
 Julv 21 
 
 
 
 
 July 22 
 
 Eggs hatche 1 
 
 
 
 Winter habits. — The common red spider passes the winter in the 
 adult and nymphal stage on winter-growing plants, or hibernates 
 among leaves or in the soil. Colonies of mites ranging from eggs to 
 adults may be found on the leaves of wild morning glory (Convolvulus 
 arvensis) during the first part of December. When the upper part 
 of the morning glory is killed by frost, the mites are forced to hibernate 
 in the soil, on underground suckers or among dead leaves. Hibernat- 
 ing mites have also been found in bean straw and on blackberry and 
 loganberry leaves where the canes were lying on the ground. Con- 
 trary to common belief, the writer has never found them underneath 
 the bark of trees. It is probable that those attempting to hibernate 
 in such situations would fall a prey to the predators that are usually 
 found there. 
 
44 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 Hibernation experiments. — To determine the location of hibernat- 
 ing mites during midwinter, soil from beneath plants which were 
 infested in the late fall was carefully removed, layer by layer, to a 
 depth of four inches and kept in a warm room. Each lot was placed 
 in a separate jar, the top edge of which was covered with tanglefoot 
 to prevent any mites entering from the outside. Seeds or uninfested 
 young seedlings of beans or other host plants were planted in these 
 jars, which were held at a temperature of 60° to 70° F. Infestation 
 was noted in almost every jar within a few days after planting, show- 
 ing that the mites were lying dormant in the soil or surface mulch of 
 leaves. The greatest number were found on the immediate surface, 
 although in one jar, with soil from a four-inch depth, a single mite 
 appeared. Experiments with bark from the trunks of trees gave 
 negative results. 
 
 Cold storage experiments. — Great resistance to freezing temper- 
 atures was shown by adults and the older nymphs of T. telarius. 
 Colonies from wild morning glory were placed in cold storage at 
 temperatures of 36°, 40°, and 50° P. for 21 days, and, when returned 
 to a temperature of 70° P., living mites were found in containers 
 from each temperature, the highest record being 66 per cent from 
 the room held at 36° F. One adult female mite from this lot was 
 placed on a young bean plant and in two days had deposited three 
 eggs, but she died on the fourth day. These eggs hatched in seven 
 days at a temperature of 65°-75° F., thus proving that the mite was 
 quite normal after an exposure of three weeks at 36° F. Four female 
 nymphs and one adult male from this same temperature were also 
 kept alive for several days. 
 
 A second and longer experiment was then tried with mites of the 
 same species, stored from November 9, 1920, to January 3, 1921, at 
 temperatures of 32°, 36°, and 45°-50° F. The per cent of living 
 mites found in the different series, when removed to a warm room, 
 was 8, 12, and 5, respectively. 
 
 In most localities in California where red spiders are serious pests, 
 it is probable that dormancy will alternate with periods of activity 
 during the winter. A few eggs may even be deposited on warm days, 
 but no material increase occurs until spring. 
 
 Food plants. — The list of recorded host plants of this mite is grow- 
 ing constantly; the following arc subject to more or less severe injury 
 in California: almond, apricot, bean (pink, whites, etc.), blackberry 
 (wild), cherry, cucumber, geranium, hop, Malva parviflora, melon, 
 morning glory, grapevine, pea, peach, pear, plum, poplar, pumpkin, 
 
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Bulletin 347] control of red spiders in deciduous orchards 45 
 
 rose, squash, strawberry, violet, walnut (English and Northern Cali- 
 fornia Black), watermelon. 
 
 In addition to this list, the following host plants have been 
 recorded in California and throughout the world : abutilon, acacia, 
 alder (red), alfalfa, Angelica, apple, arborvitae, ash, aster, beet 
 (sugar), birch, Bouvardia, cabbage, calla, carnation, castor bean, 
 celery, chard, chrysanthemum, Cinchona, citrus, clematis, Clitoria, 
 clover (red and white), corn, cotton, cowpea, cosmos, currant, cypress- 
 vine, dahlia, eggplant, elm, fever-few, flax, fuchsia, Godelia, goldenrod, 
 gooseberry, heliotrope, hemp, Hevea. holly-hock, hop-tree, horse-chest- 
 nut, iron-weed, jimson weed, larch, lettuce, lily (Easter), Manettia, 
 Manihot, maple, maize, mignonette, monkey-flower, moonflower, mus- 
 tard, okra, onion, passion-vine, Papaya, pea (sweet), peanut, pecan, 
 pepper, phlox, pink, pigweed, potato (Irish and sweet), radish, rasp- 
 berry, sage, Sesoania acgjjptiaca, smilax, snowball, sunflower, Thun- 
 bergia, tomato, turnip, verbena, willow. 
 
 In addition to the above, McGregor mentions 83 host plants that 
 are native, wild species in South Carolina, 8 perhaps none of which 
 is included among the above names. A systematic search of native, 
 California, host plants which are at least occasionally infested would 
 no doubt add many plants to the above record. 
 
 II. BROWN MITE (Brijobia praetiosa Koch) 
 
 The brown mite is a non web-spinning species which winters in 
 the egg stage. The egg hatches as the buds open in the spring and 
 the young mite feeds on the growing leaves and the blossom. The 
 mites usually congregate on the young twigs, especially the leaf scars, 
 through the day and migrate to the leaves at night ; the other two 
 species are found almost exclusively upon the leaf blade. There is 
 no distinct migration from one type of host to another, as with the 
 common red spider, the entire year usually being spent on one host. 
 They may, however, crawl from plant to plant or be carried about 
 by the wind. 
 
 Egg. — The egg is deep red, frequently with a yellowish tinge, 
 round, and barely visible to the naked eye. Eggs may be deposited 
 singly, but if abundant, are usually grouped in conspicuous patches 
 on the underside of the limb. A favored situation for ovipositing is 
 the slightly roughened bark of two or three year old wood. They are 
 also common on fruit spurs, and leaf and fruit scars. The white shells 
 of hatched eggs and molted skins of young mites are nearly always 
 found among the eggs, as seen in Pig. 2. 
 
46 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 The hatching of the winter egg varies with the host and is so inti- 
 mately associated with the development of the spring growth as to 
 imply some relationship between the activity' of the developing host 
 and the hatching time of the egg. 
 
 TABLE III 
 
 Hatching Dates of the Winter Egg of the Brown Mite (Bryonia praetiosa) 
 
 Host 
 
 Hatching Date 
 
 Bud Development 
 
 A'mond 
 
 Feb. 21-Mar. 1, 1920 
 
 One-third to full bloom. Leaf clusters 
 one-fourth to one inch long. 
 
 Plum (Grafted on 
 peach) 
 
 Mar. 24-Mar. 31, 1920 
 
 Bloom buds opening. Leaf clusters 
 three-fourths of an inch long. 
 
 Apple 
 
 Mar. 27-April 3, 1920 
 
 Cluster buds opened. Leaf clusters one- 
 third of an inch long. 
 
 The last generation of mites on the apple may complete their egg- 
 laying from two to four weeks later than on the almond, i.e., July 
 to August 15. Mites are occasionally seen in September, but females 
 are rare by the first of August. Hence a period of from seven to 
 eight months is spent in the winter egg on the apple. Egg-laying 
 on the almond in the vicinity of Davis is usually completed by July 1, 
 but on early defoliated trees this date may be as early as the last 
 of May. The winter egg stage on prematurely defoliated almond trees 
 may continue from eight to nine months, but on normal trees the 
 period would be about one month shorter. 
 
 Larva. — Bright red, body almost globular, six-legged. Contrary 
 to the habits of the larval form of the common red spider, it is very 
 active, moves about freely over the leaf-surface and usually retreats 
 to a roughened part of the bark through the day. 
 
 Nymph. — All stages of the mite after molting the first time are 
 eight-legged. The color changes from red to brown or greenish after 
 the mite has fed a short time. The complete life-history was not 
 determined, but under most conditions three to four days were spent 
 in each instar. Nymphs of the second and third instar were found 
 on March 6, but no hatching of the winter egg was observed prior to 
 February 20. 
 
 Adult. — Reddish-brown, with a green tinge. First pair of legs 
 greatly elongated. Body flattened. Male slightly smaller and with a 
 
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 47 
 
 mor^ pointed body than the female. Adults of the first generation 
 are found from March 20 to April 5. Second generation matures by 
 April 16 and the third generation by May 10. 
 
 Fig. 2. — Eggs of brown mite (Bryobia praetiosa) on rough bark. The white 
 objects are molted skins and egg shells. (X 10.) 
 
 Food plants. — The list of host plants for this mite grows constantly 
 as does that for the common red spider. The following are cultivated 
 plants subject to more or less serious attacks in California : almond, 
 prune, plum, peach, pear and apricot. 
 
 Those reported in the eastern United States, Europe, and Australia 
 are: alfalfa, barley, buckwheat, cherry, cherry (sand), citrus trees, 
 clover, gooseberry, grasses, oats, peas, quince, wheat. 
 
48 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 III. CITEUS MITE (Paratctranychus pilosus Can. and Fanz.) 
 
 The citrus mite spins a web, but less abundantly than the common 
 red spider. The life-history of this mite on deciduous trees is similar 
 to that of the brown mite in that the winter is spent in the egg stage 
 on the host of the previous summer. A full account of its life-history 
 on citrus trees has been worked out by Quayle. 10 
 
 Fig. 3. — Eggs of the citrus mite (Paratetranychus pilosus), showing strands 
 of webbing and stalk; greatly enlarged. 
 
 This mite is of a deep red color, with conspicuous white spines 
 arising from prominent tubercles on the body. The egg is slightly 
 striated, a flattened sphere in shape, with a prominent stalk about 
 twice the diameter of the egg in height. This stalk is bent over at the 
 tip and ends in a slight enlargement (see Fig. 3). Strands of webbing 
 frequently extend from the upper part of the stock to the leaf -surface 
 or to other strands. 
 
 Food plants. — This mite feeds principally on citrus trees but is 
 found on almond, pear, apple, peach, prune, and plum trees and on 
 evergreens, such as Pittosporum, in northern California. 
 
 WHY RED SPIDERS SHOULD BE CONTROLLED 
 
 The red spider, while feeding, extracts the sap and reduces the 
 amount of green coloring matter (chlorophyll) as is evidenced by 
 the pale color of the leaves. This causes, first, a mottled appearance, 
 
Bulletin 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 49 
 
 Fig. 4.- 
 
 -Sample I. Dried prunes from normal trees (top) and defoliated trees 
 (bottom). This fruit was all grown in the same orchard and under similar con- 
 ditions, except that one part of the orchard was defoliated by red spider. 
 
