UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA CONTROL OF DIABROTICA, OR WESTERN SPOTTED CUCUMBER BEETLE, IN DECIDUOUS FRUIT ORCHARDS A. E. MICHELBACHER, G. F. MacLEOD, and RAY F. SMITH BULLETIN 681 October, 1943 UNIVERSITY OF CALIFORNIA BERKELEY, CALIFORNIA CONTENTS PAGE Introduction 3 Systematic position 3 Historical summary 4 Distribution 5 Host plants 5 Life history 6 Experimental methods used 11 Injury to deciduous fruits, and economic importance 12 Factors influencing size of populations in orchards 16 Control in deciduous fruit orchards 23 Natural enemies , 31 Summary 31 Acknowledgments 32 Literature cited 33 CONTROL OF DIABROTICA, OR WESTERN SPOTTED CUCUMBER BEETLE, IN DECIDUOUS FRUIT ORCHARDS 1 A. E. MICHELBACHER, 2 G. F. MacLEOD, 3 and RAY F. SMITH* INTRODUCTION The Western spotted cucumber beetle, Diabrotica 11-punctata Mann., is one of the most destructive native insects in California. This insect is most com- monly known in California as "diabrotica." In order to avoid the frequent use of the more cumbersome name, the term diabrotica will be used in this bul- letin. It attacks a great many cultivated crops and both the larvae and adults injure plants. The adults, however, are the more important and each year large sums of money are expended trying to control them. They are particularly destructive to some of the cucurbits, to beans, corn, and other truck crops. Under certain conditions deciduous fruits are seriously attacked. The degree of attack varies not only from year to year but between districts. Under severe conditions nearly all the ripening fruit may be injured. The beetles not only damage the fruit by direct feeding, but the injuries are a portal of entry for the brown rot organism. Request for the investigation of this pest as it affects deciduous fruits was made following the serious outbreak of 1938. The study was undertaken late that season and has been continued up to the present time. A preliminary report on the early work has been published (Michelbacher and associates, 1941 ) . 5 The investigation has now progressed to a point where con- siderable information on life history, habits, and control has been accumu- lated, and is summarized in this bulletin. In this publication diabrotica is considered only from the standpoint of its destructiveness to deciduous fruits, and the control measures given pertain to orchard crops only. SYSTEMATIC POSITION Diabrotica 11-punctata Mannerheim (1843) Galleruca 11-punctata Eschscholtz (MS) Diabrotica soror LeConte (1865) The genus Diabrotica is one of the largest genera in the family Chrysome- lidae. Many of its species are of economic importance. When D. 11-punctata was collected by the Russians in California, it was determined as D. 12-punc- tata (Fabr.). However, Eschscholtz in a letter to Mannerheim called it Galle- ruca 11-punctata. Later Mannerheim (1843) clearly distinguished it, but considered it a variety of D. 12-punctata and recorded it from California. LeConte (1865) described D. soror from specimens collected by John Xantus at Fort Tejon in 1857-58. The western spotted cucumber beetle has generally 1 Received for publication January 2, 1943. 3 Assistant Entomologist in the Experiment Station. 3 Professor of Entomology and Entomologist in the Experiment Station. 4 Senior Laboratory Assistant. 5 See "Literature Cited" at the end of the bulletin for full data on citations which are referred to in the text by author and date of publication. [3] 4 University of California — Experiment Station been known under the name D. soror LeConte (1865). As pointed out by the present authors (1941), however, the older name D. 11-pwnctata Mannerheim is available. HISTORICAL SUMMARY This beetle was first collected in California by the early Russian exploring expeditions. Eschscholtz took it in 1824 and later Tschernikh collected it on the expedition of 1833-1835 (Essig, 1931) . This native insect must have been a pest of agriculture from early times although the first published record of it as a pest does not occur until 1877. In January of that year a letter from H. G. Newton of Pasadena, California, was published in the Pacific Rural Press complaining of injury to oranges and roses. The reply by the editor indicated that the insect had been a pest many times before. The earliest report of injury to deciduous fruit was made by Comstock (1880) who reported considerable damage to apricots from Dixon, California. Cooke (1883) also recorded damage to apricots. Practically no important papers have been published on diabrotica but a large number of isolated notes have been made on life history, habits, and control. Most workers have been content to assume that what applied to the eastern species of Diabrotica held true for 11-punctata. Craw (1892), Lelong (1890), Koebele (1890, 1891), Coquillett (1890), Marsh (1907), Wymore (California Agricultural Experiment Station, 1927, 1929, 1930), Woodworth (1895), Campbell and Nixon (1921), Mote (1926),Besse (1936), Elmore and Campbell (1936), all mention the insect, usually as part of a general treatise. As editor of the Pacific Rural Press, Wickson (1877-1923) published a large number of letters reporting damage to various crops by this beetle. The most significant thing brought out by that series of complaints was the great varia- tions in abundance of the beetles. Doane (1897) was the first person actually to studj^ our western species. He conducted life-history studies and described the egg, larva, and pupa. His brief notes on the habits of the immature forms agree very closely with those obtained by the present authors under controlled conditions. Chittenden (1910) figured the egg and adult. Marsh (1907, 1908) men- tioned considerable damage to cucurbits and other truck crops. Essig (1913) stated that there are two distinct generations whose broods overlap through- out the summer and he gave a partial host list. Later, Essig (1915) gave a brief review of the status of the beetle, figured the injury, gave a host list and control measures. Lovett (1913) drew heavily on the published information concerning the eastern species of this genus in his work on Diabrotica 11-punc- tata. His notes have been repeated many times in later Oregon publications. Sell (1915) undertook to study the life history of diabrotica but was not very successful and encountered many difficulties. He discussed food habits, migration, enemies and "color phases." lie did not obtain any stages beyond the egg. Wilson (1915) reported diabrotica as feeding on the leaves of young almond and apple trees and the fruit of peaches in Oregon. Urbahns (1926) reported diabrotica as a pest of apricots and early peaches and suggested that the prevalence of these beetles was probably due in part to the late rains coming in June. Mackie (1936) reported the beetles as seriously Bul. 681] Control of Diabrotica 5 injuring apricots in 1936. A few notes were made on the habits of the insect in the orchards during the previously reported study by Michelbacher and asso- ciates (1941), but no complete study of D. 11-punctata has ever been under- taken and, until the work of the present authors, no one had reared the insect through all the stages. DISTRIBUTION The genus Diabrotica is almost entirely confined to the two American con- tinents and the West Indies. Over 800 species have been described and prob- ably many more yet remain to be described ( Weise, 1924) . The largest part of the genus is confined to the tropical part of the Americas ; a few species occur in the more temperate regions. North of Mexico there are 19 species and several varieties. About two thirds of these are restricted to the most southern regions of the United States, only 2 or 3 species finding their way into the north. Four species of Diabrotica occur in California, namely D. 11-punctata Mann., D. trivittata Mann., D. balteata Lee, and D. 12-punctata (Fabr.). Diabrotica 11-punctata is most abundant in California and Oregon. It also extends into Washington, Arizona