50 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 and, finally, a paling of the entire leaf surface, which is frequently 
 followed by defoliation. Restoration from this type of injury to a 
 normal condition is rapid if the mites are destroyed before the leaves 
 are badly damaged (see Fig. 10). This recovery has been more pro- 
 nounced after attacks by the brown mite than by the common red 
 spider, probably because the former is more exposed and hence readily 
 killed by contact sprays. The seriousness of the injury is due to the 
 fact that substances are found in the leaf, associated with the green 
 coloring matter, by the aid of which the plant manufactures sugar 
 and starch from which the plant itself is developed. These materials 
 constitute more than four-fifths of the dry weight of all fruit and 
 even of the tree itself. If this green coloring matter is destroyed, the 
 plant is unable to feed a growing fruit crop properly or to mature 
 a normal number of strong buds for the coming year. The tree which 
 in midsummer has pale yellow leaves or is prematurely defoliated has 
 suffered a very serious reduction in its producing powers even though 
 it may mature a small fruit crop. The fruit must of necessity be of 
 small size, with an unduly large amount of pit in proportion to pulp — 
 a grade which is always a drug on the market. Trees that have been 
 prematurely defoliated by red spiders or other causes and then irri- 
 gated in the late summer will usually throw out new leaves (see 
 Fig. 7, d) and possibly bloom, from buds developed for next year's 
 growth. Other buds which have not been forced into foliage may 
 have so much of their stored starch withdrawn that they will shatter 
 off. A late second crop of leaves will manufacture and store more 
 sugar and starch but it is doubtful if it can restore a tree to normality 
 or develop fruiting buds. 
 
 The foliage must be retained in a normal condition until the last 
 of September or the first of October to enable deciduous fruit trees 
 to yield maximum returns. Foliage injury in any or all parts of the 
 tree reduces, in proportion to the extent of the injury, the amount of 
 food which the tree can manufacture, as shown by Harvey and Mur- 
 neek 7 and other investigators. Such weakness is manifest in many 
 ways : by scanty foliage, probably resulting in sunburn, infertile 
 bloom, and weakened growth. Common orchard troubles, usually 
 attributed to the need of fertilizers, irrigation, and cross-pollination, 
 may in some instances be due to the injury caused by red spiders, 
 and should be studied from this standpoint. 
 
 Effect of defoliation on the prune. — Data on the red spider injury 
 to the prune crop were obtained through the assistance of H. P. 
 Stabler, County Horticultural Commissioner of Sutter County. He 
 collected average, orchard samples during the last two years from 
 
BULLETIN 347] CONTROL OF RED SPIDERS IX DECIDUOUS ORCHARDS 51 
 
 which the following grades and weights were determined. The first 
 sample contained 300 primes and the second 400. The two types of 
 fruit in each sample were grown under the same conditions, except 
 that of defoliation by red spider in a part of the orchard. Sample I 
 had been dried at the orchard, sample II was received in a fresh con- 
 dition and afterwards dried. 
 
 
 TABLE IV 
 
 
 
 Comparative Weights 
 
 of Fresh and Dried PRr 
 
 xes from Normal axd 
 
 , 
 
 Defoliated Trees 
 
 
 
 
 Average weight Average weight 
 of fresh fruit of dried fruit 
 
 Xumber of 
 of fruits to lb. 
 
 Percentage 
 of pits 
 
 
 ounces ounces 
 
 
 % 
 
 Sample I 
 
 
 
 
 
 Normal 
 
 
 0.403 
 0.22 ' 
 
 40 
 72 
 
 11.4 
 
 Defoliated 
 
 
 13.8* 
 
 
 
 
 Sample II 
 
 
 
 
 Normal 
 
 0.565 
 
 0.259 
 
 64 
 
 13 
 
 Defoliated 
 
 0.438 
 
 0.205 
 
 78 
 
 17 
 
 * The small proportion of pits found in sample I indicates a better quality of 
 fruit than that in sample II, which may account for the difference in the effect 
 on the two grades. 
 
 The trees defoliated by red spider in sample I showed a crop loss 
 of 44 per cent by weight and a drop of 3 points in the grading. At 
 this rate of decrease a crop of dried prunes (40-50 grade) of 3 tons 
 per acre would have lost in weight 1% tons. The remaining 1% 
 tous would have depreciated in value from 11% cents per pound for 
 40-50 's to 744 cents per pound for 70-80 's, a total loss of approxi- 
 mately $453 per acre. Sample II showed a total crop loss from red 
 spider injury of $139.18 per acre (on the basis of three tons of dried 
 fruit per acre"), including both the reduction in weight and the 
 depreciation in value of the remainder. The injury to next year's 
 buds, also due to the work of red spider, causes a further loss. 
 
 A photograph of the dried prunes of sample I is shown in Figure 4 ; 
 the six at the top are from normal trees. The proportionate number 
 of the different sized fruits in Figure 5 is roughly indicated by the 
 number of individual prunes. For example, ten out of every hun- 
 dred fruits, from the defoliated trees, were shriveled like the one at 
 the left of row three from the top. The four to the right of this one 
 indicate the high proportion of low-grade fruit from the part of the 
 orchard defoliated by red spider. These trees had been absolutely 
 
52 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 / 
 
 2 
 
 3 
 
 4 
 
 o 
 
 6 
 
 Fig. 5. — Sample II. Fresh prunes from normal trees (top) and defoliated 
 trees (bottom) (see Fig. 6). This fruit was all grown in the same orchard and 
 under similar conditions except that one part of the orchard was defoliated by red 
 spider. 
 
BULLETIN 3-17] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 53 
 
 Fig. 6. — Normal and defoliated prune twigs showing the condition of the 
 trees from which the fruit of Fig. 5, Sample II, was gathered. The fruit and 
 twigs at the top of Figs. 5 and 6 are from normal and the lower ones from defoli- 
 ated trees. 
 
54 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 defoliated by red spider three to four weeks before harvest, while the 
 remainder of the orchard still retained a fair amount of foliage. The 
 entire orchard was dry and in poor condition. A small proportion of 
 large prunes will be noted in the fresh fruit sample but none in the 
 dried; this may have been caused by varying conditions in the two 
 orchards from which the samples were drawn, or a low sugar content 
 in the large prunes from defoliated trees may have resulted in a 
 greater drying loss. 
 
 RELATION BETWEEN IRRIGATION AND RED SPIDER INJURY 
 
 Attack by red spider and drought have long been associated together. 
 In certain districts, the only protective measure used is abundant 
 irrigation, a practice that is not, however, entirely adequate in many 
 instances. But irrigation must always be considered of primary im- 
 portance, for an orchard suffering from drought is already in an 
 unthrifty condition which may be intensified by an uncontrolled 
 attack of red spider. Spraying is a waste of money or at best of 
 slight advantage unless an adequate supply of moisture is assured. 
 
 Bud development. — Much of the experiment work at the Univer- 
 sity Farm was with almonds, and since the orchard bore irregularly, 
 it was necessary to judge results by a study of the development of 
 buds rather than of production. Buds were selected in October in 
 certain sprayed and irrigated plots, to determine their relative devel- 
 opment under different types of treatment. The methods of treating 
 the various plots and the percentage of moisture found October 9 in 
 the first six feet of soil are shown in Table V. 
 
 TABLE V 
 
 Description of Sprayed and Irrigated Almond Plots 
 
 No. 
 
 Date of 
 Treatment 
 
 A 
 
 June 10 
 
 B 
 
 June 8 
 
 C 
 
 June 19 
 
 D 
 
 June 8 
 
 E 
 
 June 5 
 
 F 
 G 
 
 July 20 
 
 14-inch irrigation — no spraying 
 
 Sprayed with lime sulfur solution 2 gallons, sulfur 5 
 
 pounds, dry prepared flour paste 2 pounds, in 100 
 
 gallons of mixture '. 
 
 Sprayed with crude oil emulsion 15 gallons of oil in 100 
 
 gallons of mixture 
 
 Sprayed with 5 pounds of sulfur, plus 2 pounds of dry 
 
 prepared flour paste in 100 gallons of mixture 
 
 Dusted with sulfur 
 
 6-inch irrigation — no spraying 
 
 Untreated 
 
 11.51 
 
 8.81 
 
 t 
 
 t 
 
 12.23 
 
 7.78 
 
 fSimilar to plot B. 
 
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 55 
 
 The efficiency and more details as to preparation and use of the 
 spray mixture used in plots B, C and D will be found in the discussion 
 of Tables VI and VIII. It should be noted that the control in plot C 
 is for the brown mite, no attempt being made to check the red spider 
 attack during midsummer. In plots B and D the application was made 
 too late to give much protection against the brown mite, but it did 
 control the common red spider. An irrigation of approximately six 
 inches of water was given in July to plots B, C, D, E, and F. From 
 the contour of the ground it was impossible to apply a uniform depth 
 of water over all the plots and judging from the amount of soil moist- 
 ure present in October, plot F received considerably more than the 
 average depth. The comparative data given are possible from the acci- 
 dental joining of irrigation and spraying plots. Plot A was chosen as 
 a typical well-irrigated plot from a series of experiments planned by 
 the Division of Irrigation Practice. 
 
 TABLE VI 
 
 Comparative Numbers and Weights of Buds from Sprayed and Irrigated 
 
 Almond Plots in October 
 
 (Orchard attacked by both the brown mite and the common red spider.) 
 
 Plot 
 No. 
 
 Type of treatment 
 
 Total No. 
 of buds 
 on spurs 
 
 Buds per 
 spur 
 
 Rank by- 
 No. of 
 buds 
 
 Total 
 
 weight 
 
 of buds 
 
 on spurs 
 
 Weight 
 per bud 
 
 Rank by 
 bud size 
 
 Rank by 
 
 average 
 
 of weight 
 
 and No. 
 
 A 
 
 Early summer 
 irrigation 
 
 336 
 
 3.36 
 
 1 
 
 1.6429 
 
 4.89 
 
 1 
 
 1.0 
 
 B 
 
 Slimmer spray 
 (Lime-sulfur) 
 
 254 
 
 2.54 
 
 3 
 
 1.1221 
 
 4.42 
 
 2 
 
 2.5 
 
 C 
 
 Winter spray 
 (crude oil) 
 
 275 
 
 2.75 
 
 2 
 
 1.1752 
 
 4.27 
 
 3 
 
 2.5 
 
 D 
 
 Summer spray 
 (sulfur) 
 
 177 
 
 1.77 
 
 5 
 
 0.7133 
 
 4.003 
 
 4 
 
 4.5 
 
 E 
 
 Summer dust 
 (sulfur) 
 
 191 
 
 1.91 
 
 4 
 
 0.5281 
 
 2.76 
 
 6 
 
 5.0 
 
 F 
 
 Late summer 
 irrigation 
 
 55 
 
 0.55 
 
 6 
 
 0.1834 
 
 3.33 
 
 5 
 
 5.5 
 
 G 
 
 Untreated 
 
 38 
 
 .38 
 
 7 
 
 0.0846 
 
 2.23 
 
 7 
 
 7.0 
 
 The selection of buds, for the data given in Table VI, was made 
 as follows : Ten fruit spurs were selected from each of ten trees in 
 the different plots, the buds were then removed from these, counted, 
 and weighed. The total number of buds from the ten spurs taken 
 from each plot is given in column three of Table VI, the average num- 
 ber in column four, while in column five the rank or order according 
 to the total number of buds as listed. That is, the plot showing the 
 
56 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 highest number of buds ranks first in column five. The same plan is 
 followed in grouping the data for the weights. The bud development 
 on the first of October is seen with typical foliage in Figure 7. 
 
 It should be noted that this comparison is not alone between spray- 
 ing and irrigation but also shows comparative control values for two 
 species of mites, the brown mite, and the common red spider. Plot 
 C represents the control of the brown mite alone ; in plots B, D, and 
 E, this mite had almost ceased feeding for the year, hence the value 
 of the treatment applies almost entirely to the control of the common 
 red spider. 
 
 From the data in Table VI it will be seen that the trees were 
 suffering from a combination of drought and red spider injury, as 
 irrigation at the proper time, but without spraying, resulted in even 
 better bud formation than where the mites were controlled but where 
 the trees suffered from drought. The combined injury from drought 
 and red spider resulted in the feAvest number of buds, as shown in 
 plot G. The next lowest in rank was plot F. The latter had been 
 defoliated rather early in the summer and, when irrigated, sent out 
 a heavy foliage at the expense of next year 's buds ; many of the latter, 
 which had been formed earlier in the year, withered and dropped off 
 at this time. This plot had practically no bloom the following spring. 
 It should also be noted that the summary of bud development for 
 plot C, where the brown mite alone was controlled, was identical with 
 that in plot B, in which the attack of the brown mite was unchecked 
 except for a very short time, while the red spider was thoroughly con- 
 trolled. Apparently the attack of the brown mite is as serious on the 
 almond as that of the red spider, a theory which is in accord with the 
 belief of many almond growers. 
 
 DISPERSAL OF RED SPIDERS 
 
 The usual ways of dispersal of the different species of red spiders 
 are: (1) moving from leaf to leaf; (2) crawling over the ground; 
 (3) wind carriage either of the individual mite or of a falling leaf 
 bearing a colony of spiders; (4) accidental carriage by insects or 
 birds which have rested upon an infested twig and then flown to a 
 new host. 
 
 Wind carriage is perhaps the most important of the four ways 
 under California conditions. Tests with sticky tanglefoot paper have 
 repeatedly shown that mites may be carried hundreds of feet even 
 by a light wind. 11 But the fact that fche falling leaf may be a carrier 
 of entire colonies of mites has apparently been overlooked. This seems 
 to be a natural means of infestation and was adopted as the basis for 
 
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 
 
 57 
 
 making cultures in laboratory practice. A small section of an infested 
 leaf, pinned to another plant, soon dries, thus forcing a migration of 
 the mites to the new host. 
 
 Migration is more pronounced with the common red spider than 
 with the other two species. This is especially noticeable during the 
 hottest and driest months of the year, July and August. Voracious 
 
 Fig. 7. — Prune twigs showing bud development in sprayed and irrigated plots. 
 
 (a) Winter application of crude oil emulsion to control the brown mite, 
 Bryobia. 
 
 (b) Early summer irrigation. 
 
 (c) Summer application of lime-sulfur solution, sulfur and flour paste to con- 
 trol the common red spider, T. telarius. 
 
 (d) Late summer irrigation, causing a second growth of foliage. These trees 
 did not bloom the following year. 
 
 (e) Check. No spraying or irrigation. Note Bryobia eggs on the spurs. 
 
 feeding withers the leaves of the host plant and if it be those of a 
 small annual the entire plant soon dies. The mites then migrate or 
 are blown away by the wind. Defoliation of tall trees and the brush 
 along levees facilitates wind carriage and the infestation may advance 
 hundreds of feet in a few hours. 
 
58 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 PREVENTIVE AND CONTROL MEASURES 
 
 I. COMMON RED SPIDER (Tetranychus telarius Linn.) 
 
 The control of the common red spider is frequently considered to 
 be dependent alone upon spraying or dusting. There are, however, 
 certain farm practices whose preventive value should be recognized. 
 Such measures alone, however, cannot be depended upon to control 
 the red spider under conditions favorable to its development. But 
 increasing the thriftiness of the tree and delaying somewhat the attack 
 on cultivated crops may be of great value in supplementing the general 
 spray practice. The postponement of the attack even for two or three 
 weeks during midsummer lessens the injury to any orchard. 
 
 Preventive Measures 
 
 1. Abundant soil moisture. — The advantages of timely irrigation 
 are shown on pages 54-56. 
 
 2. Selected cover crops. — The cover crops commonly used in Cali- 
 fornia, including legumes, grains, alfilaree, mustard, and the grasses, 
 are almost free from attack by red spiders. The common red spider 
 has been found feeding and ovipositing upon bur clover (Medicago 
 hispida and M. arabica), but apparently it does not thrive upon 
 either species. Any of the above-named plants may, so far as has 
 been observed, be safely used as cover crops, where such practice is 
 desirable. They are of value not only for green manuring but may 
 prevent the growth of host plants of the red spider. 
 
 3. Cultural practices. — Early plowing or other cultural practices 
 which kill or retard the growth of early spring host plants, such as 
 wild morning glory, and the cheese weed (Malva spp.), may aid 
 materially in retarding the development of the common red spider. 
 The mites are much reduced in numbers through the winter but 
 multiply slowly during the spring months on weeds and cultivated 
 plants. Migration of the mites from the spring to the summer host, 
 whether orchards, hops, or a truck crop, does not usually occur until 
 the early host has matured or succumbed to their attacks. Killing or 
 checking the spring host may prevent a great increase of the mites 
 and thus lessen the number of migrants. 
 
 4. Selected intercrops. — Intercrops for young orchards, subject to 
 red spider attack, can be chosen with the view of minimizing the 
 attack. If this is not done and very susceptible intercrops are grown 
 in the orchard in regions subject to attack by red spider, both the 
 orchard and the intercrop may suffer more than if each crop were 
 
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 
 
 59 
 
 grown alone. Tomatoes and the various varieties of corn and sorghum 
 have been found safe to use as intercrops. 
 
 The above remarks concerning host plants apply only to the 
 common red spider, not to the brown mite or to the citrus mite. For 
 the control of the latter two species, winter or summer spra}4ng and 
 abundant soil moisture constitute the main dependence. 
 
 Control Measures 
 
 Spraying versus dusting. — Sulfur applied as a dust has long been 
 used in California for the control of red spiders, but in recent years 
 the practice has become increasingly unsatisfactory. Criticism of 
 dusting as a means of applying sulfur dates back at least to 1903 
 when Volck recommended a potassium sulfid spray as a substitute for 
 dusting in red spider control in Sutter County. 13 Sulfur dusting is 
 more satisfactory in regions where the air is somewhat humid* and 
 the winds light — conditions which are common throughout much of 
 the coast region. Dusting is of decidedly less value in the dry, windy, 
 interior valleys where the percentage of relative humidity runs very 
 low as will be seen in Table VII. Under such conditions, dependence 
 must be placed largely on spraying, preferably using a spreading and 
 adhesive material in the mixture. These general deductions have their 
 exceptions where the reverse of the above climatic features are found 
 during certain seasons of the year or in certain restricted localities. 
 
 TABLE VII 
 
 Monthly Mean of Relative Humidity at 5 p.m. During June, July, and 
 
 August, 1919 and 1920 
 
 Location 
 
 June 
 
 July 
 
 August 
 
 San Jose { 1919 
 
 (Coastal Region) 1 1920 
 
 % 
 
 50 
 49 
 30 
 30 
 16 
 23 
 
 % 
 
 56 
 44 
 32 
 30 
 16 
 20 
 
 % 
 
 54 
 51 
 
 Sacramento J 1919 
 
 (Interior valley) 1 1920 
 
 29 
 27 
 
 PpH "Rlnff 1919 
 
 17 
 
 (Interior valley) 1 1920 
 
 18 
 
 
 
 The greatest advantage of dusting over spraying is the speed of 
 application ; where climatic or other conditions permit its use it is a 
 valuable supplement to the slower work of spraying. One man with 
 
 * A distinction must be made between the amounts of condensed moisture on 
 leaf surfaces relative to the adherence of dusts. Moisture in sufficient quantity 
 to wet the surface of the leaf or condense in drops may actually prevent the 
 adherence of sulfur and cause it to run off onto the lower edge of the leaf. A 
 damp surface, however, will cause a more uniform adherence than a dry one. 
 
60 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 a hand duster may cover from two to ten acres per day, according 
 to the size of the trees and the thoroughness of the application. With 
 a power blower, two men may cover from 20 to 30 acres per day, pro- 
 vided there is not sufficient wind to prevent dusting all day. Spraying 
 is very much slower, the range being from two and one-half to six 
 acres per day. 
 
 A serious objection to the dusting method is that two or more 
 applications are usually necessary, even where success is obtainable at 
 all in this way. One careful application of a liquid spray before the 
 attack is severe at any point should give a long period of immunity 
 from injury and is frequently sufficient for the entire season. 
 
 Sulfur Mixtures for Spraying 
 
 Sulfur may be applied as a spray in a pure state but only when 
 combined with a spreading or adhesive substance, such as casein* 
 or glue water, forming the so-called "wettable sulfurs" which are 
 similar to the commercial sulfur pastes. It may also be applied as 
 a caustic spray in chemical combination with other substances, for 
 example, a lime-sulfur solution either with or without the addition 
 of pure sulfur. Sulfur applied as a spray has apparently the same 
 effect as when applied as a dust and adheres better. Its action is 
 slow but extends over a period of several days. 
 
 The caustic solution of lime-sulfur acts as a "contact spray, ' : 
 killing the mites within 24 hours, provided they are well covered. 
 This spray gives immediate relief from injury and hence should always 
 be used when the mite is in evidence. A spreader, such as casein or 
 flour paste, is especially useful with all caustic sprays to facilitate 
 wetting all the foliage. 
 
 Recommended practice. — The following formulas have been tested 
 and found very successful for the past two years at the University 
 Farm and in a number of private orchards in Sutter and other 
 counties of northern California. To get the best results they must 
 be applied at the right time and with painstaking effort to cover both 
 the upper and lower sides of every leaf. Careless, scanty, or hasty 
 spraying is of almost no value. A large almond tree will require from 
 eight to 12 gallons to thoroughly cover it; medium-sized prune trees, 
 from four to eight gallons per tree. 
 
 Sulfur spray, Formula 1, is to be applied just before the time when 
 the mites usually began feeding in the orchard in previous years, but 
 before they are found on the trees. Should the work be delayed until 
 red spiders are present, then use Formula 2. 
 
 * There are now other forms of commercial spreaders, developed since this 
 experimental work was completed, which may be substituted for calcium caseinate. 
 
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 61 
 
 Formula 1 
 
 Sulfur (sublimed or powdered) 5 pounds 
 
 *Calcium casemate £ pound 
 
 Water to make 100 gallons 
 
 Prepare a paste of the sulfur and casemate as directed on page 
 62. Add this to the spray tank with the agitator running. The pres- 
 ent commercial form of casein is given preference as it is a better 
 spreader than the others suggested and is very convenient to use. 
 
 The first of July is the usual date for this application in the Sac- 
 ramento Valley. It maj^ be necessary to apply it in June in orchards 
 where the spring host plants are very abundant or if drought has 
 hastened the death of the first host. If the work is done early and 
 thoroughly, it usually holds the mite in check for the entire season. 
 
 Contact sprays. — Actual infestations by the common red spider or 
 the brown mite require a quicker acting spray than sulfur alone. 
 A spray is required for this type of work that will kill the mites on 
 contact and also one which contains sufficient sulfur to act upon the 
 3 r oung mites as they hatch from the eggs. The following formula has 
 been found very satisfactory, but if desired, dry lime-sulfur or other 
 lime-sulfur substitutes may be used in place of the solution. These 
 materials are added in the liquid form after the paste is prepared as 
 under formula 1. 
 
 Formula 2 
 
 Lime-sulfur concentrate (31°-34° Baume) on almond 2 gallons 
 
 prune 1 gallon 
 
 peach 1 gallon 
 
 Calcium caseinate \ pound 
 
 Sulfur (powdered or sublimed) 5 pounds 
 
 Water to make 100 gallons 
 
 Heavy infestations of red spider resulting from delayed control 
 cannot be entirely checked by a single spraying. One application will 
 be effective for about three weeks, when it should be repeated, unless 
 the attack begins very late in the season. If possible, spraying should 
 never be delayed until injury from the mites is noticeable, as it is 
 almost impossible to get as good results from late spraying as if the 
 work had been done when the attack usually begins. 
 
 * The glue-water formula, flour paste, or other commercial spreaders, may be 
 substituted for casein, if desired, according to the formulas given on p. 62. 
 
62 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 Caustic sprays are more dangerous to the foliage than those of 
 sulfur alone, hence particular care should be taken to work at tem- 
 peratures below 100° F., if possible. It might be possible in limited 
 areas to spray when the sun is not shining directly on the foliage that 
 is being drenched. Much stronger concentrations can be used at such 
 a time. 
 
 It has been reported from the citrus districts of southern Cali- 
 fornia that the use of caustic sprays has killed the insects that feed 
 upon the red spider. This observation has not been substantiated in 
 northern California; moreover, parasitic or predacious enemies are of 
 little importance at the height of the mite attack. 
 
 Preparation of sulfur pastes. — (-a) Casein in its present commer- 
 cial form of calcium caseinate is very satisfactory for mixing with 
 sulfur in preparing liquid sprays. It aids in wetting the sulfur and 
 also acts as a spreader and adhesive on the leaf surface. One-half 
 pound of the commercial powder is sufficient for preparing 100 gallons 
 of mixture. Add the casein to one and one-half gallons of water, into 
 which stir five pounds of sulfur (powdered or sublimed) until a stiff 
 paste without lumps is formed, adding more water if necessary. Wash 
 this through a fine screen into the spray tank with the agitator run- 
 ning. The material is then ready for application. 
 
 (b) The glue-water formula suggested by Gray 6 has been widely 
 adopted but on account of the time required for melting the glue it is 
 less convenient than the casein preparation. The formula is : 
 
 Formula 3 
 
 Powdered glue f ounce 
 
 Hot water 1£ gallons 
 
 Powdered sulfur 5 pounds 
 
 Water to make : 100 gallons 
 
 After melting the glue in hot water, it is diluted to one and one-half 
 gallons and this water used in mixing the sulfur into a paste, free from 
 lumps. Wash the paste through a sieve into the spray tank, while 
 the agitator is running. For small amounts of this mixture the more 
 expensive form of liquid glue may be substituted for the dry glue, 
 thus saving the time and work required in melting the powdered glue. 
 
 (c) Prepared flour pastes, wet (jelly) or dry, such as bill posters 
 use are also good mediums to use in mixing dry sulfur with water. 
 Home made flour paste (see p. 70) is also a good spreader to use 
 with caustic solutions of lime sulfur. Two pounds of the powdered 
 paste or three pounds of the wet paste (jelly) is sufficient for 100 
 gallons of mixture. Flour pastes are less convenient than casein in 
 the preparation of sulfur mixtures. 
 
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 
 
 63 
 
 (d) All mixtures of "wettable sulfurs' ;i should be used the day 
 they are prepared. If allowed to stand over night there is a tendency 
 to '.'set" and form a solid layer which is difficult to bring into sus- 
 pension. 
 
 TABLE VIII 
 
 Efficiency of Sulfur in Different Types of Liquid Sprays as a Control of 
 the Brown Mite and the Common Red Spider 
 
 (Applied June 8 on almond trees.) 
 
 (Amounts used are for 100 gallons of spray mixture.) 
 
 
 Material used 
 
 Decrease in infestation* 
 
 
 Plot 
 
 No. 
 
 observed 
 6/18 
 
 observed 
 8/11 
 
 observed 
 9/20 
 
 Remarks 
 
 A 
 
 Lime-sulfur solution (31° Baume) 
 2 gallons (1-50) plus 2 pounds of 
 dry flour paste and 5 lbs. of flow- 
 ers of sulfur 
 
 % 
 
 96 
 
 % 
 
 90 
 
 % 
 
 90 
 
 Control satisfactory for 
 entire season. 
 
 
 
 
 B 
 
 "Soluble Sulfur" (Niagra Sprayer 
 Co.) 3 lbs. plus 2 lbs of dry flour 
 paste 
 
 92 
 
 60 
 
 15 
 
 Apparent control but 
 results not permanent. 
 Slight foliage injury. 
 
 
 
 
 C 
 
 Dry lime-sulfur (Shervvin & Wil- 
 liams) 13^ lbs. plus 2 lbs. of dry 
 flour paste 
 
 85 
 
 42 
 
 5 
 
 Control unsatisfactory. 
 Results not perma- 
 nent. 
 
 
 
 
 D 
 
 Dry lime-sulfur (Sherwin & Wil- 
 liams) 3 lbs., plus 2 lbs. of dry 
 flour paste 
 
 93 
 
 73 
 
 25 
 
 Apparent control but 
 results not permanent. 
 
 
 
 
 E 
 
 "Milled Sulfur" (California Spray 
 Chemical Co.) 5 lbs 
 
 92 
 
 88 
 
 80 
 
 Slower in action than 
 caustic sprays but re- 
 
 
 
 sults permanent. 
 
 F 
 
 Sulfur (flowers of sulfur) 5 lbs. 
 plus 2 lbs. of dry flour paste 
 
 88 
 
 88 
 
 90 
 
 Similar to No. 5. 
 
 
 
 
 G 
 
 Dry lime-sulfur (Sherwin & Wil- 
 liams) 5 lbs. plus 4 lbs. of wet 
 flour paste and 5 lbs. of flowers of 
 sulfur 
 
 95 
 
 86 
 
 90 
 
 Equal to that of No. 1. 
 
 
 
 
 H 
 
 Dry lime-sulfur (Sherwin & Wil- 
 liams) 3 lbs. plus 4 lbs. of wet 
 flour paste and 5 lbs. of flowers of 
 sulfur 
 
 93 
 
 85 
 
 70 
 
 Slightly inferior to 
 No. 7. 
 
 
 
 
 Check (untreated). 
 
 Percentage of live mites 95 to 98. 
 
 * These percentages are a comparison with the check plots in which the number 
 of dead mites is normally from 2 to 5 per cent. 
 
64 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 A number of commercial sulfur pastes are now on the market which 
 can be substituted for the above mixtures. These pastes are mixtures 
 of sulfur and other substances so combined as to give a wettable sulfur 
 with adhesive qualities. 
 
 Fig. 8. — Almond trees at the University Farm sprayed with 2 gallons of lime- 
 sulfur solution, 5 pounds of sulfur, and 2 pounds of dry prepared flour paste in 
 100 gallons of spray. Sprayed June 8, photographed August 4. Compare with 
 Fig. 9. 
 
 Experimental spraying at the University Farm, Davis. — Orchard 
 and laboratory experiments with a large number of sprays were con- 
 ducted at Davis and in the immediate vicinity during 1920. This 
 work was done principally on almonds as there were no large blocks 
 of prune orchard available. The more promising experiments have 
 been tested during 1921 on prunes at the University Farm and in 
 private orchards with similar results. Hence the data in Table VIII 
 may be taken as indicative of the behavior of such sprays both on 
 prunes and almonds. 
 
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 
 
 65 
 
 The plots received but one irrigation, the latter part of July, so 
 that all suffered from drought. The number of trees to an experiment 
 ranged from 14 to 40. The trees are about the size of the average full- 
 grown almond. Dry prepared flour paste, such as bill posters use, was 
 
 
 \ 
 
 4 
 
 W ' \ •X, 
 \iXJ 
 
 
 ^ A - 
 
 
 
 Fig. 9. — Almond trees defoliated by the red spider, T. telarius. These trees 
 were grown under the same conditions as those shown in Fig. 8 except that they 
 were unsprayed. Photographed August 4. 
 
 combined with all the caustic sprays to increase their spreading quali- 
 ties. The sulfur spray in plot F was prepared with the same paste, 
 instead of glue water, so as to make the series comparable. The amount 
 of spray mixture per tree was from 8 to 12 gallons, 10 gallons being 
 the average. 
 
66 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 These applications were made toward the close of the brown mite 
 attack and just as the common red spider was beginning to show in 
 the orchard. The foliage at this time was becoming yellow and begin- 
 ning to drop, the combined result of the brown mite attack and 
 drought. Within ten days after spraying, the plots where the caustic 
 solutions were used (letters A to D inclusive, and G and H) were 
 regaining their normal color. Plots E and F did not recover until more 
 than a month after treatment. The quick action of the caustic solution 
 resulted from its effect as a "contact" spray. A large percentage 
 of the brown mites were killed within 24 hours after spraying and witli 
 this heavy drain removed the green coloring matter was quickly 
 replaced in the leaves. The sulfur used in plots E and F acted more 
 slowly but just as surely, so that in the final summary all plots which 
 had received sprays containing approximately five pounds of sulfur 
 per hundred gallons were almost free from red spider. Plots B, C, 
 and D, which received less than five pounds of sulfur per hundred 
 gallons of mixture were much more heavily infested from August 
 until the end of the season. It would seem from these experiments that 
 the red spider may be checked on almonds by a single thorough spray- 
 ing at the beginning of the season. The type of spray mixture seems 
 to be of less importance than the amount of sulfur used. All sprays 
 containing approximately five pounds or more of sulfur per 100 gallons 
 resulted in a fair degree of success. 
 
 Sulfur Dusting 
 
 Powdered or sublimed sulfur has frequently proved satisfactory 
 as a control for the different species of red spiders, but only when it 
 adheres well and is in close proximity to the mite. Quayle has shown 
 that the distance over which it is effective is but a very small fraction 
 of an inch. 10 Lack of adherence, poor distribution, and the heavy 
 web of the common red spider are the causes of many of the failures 
 attributed to this manner of application. 
 
 Comparative laboratory tests of dry and liquid applications have 
 shown an almost uniform efficiency in the action of the two methods. 
 These tests were made with infested twigs sprayed or dusted on the 
 same day and in the same way as the work was done in the orchard. 
 The twigs were then placed in jars partly filled with water and each 
 jar set in the center of a sheet of tanglefoot fly paper. The latter 
 precaution prevented the loss of dead mites as they dropped from 
 the twigs. The laboratory temperature was practically that of the 
 orchard, the greatest difference in the two situations being that the 
 
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 67 
 
 laboratory was free from currents of air sufficiently strong to dislodge 
 the sulfur from the leaves. Under these conditions, dusting experi- 
 ments requiring thousands of mites showed an efficiency within 1 or 
 2 per cent as great as that on sprayed twigs, while in the orchard a 
 difference of 12 to 40 per cent was common. Sulfur applied as a 
 dust in the orchard apparently does not adhere sufficiently long to 
 be effective, especially with the little pubescence common on almond 
 foliage. 
 
 Control by dusting seems more practical for the brown mite than 
 for the common red spider. The work is done earlier in the year 
 when the humidity is higher and the freedom from webbing in the 
 case of the brown mite makes it easier to bring the sulfur in intimate 
 contact with the mites. The most successful dusting at the Univer- 
 sity Farm was on a still, foggy morning in April on the brown mite. 
 This application of sulfur adhered through a three-day heavy north 
 wind, and ten days after the application there were, by actual count, 
 from 81 to 102 thousand grains of sulfur per square inch of leaf 
 surface. This one application controlled the brown mite for the entire 
 season. Other applications made on dry windy days showed scarcely 
 a trace of sulfur 24 hours after the dusting. Such applications are 
 of course worthless. 
 
 Sulfur fillers. — Sulfur for dusting purposes must be sufficiently 
 fine so that it will adhere well to foliage. Sulfur refiners are now 
 preparing a very finely powdered material for this purpose, but such 
 sulfur used alone has a tendency to pack and become lumpy.* This 
 material does not spread well either from hand or power blowers, 
 unless a very efficient agitator is used, as it has a tendency to clog the 
 machine and to spread very unevenly. This difficulty may be over- 
 come by mixing an inert material, such as hydrated lime or kaolin, 
 with the sulfur. The best proportionate amounts of sulfur and filler 
 were determined in the experiments reported in Table IX. This work 
 was done in a closed laboratory where the possibility of air currents 
 dislodging the sulfur was reduced to a minimum. Almond twigs 
 infested with red spider were dusted with a hand blower, and placed 
 in jars containing water. These jars were set on sheets of tanglefoot 
 paper so that a record could be kept of the mites as they died and 
 dropped off. The tests were made in duplicate with two check jars 
 for comparison. 
 
 * Eecent developments in the preparation of sulfur dusts are overcoming this 
 trouble, hence it may not always be necessary to use fillers. 
 
68 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 TABLE IX 
 
 Comparative Efficiency of Sulfur with Varying Amounts of Filler 
 (Laboratory test — applied in August) 
 
 Proportions of sulfur 
 
 (1) Pure sulfur 
 
 (2) 90% sulfur— 10% hydrated 
 
 lime 
 
 (3) 80% sulfur— 20% hydrated 
 
 lime 
 
 (4) 70% sulfur— 30% o hydrated 
 
 lime 
 
 (5) 50% sulfur— 50% hydrated 
 
 lime 
 
 (6) Check (untreated) 
 
 Per- 
 centage 
 of red 
 spiders 
 killed 
 August 
 25 
 
 Per- 
 centage 
 
 of 
 
 mites 
 
 killed 
 
 August 
 
 28 
 
 90 
 
 98 
 
 90 
 
 98 
 
 85 
 
 97 
 
 77 
 
 85 
 
 65 
 
 65 
 
 95 
 
 (alive) 
 
 95 
 
 (alive) 
 
 Remarks 
 
 Spreads unevenly; packs on standing. 
 
 Spreads more evenly. Requires less 
 material to cover twigs than pure 
 sulfur. 
 
 Slow action. Effective in laboratory 
 but of doubtful value in orchard. 
 
 Slow action. Inferior to larger pro- 
 portions of sulfur. 
 
 Inefficient. 
 
 From the above data, it is concluded that dusting sulfur with a 
 ten per cent filler is superior to the pure material in that it spreads 
 more evenly and gives a better distribution than is secured with pure 
 sulfur. There was little difference of efficiency between the pure 
 sulfur and that with a ten per cent filler. It will also be noted that 
 the 90-10 dilution was more rapid in action than any of those con- 
 taining smaller amounts of sulfur. This point is very important, for 
 the great weakness in the use of dusting sulfurs is their tendency to 
 shatter off from the leaves. Hence a slow acting sulfur may not be 
 efficient because much of it is dislodged before becoming effective. 
 The 80-20 dilution was almost as efficient as the 90-10 after the lapse 
 of 7 to 12 days, but for shorter periods than this was less effective. 
 In orchard practice, therefore, the 20 per cent filler would not be 
 desirable unless applied when the leaves were damp or combined with 
 an adhesive material. The 70-30 dilution was very much slower in 
 action than the first three materials tested, and the 50-50 dilution 
 gave such poor results that it could hardly be considered worth 
 applying. 
 
 The cost of dusting material with a ten per cent filler should be 
 about the same as the cost of pure sulfur. It is doubtful whether 
 the decreased cost obtained by using 20 per cent of lime would be 
 an economical practice, unless the material can be applied under the 
 most favorable circumstances. 
 
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 
 
 69 
 
 Field tests of sulfur with varying amounts of filler confirmed the 
 laboratory results, viz., that a ten per cent filler increased the mechan- 
 ical action of dusting sulfur without a loss in efficiency, and that a 
 50-50 dilution was decidedly inferior to blends with higher percent- 
 ages of sulfur. 
 
 Intervals between dustings. — It is the common experience in sulfur 
 dusting that the applications must be repeated at intervals. The 
 length of time elapsing between applications varies with the adhesion 
 of the sulfur and to a certain extent with the temperature. At 75° F. 
 and above, the effect of sulfur is more toxic, and the rate of hatching 
 of the egg is shorter than at 70° F. and below. To determine the exact 
 intervals between spraying, the following experiments on the brown 
 mite were made on almond from April 20 to June 4. The sulfur used 
 was a mixture of a special dusting brand containing a ten per cent 
 filler of hydrated lime. The applications were made at intervals 
 ranging from 5 to 20 days and repeated from 2 to 9 times. All 
 work was done as uniformly as possible. The sulfur was applied with 
 the same knapsack blower in all the experiments, the work being done 
 between 7 :00 a.m. and 9 :00 a.m. This is usually the stillest time of 
 the day with a comparatively high humidity, conditions which favor 
 this type of work. 
 
 TABLE X 
 
 The Number and Frequency of Sulfur Dustings Necessary to Control the 
 
 Brown Mite (Bryobia) on Almond 
 
 (The first dusting in all plots was on April 20.) 
 
 
 No. of 
 applications 
 
 Days elapsing 
 between dustings 
 
 Observations 
 
 No. 
 
 May 17 
 
 Percentage of 
 
 mites dead 
 
 June 14 
 
 Relative number of 
 
 live mites 
 
 
 
 
 % 
 
 % 
 
 1 
 
 9 
 
 5 
 
 98 
 
 1 
 
 2 
 
 5 
 
 10 
 
 96 
 
 2 
 
 3 
 
 3 
 
 15 
 
 95 
 
 12 
 
 4 
 
 3 
 
 20 
 
 70 
 
 2 
 
 5 
 
 2 
 
 5 
 
 70 
 
 35 
 
 6 
 
 2 
 
 10 
 
 65 
 
 35 
 
 7 
 
 2 
 
 15 
 
 65 
 
 30 
 
 8 
 
 2 
 
 20 
 
 70 
 
 30 
 
 Check 
 
 
 
 
 
 (untreated) 
 
 
 
 4 
 
 96 
 
 
 
 
70 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION 
 
 The data given in Table X show that at least three dustings are 
 necessary under the existing conditions, to gain a satisfactory control 
 of this species of red spider. A decided decrease in the degree of 
 efficiency is shown in all plots receiving but two applications. 
 
 A fair degree of control is seen in all of the first three applications 
 where the time between dustings ranged from 5 to 15 days, but 
 applications at 20-day intervals were inferior to those of 15 days 
 or less. 
 
 The data indicate that the range should be from 10 to 15 days 
 rather than 15 to 20 days, with 3 as the total number of applica- 
 tions. The sulfur adhered better on the plots dusted early in the 
 morning than when applied during a hot, windy time of the day, 
 particularly in midsummer. Sulfur in noticeable quantities was 
 present in plots one to three, inclusive, throughout the entire experi- 
 ment. 
 
 It should be noted that this work was done on the brown mite, 
 which is not a web spinner, thus making possible a much better dis- 
 tribution of the sulfur among the mites than in the case of the com- 
 mon red spider, which spins a heavy protecting web. The work was 
 also begun in the spring when the humidity is higher than in the 
 summer, consequently the sulfur adhered better than in drier weather. 
 For these reasons we would not expect so good control of the com- 
 mon red spider through midsummer as is shown in the above table. 
 
 SUBSTITUTES FOE SULFUE 
 
 It has long been recognized that there is an element of danger in 
 an indiscriminate use of sulfur on certain trees and plants. The 
 apricot and apple in certain districts are usually conceded to be par- 
 ticularly susceptible to "sulfur sickness." But fortunately the stone 
 fruits, which are the heaviest sufferers from red spider, are not very 
 susceptible to injury from this chemical. The caustic solutions of 
 lime or soda and sulfur are much more dangerous to foliage than 
 solutions of sulfur alone. But the application of weak caustic solu- 
 tions is seldom attended with foliage injury unless applied at tem- 
 peratures of 100° F. or more. Even in such cases it is quite probable 
 that the injury from an unchecked attack of red spider would be very 
 much greater than that resulting from spray injury. 
 
 Substitutes for sulfur that have been suggested are wheat flour 
 paste (cooked), consisting of one pound of flour to one gallon of water 
 and diluted at the rate of one part of paste to nine parts of water. 9 
 The time and work required to cook the paste have been a great deter- 
 rant to the use of the formula, but prepared flour pastes, which are 
 
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 
 
 71 
 
 sold at prices comparable to that of a good grade of flour, make this 
 material of practical value. Where the use of sulfur is attended with 
 considerable risk, flour paste mixtures alone might be used with 
 advantage. 
 
 Spraying with water is sometimes resorted to in regions where 
 sulfur applications are objectionable. Such sprays are of value in 
 breaking down the web and washing large numbers of the mites off 
 the leaves, but of course are less effective than those containing at 
 least one gallon of lime-sulfur solution or its equivalents (see p. 74). 
 
 Linseed-oil emulsion has been recommended by Vinal particularly 
 for greenhouse work. 12 The oil is emulsified with soap and used at the 
 rate of one or two gallons per 100 gallons of spray. This mixture is 
 effective on cucumbers and violets but apparently has not been tested 
 as an orchard spray. 
 
 Fish-oil soaps have frequently been recommended and are efficient, 
 but foliage injury frequently occurs from their use. A resin wash 
 has been highly recommended by McGregor for use in South Caro- 
 lina, 8 but has received little attention in California. 
 
 None of these materials has come into general use, perhaps from 
 inefficiency or lack of convenience in preparation. Fish-oil soap, on 
 account of its variable nature and the danger which attends its use 
 on foliage, cannot be recommended. Nicotine applied in the sulfate 
 form has frequently been suggested, but after careful tests, was found 
 inefficient both as a liquid spray and dust. A summary of the results 
 with dusts is given in Table XI. Weak emulsions of the safer forms 
 of petroleum oils are also being used. 
 
 . ' TABLE XI 
 
 Experiments with Nicotine Dusts* on Adults and Nymphs of the 
 
 Brown Mite 
 (Laboratory experiments applied May 8, observed May 13, 1920.) 
 
 Material used 
 
 Number and percentage of 
 dead mites 
 
 Number and percentage 
 of living mites 
 
 2% "Nicodust" 
 
 480 
 
 68.4% 
 685 
 
 87.2% 
 610 
 
 89.6% 
 
 152 
 
 (.8% nicotine) 
 
 5% "Nicodust" 
 
 31.6% 
 
 88 
 
 (2.0% nicotine) 
 
 4% "Nicodust" (1.6% nico- 
 tine) and 60% Sulfur 
 
 12.8% 
 70 
 
 10.4%. 
 
 Check (untreated) 
 
 2% 
 
 98% 
 
 * The nicotine dusts used in these experiments were prepared by the California 
 Walnut Growers' Association, under the direction of Professor Ralph E. Smith. 
 They are composed of a carrier of hydrated lime or kaolin combined with definite 
 amounts of nicotine sulfate. The percentage of nicotine can be approximated 
 from the amount of nicotine sulfate. For example a 2 per cent nicodust is sup- 
 posed to contain 0.8 per cent nicotine. 
 
72 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 Banding trees with tanglefoot or other substances which might 
 prevent the mites from crawling up the trunk has proved of little 
 value. Such measures have been tried repeated^, but with so little 
 success that they are seldom used. 
 
 Natural enemies. — Red spiders are subject to attack by a number 
 of insects, including predacious thrips, coccinellids. a coniopterygid, 
 syrphid fly larvae, and predacious bugs. The last group only has 
 been recorded at all commonly during the height of the attack by the 
 common red spider, and then in too few numbers to be of practical 
 value. A bug (Triphleps tristicolor) has been noted more frequently 
 during the summer than any other predator. 
 
 The mites are so well adapted to the heat and aridity of the interior 
 valleys during the summer months and so prolific under these con- 
 ditions that no natural control has been found that can cope with them 
 at this season. Natural control by parasitic or predacious insects in 
 the early spring seems much more feasible. The mites are greatly 
 reduced in numbers during the winter and at the low temperatures 
 of March and April increase very slowly. One attack on the common 
 red spider at this time of the year was noted, apparently the work 
 of a predacious thrip, in which the mites in every colony examined 
 were killed and the contents of the eggs eaten. It was months after- 
 wards before a single colony of mites could be found in this orchard, 
 and during the entire year the red spider was less abundant in this 
 orchard than ever before noted. 
 
 II. BROWN MITE (Bryobia praetiosa Koch.) 
 
 Spraying experiments at the University Farm and at Durham 
 show that the most effective control for the brown mite is an applica- 
 tion of crude-oil emulsion or lime-sulfur solution for the winter egg, 
 during December, Januar}^, or the first of February. The oil emul- 
 sions have in all cases killed 99 per cent or more of the winter eggs and 
 the efficiency of the lime-sulfur solution has averaged 95 per cent. 
 
 Crude-oil emulsions were at first used at 12 or 15 per cent strength. 
 Later experiments have shown that these percentages are needlessly 
 high and that emulsions containing but four per cent of oil (four gal- # 
 Ions in 100) have been as effective as an ovicide at stronger dilutions. 
 The latter dilution might be inefficient against insect eggs or scale 
 insects, which might be present, hence unless the object is to control 
 only brown mite eggs, it would be better in most instances to use ten 
 gallons or more of oil in 100 gallons of mixture. Two disadvantages 
 
Bulletin 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 
 
 73 
 
 TABLE XII 
 
 Comparative Value of Oil Emulsions and Lime- Sulfur Solutions as a 
 
 Control of the Brown Mite (Bryobia praetiosa)i in the 
 
 Winter Egg Stage. Orchard Test* 
 
 (Observed 3/17) 
 
 Date 
 
 Applied 
 
 Material 
 used 
 
 Dilution 
 
 No. of 
 
 trees 
 
 sprayed 
 
 Percentage 
 
 of eggs 
 
 hatching 
 
 Percentage 
 of hatched 
 
 mites 
 
 living 
 
 Percentage 
 
 of 
 efficiency 
 
 Remarks 
 
 
 Crude oil 
 
 15 gals. 
 
 
 % 
 
 % 
 
 % 
 
 Shot hole 
 
 1-19-20 
 
 emulsion 
 (Home-made) 
 
 of oil 
 in 100 
 
 24 
 
 1 
 
 30 
 
 99.7 
 
 fungus 
 unchecked 
 
 
 Crude oil 
 
 15 gals. 
 
 
 
 
 
 Shot hole 
 
 1-19-20 
 
 emulsion 
 ("Ortho") 
 
 of oil 
 In 100 
 
 26 
 
 1 
 
 25 
 
 99.75 
 
 fungus 
 unchecked 
 
 
 Crude oil 
 
 15 gals. 
 
 
 
 
 
 Shot hole 
 
 1-20-20 
 
 emulsion 
 ("Buggo") 
 
 of oil 
 in 100 
 
 6 
 
 1 
 
 25 
 
 99.75 
 
 fungus 
 unchecked 
 
 
 Lime-sulfur 
 
 10 gals. 
 
 • 
 
 
 
 
 Shot hole 
 
 1-20-20 
 
 (Home-made) 
 32° Baume 
 
 in 
 100 
 
 23 
 
 50 
 
 8 
 
 96.0 
 
 fungus 
 checked 
 
 
 Distillate 
 
 Q l A gals. 
 
 
 
 
 
 Shot hole 
 
 1-20-20 
 
 Emulsion 
 
 ("Spra- 
 
 mulsion") 
 
 of 
 
 emulsion 
 
 in 100 
 
 10 
 
 14 
 
 90 
 
 87.4 
 
 fungus 
 unchecked 
 
 
 Distillate 
 
 Q% gals. 
 
 
 
 
 
 Shot hole 
 
 1-20-20 
 
 emulsion 
 ("Zeno") 
 
 of 
 
 emulsion 
 
 in 100 
 
 12 
 
 16 
 
 90 
 
 85.6 
 
 fungus 
 unchecked 
 
 
 Distillate 
 
 7 gals. 
 
 
 
 
 
 Shot hole 
 
 1-20-20 
 
 emulsion 
 (" Distillate 
 Emulsion") 
 
 of 
 
 emulsion 
 
 in 100 
 
 12 
 
 18 
 
 90 
 
 83.8 
 
 fungus 
 unchecked 
 
 
 Check 
 
 
 
 
 
 
 Shot hole 
 
 
 (untreated) 
 
 
 
 90 
 
 92 
 
 
 fungus 
 unchecked 
 
 
 
 
 
 
 
 
 * Duplicate orchard and laboratory experiments in large series were made with 
 the materials mentioned, with results so similar that the above data may be con- 
 sidered as typical. 
 
 t Garman reports similar results on the egg of P. pilosiis. 50 
 
74 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 in the use of crude-oil sprays are, first, their lack of fungicidal action 
 (for example on "Shot-Hole Fungus") and second, their somewhat 
 low efficiency on the twig borer (Anarsia linmtella). These faults can 
 be largely overcome by the use of a specially prepared crude-oil emul- 
 sion, which can be safely combined with lime-sulfur solution. This 
 mixture in the one year that it was tested showed the superior qualities 
 of both the oil and lime-sulfur preparations at the dilutions of four 
 gallons of crude oil and one gallon of lime-sulfur solution in ten of 
 the mixture. 
 
 Proprietary and homemade crude-oil emulsions proved equally 
 efficient. A satisfactory type of oil for the preparation of emulsions is 
 one testing 18° to 21° Baume. Very light crude oil was slightly less 
 efficient than the heavier types. 
 
 TABLE XIII 
 
 Comparative Value of Lime-Sulfur Solution and its Substitutes* and 
 Crude Oil Emulsion as a Control of the Brown Mite 
 (Bryobia praetiosa) in the Winter Egg Stage 
 
 (Laboratory tests applied January 18, 1921.) 
 
 No. 
 
 1 
 2 
 
 3 
 
 *4 
 
 5 
 
 *6 
 
 7 
 8 
 
 Material used 
 
 Observed February 23 
 
 Percentage of 
 
 eggs 
 
 hatching 
 
 Lime-sulfur, 1 in 10 
 
 Lime-sulfur, 1 in 10; Nicotine 
 sulphate, 1 in 800 
 
 Barium-sulfide, 26 lbs. in 100 
 gals 
 
 Barium-sulfide (B.T.S.), 64 lbs. 
 in 100 gals 
 
 Dry lime-sulfur, 20 lbs. in 100 
 gals ' 
 
 Dry lime-sulfur, 44 lbs. in 100 
 gals 
 
 Crude oil emulsion, 4% 
 
 Crude oil emulsion, 4%; Lime- 
 sulfur, 1 in 10 
 
 Check (not sprayed) 
 
 Percentage of 
 
 hatched 
 
 mites dying 
 
 65 
 
 70 
 
 60 
 
 55 
 
 60 
 
 65 
 
 
 4 
 95 
 
 10 
 15 
 
 
 
 
 
 5 
 
 
 10 
 2 
 
 Observed March 1 
 
 Percentage of. Percentage of 
 
 eggs 
 hatching 
 
 80 
 95 
 95 
 65 
 
 85 
 
 90 
 
 
 
 12 
 95 
 
 hatched 
 mites dying 
 
 100 
 100 
 
 91 
 
 83 
 
 81 
 
 
 100 
 12 
 
 * The substitutes for lime-sulfur solution are here compared on their poly- 
 Blllfide content. On this basis it requires 6.4 pounds of dry barium-sulfld or 4.4 
 pounds of dry lime sulfur to equal 1 gallon of lime-sulfur concentrate, 33° Baume. 
 
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 
 
 75 
 
 HOME-MADE CEUDE OIL EMULSION 
 Formula 4 
 
 Crude Oil - 4 to 12 gallons 
 
 Fish Oil Soap (hard) 4 to 7 pounds 
 
 Fish Oil Soap (liquid) 1£ to 3 gallons 
 
 Lye or caustic soda 12 to 16 ounces 
 
 Water to make - 100 gallons 
 
 No injury resulted to the tree from airy of the crude-oil sprays, 
 except that the one applied just before blooming slightly injured the 
 tips of the new leaves. It will probably be found safer to apph r oil 
 
 Fig. 11. — Development of almond buds January 20, when the second series 
 of winter sprays was applied to kill the winter egg of the brown mite {Bryooia 
 praetiosa) . 
 
 sprays while the tree is quite dormant. Light oils, such as distillates, 
 including both homemade and commercial emulsions and those listed 
 in Table XII as "Zeno" and " Spra-Mulsion, " were decidedly inferior 
 to crude-oil emulsion. 
 
 The time for applying crude-oil emulsions is during the dormant 
 period of the tree. There is no apparent advantage in late spraying, 
 as there is with lime-sulfur solutions. Crude-oil emulsions have been 
 applied during December, January, and the first week of February 
 and even after the buds were opening on almonds, without any injury 
 to the tree. It is probably safer to spray with oil on almonds and 
 prunes during December and January rather than later in the year. 
 
76 
 
 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 Avoid spraying with oil when the trees are very dry, for example, 
 following a heavy north wind. At such time there is greater possi- 
 bility of absorption of oil by the trees since the amount of moisture 
 in the twigs has been reduced. 
 
 TABLE XIV 
 
 Results of the Combined Effect of Sprays and Mite Attack 
 
 (Observed May 21) 
 
 Material used 
 
 Applied 
 
 Months 
 
 after 
 
 application 
 
 Percentage 
 
 of 
 decrease in 
 infestation 
 
 Remarks 
 
 Crude oil emulsion, 
 
 1/19 
 
 4 
 
 98 
 
 Foliage a deep green, except 
 
 15% 
 
 
 
 
 where mite is present. Side 
 
 
 
 
 
 next to check row shows a 
 
 
 
 
 
 15% increase in infestation. 
 
 Lime-sulfur, 1-10 
 
 1/20 
 
 4 
 
 92 
 
 Foliage a lighter green than 
 where crude oil was used, but 
 of a better appearance than 
 early spring app^cations. 
 
 Lime-sulfur, 1-50 
 
 3/6 
 
 2.5 
 
 88 
 
 Leaves small; beginning to fall. 
 
 Distillate oil, 7% .... 
 
 1/20 
 
 4 
 
 70 
 
 Leaves small and yellow, com- 
 parable to check row. 
 
 Lime-sulfur, 1-50 
 
 5/8 
 
 0.5 
 
 99 
 
 Leaves small but regaining a 
 deep green color. 
 
 Check (untreated).... 
 
 
 
 3 
 
 Leaves small and vellow; de- 
 
 
 
 
 
 foliated very rapidly. 
 
 Lime-sulfur solution used at winter strength (one in ten for concen- 
 trates testing about 33° Baume) was inferior to crude oil in killing 
 the egg of the brown mite, but since this spray is also of value in 
 controlling "Shot-Hole Fungus" {Cercospora circumscissa) and the 
 twig borer (Anarsia lineatella) , when applied just prior to blooming, 
 its total value in most orchards might be greater than crude oil. The 
 percentage of mite eggs actually killed by the lime-sulfur solution is 
 low, ranging from 10 to 70 per cent, but it will be seen from Table XII 
 that a large number of mites which hatched died within a few days 
 after emerging. The total efficiency (i.e., including both the eggs and 
 young mites that were killed) of the lime-sulfur spray applied at 
 different times is as follows : 
 
 December 29, 1919, total efficiency 92.3 per cent 
 
 January 20, 1920, total efficiency 96.0 per cent 
 
 February 11, 1920, total efficiency 97.8 per cent 
 
 The latter date is from seven to ten days before blooming. It will be 
 seen from this that the later in the winter this spray is applied the 
 more effective it is in control. 
 
Fig. 10. — (Color plate.) Natural color of foliage restored after checking the 
 brown mite (Brijobia) attack. The twig at the right was photographed thirty-six 
 days after being sprayed. The twig at the left shows the typical color of the 
 uiisprayed trees. Spray applied May 8. 
 
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 77 
 
 Spring and early summer control of the broivn mite is best effected 
 by the caustic spray solution as recommended for the common red 
 spider (page 61). Dusting with sulfur is more effective against this 
 
 Fig. 12. — Development of almond buds February 11, when the last series of 
 winter sprays was applied to kill the winter egg of the brown mite (Bryobia 
 praetiosa.) 
 
 mite than against the common red spider, but is slower in action than 
 the lime-sulfur solution. The almond tree responds very quickly after 
 the mites are killed and within a month's time it regains its normal 
 color (see Fig. 10). 
 
78 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 III. CITEUS MITE (Paratetranychus pilosus Can. and Fanz.) 
 
 The control of the citrus mite on deciduous trees is similar to that 
 of the brown mite. Winter sprays applied to the egg are desirable 
 but, if control is delayed until spring, the use of a dilute lime-sulfur 
 solution with the addition of five pounds of powdered sulfur is the 
 quickest means of control (see p. 61). 
 
 SPRAY PROGRAM FOR DECIDUOUS ORCHARDS 
 
 Almond: 
 
 To kill the winter egg of the brown or almond mite spray in 
 December or January with crude-oil emulsion 4 to 12 per cent strength 
 (formula 4, p. 75). 
 
 If the twig-borer or shot-hole fungus is present in addition to red 
 spider eggs, use a combination of crude-oil emulsion and lime-sulfur 
 (see p. 74), or lime-sulfur solution alone at winter strength (or its 
 substitutes), preferably just before the buds open (Table XIII and 
 pages 73 and 76). 
 
 For the spring attack of the brown mite, spray with lime-sulfur 
 or its substitutes (formula 2, p. 61), or dust every 10 or 15 days with 
 sulfur (pp. 66-70). 
 
 The common red spider is controlled in the same way as on the 
 prune in the interior valleys. 
 
 Prune: 
 
 Interior valleys. — Spray all orchards subject to infestation during 
 the first two weeks of July, or earlier if attacks in previous years 
 have begun before this date. Do not wait until the red spiders are 
 seen before beginning to spray. 
 
 Use formula 1 (p. 61), or dust carefully three times (usually) 
 every ten or fifteen days, with sulfur containing a ten per cent filler. 
 The latter method is frequently less satisfactory than spraying, 
 especially if much web has been spun (pp. 59-60, 66). 
 
 If the mites are well established in any part of the orchard, spray 
 with formula 2 (p. 61). 
 
 Should the brown or almond mite attack, use the methods recom- 
 mended for this mite under Almond. 
 
 Coastal regions. — Prunes in these localities are more subject to 
 attack by the brown mite than in the interior. For orchards that are 
 commonly infested or where the winter egg is found in abundance, 
 use the measures recommended under Almond. 
 
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 79 
 
 The common red spider is not so troublesome as in the interior 
 valleys, but may require control, especially in dry years. For this 
 mite use the same sprays as in the interior, though dusting is more 
 successful in these localities than in the interior valleys. 
 
 Peach : 
 
 Red spiders seriously injure peaches in parts of the interior, par- 
 ticularly the San Joaquin Valley. This injury is principally due to 
 the common red spider. Use the control measures recommended for 
 prunes in interior valleys (p. 78). 
 
 COST OF SPRAYING AND DUSTING 
 
 Spraying.— To thoroughly wet a full-grown tree in foliage will 
 require from 7 to 11 gallons of spray, the latter amount for large 
 trees like almonds. Allowing 70 trees to the acre and an average of 
 9 gallons per tree, 630 gallons per acre would be required. With 
 sulfur at 5 cents a pound, lime-sulfur solution, 20 cents a gallon, 
 and calcium casemate, 23 cents a pound, the cost of the materials in 
 100 gallons of spray and per acre is as follows : 
 
 Per 100 gallons 
 
 Formula I $0.36 
 
 Formula II 0.56 to 0.76 
 
 Formula III 0.26 
 
 Formula IV 0.82 to 1.48 
 
 The cost of application fluctuates with the price of labor and the 
 type of apparatus used. A power sprayer requires a team and two 
 or three men to operate it, according to whether a "spray gun" or 
 the regular nozzle is used, and possibly the cost of a supply wagon 
 must be included. The amount of spray applied per day varies from 
 800 to 1800 gallons per day. 
 
 Dusting. Knapsack Duster 
 
 Sulfur (15 to 20 pounds per acre) $0.75 to $1.00 
 
 Labor cost (1 man, $2.50 per day) (2 to 10 acres per day) 0.25 to 1.25 
 
 Per 
 
 acre 
 
 $2.28 
 
 
 
 3.58 
 
 to 
 
 4.84 
 
 1.67 
 
 
 
 5.20 
 
 to 
 
 9.37 
 
 'Range of total cost per acre for labor and materials $1.00 to $2.25 
 
 * Acreage covered based on trees of five years ' age or more. It should always 
 be noted in the comparison of the cost of dusting versus spraying as a control for 
 red spider, that for most situations it is necessary to make two or three applica- 
 tions of dust to one of a liquid spray, and even then the repeated dustings may 
 be less efficient than one thorough spraying. 
 
80 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 
 
 Power Duster 
 
 Sulfur (20 to Qo pounds per acre) _— $1.00 to $3.25 
 
 Labor cost (2 men and 1 team per day, $5.00) 
 (Machine operation, per day, $1.00) 
 
 $9.00 
 Labor per acre (20 to 30 acres for an all-day run) 0.30 to 0.45 
 
 Range of total cost per acre for labor and materials $1.30 to $3.70 
 
 SUMMARY 
 
 Three species of red spiders are common in deciduous orchards : 
 the common red spider or two-spotted mite which attacks the trees 
 during midsummer ; the brown mite or almond mite, which passes the 
 winter in the egg stage on the tree, feeds from the first of March to 
 August ; and the citrus red spider, also found on the tree during the 
 winter in the egg stage. 
 
 The common red spider winters in a dormant condition or feeds 
 upon hardy weeds and cultivated ^plants. There is a slow increase in 
 numbers during the spring on wild morning-glory, Malva, and other 
 weeds. Migration to orchard trees occurs in June or the first of July. 
 
 Red spider attack results in pale, yellow leaves, followed by defolia- 
 tion. Trees so affected are incapable of producing the required food 
 for maturing the year's crop and buds for the following year. 
 
 Red spider attack decreased the crop value in two prune orchards, 
 $113 and $453 per acre, respectively. This included loss in weight of 
 crop and reduction in grade. 
 
 Drought is usually associated with red spider injury. Thrifty trees 
 with abundant moisture are less liable to this type of loss, but ample 
 soil moisture is not absolute protection against red spider attacks. 
 
 Timely irrigation or the control of either the spring or summer 
 attacks of red spiders caused a marked increase in the number and 
 the size of fruit buds in orchards attacked by red spider. 
 
 Spraying as a control for the common red spider should be done at 
 the time the attack usually began in previous years, even though the 
 mites cannot be seen. The spraying must be done very carefully, 
 using five pounds of sulfur (made into a paste with calcium casemate 
 or glue water) to 100 gallons of water. 
 
 Should the spraying be delayed until the red spider is abundant, 
 two or more applications at three weeks' intervals are usually neces- 
 sary. 
 
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 81 
 
 Sprays, applied after the common red spider is numerous in the 
 orchard or for severe attacks of the brown mite or citrus mite, should 
 contain one gallon of lime-sulfur concentrate (or its substitutes) to 
 100 gallons of mixture. 
 
 The control of any species of red spider by spraying requires 
 timely and extremely careful applications. Careless, scanty spraying 
 is almost worthless. Cover both sides of the leaves on every part of 
 the tree, using from five to ten gallons per tree. An early applica- 
 tion, usually about the first of July, is of far more value than late 
 sprayings. 
 
 Sulfur dusting is of the greatest value in regions of light wind and 
 moderate humidity. Use a 10 per cent filler of hydrated lime in all 
 dusting sulfurs and apply every 10 or 15 days. Three applications 
 are usually necessary unless all conditions are very favorable. 
 
 The brown mite is most easily controlled in the egg stage by winter 
 spra} T s of crude-oil emulsion or lime-sulfur solution, winter strength. 
 The oil emulsion may be applied at any time during the winter, but 
 lime-sulfur is more effective as the cluster buds are opening. 
 
 Lime-sulfur in the dry or solution form is also of value in control- 
 ling the peach twig borer and shot-hole fungus. If the latter pests are 
 present, it is better to use lime-sulfur than crude-oil emulsion, even 
 though the latter is more effective against the egg. 
 
82 UNIVERSITY OP CALIFORNIA EXPERIMENT STATION 
 
 BIBLIOGRAPHY OF LOCAL INTEREST 
 
 i Banks, Nathan. 
 
 1900. "The Eed Spiders of the United States." U. S. Dept. Agr., Div. 
 Ent., Tech. Ser. 8, pp. 65-79, figs. 1-16. 
 
 2 DE Ong, E. R. 
 
 1918. "Control of Red Spiders." Mo. Bull. Calif. St. Com. Hort,, vol. 7, 
 
 no. 3, pp. 111-118. 
 
 1919. "The Red Spider." Mo. Bull. Calif. St. Com. Hort., vol. 8, nos. 
 
 11-12, pp. 679-680. 
 
 1921a. "Controlling the Red Spider." In Univ. Calif. Jour. Agr., vol. 7, 
 no. 1, pp. 3-4, 23-24. 
 
 1921b. "Suggestions for the Control of Red Spiders in Deciduous Or- 
 chards." Mo. Bull. Calif. St. Dept. Agr., vol. 10, nos. 5-6, pp. 
 186-191. 
 
 1922. < < Summary of Red Spider Control. ' ' Mo. Bull. Calif. St. Dept. Agr. 
 (in press). 
 
 s Essig, E. O. 
 
 1913. "Injurious and Beneficial Insects of California." Mo. Bull. Calif. 
 
 St. Com. Hort., vol. 2, nos. 1 and 2. Reference to red spiders, 
 pp. 6-10, figs. 9-11. 
 1915. "Injurious and Beneficial Insects of Calif." Supplement to Mo. 
 
 Bull. Calif. St. Com. Hort., vol. 4, no. 4, pp'. 13-17. 
 1922. "The European Red Mite." Mo. Bull. Calif. St. Dept. Agr., vol. 11, 
 
 no. 4, pp. 409-411. 
 
 * Ewing, H. E. 
 
 1914. "The Common Red Spider or Spider Mite." Oregon Agr. Exp. Sta., 
 
 Bull. 121, 95 pp. 
 1921. "New Nearctic Spider Mites of the Family Tetranychidae. " Proc. 
 U. S. Nat. Museum, no. 2394, vol. 59, pp. 659-666. 
 
 s Garman, Philip. 
 
 1921. "The European Red Mite (Paratetraiiychus pilosus Can. & Fanz.) in 
 Connecticut. ' ' In Jour. Econ. Ent., vol. 14, no. 4, pp. 355-359. 
 
 5° Garman, Philip. 
 
 Same, hi Report Conn. St. Entomologist, 1921, pp. 146-152. 
 
 6 Gray, Geo. P. 
 
 1918. "Wettable sulfurs." Mo. Bull. Calif. St. Com. Hort., vol. 7, no. 4, 
 pp. 191-192. 
 
 7 Harvey, E. M., and Murneek, A. E. 
 
 1921. "The relation of Carbohydrates and Nitrogen to the Behavior of 
 Apple Spurs." Oregon Agr. Exp. Sta. Bull. 176, 47 pp. 
 
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 83 
 
 s McGregor, E. A. 
 
 1916. "The Red Spider on Cotton and How to Control It." U. S. Dept. 
 
 Agr., Farmers' Bull. 735, 12 pp., 10 figs. 
 
 McGregor, E. A., and McDonough, F. L. 
 
 1917. "The Red Spider on Cotton." U. S. Dept. Agr. Bull. 416, 72 pp. 
 
 9 Parker, W. B. 
 
 1913. "The Red Spider on Hops in the Sacramento Valley of Calif ornia. ' ' 
 U. S. Dept. Agr., Bur. Ent., Bull. 117, 41 pp. 9 figs., 6 pis. 
 
 1913. "Flour Paste as a Control for Red Spiders and as a Spreader for 
 Contact Insecticides. ' ' U. S. Dept. Agr., Bur. Ent., Circ. 166, 
 5 pp., 2 figs. 
 
 io Quayle, H. J. 
 
 1912. "Red Spiders and Mites of Citrus Trees." Univ. Calif. Agr. Exp. 
 
 Sta., Bull. 234, pp. 483-530, 35 figs. 
 
 1913. "Some Natural Enemies of Spiders and Mites." In Jour. Econ. 
 
 Ent., vol. 6, no. 1, pp. 86-88. 
 
 ii Stabler, H. P. 
 
 1913. "Red Spider Spread by Winds." Mo. Bull. Calif. St. Com. Hort., 
 vol. 2, no. 12, pp. 777-780. 
 
 12 Vinal, Stuart C. 
 
 1917. "The Greenhouse Red Spider Attacking Cucumbers and Methods for 
 Its Control." Mass. Agr. Exp. Sta., Bull. 179, p. 33. 
 
 is Volck, W. H. 
 
 1903. "Sulfur Sprays for Red Spiders." Univ. Calif. Agr. Exp. Sta., 
 Bull. 154, 10 pp. 
 
 1913. < < The Control of Red Spiders. ' ' Mo. Bull. Calif. St. Com. Hort., vol. 
 2, nos. 3-4, pp. 356-363. 
 
 14 Written, J. C. 
 
 1919. "Frost Control and Related Factors." Mo. Bull. Calif. St. Dept. of 
 Agr., vol. 8, no. 11-12, pp. 675-678. 
 
 15 WOODWORTH, C. W. 
 
 1902. "The Red Spider of Citrus Trees." Univ. Calif. Agr. Exp. Sta. 
 
 Bull. 145, 19 pp. 
 
 1903. Entomology. In Univ. Calif., Agr. Exp. Sta., Bpt., 1901-1903, pp. 
 
 104-110. (Red Spider Remedies, p. 105.)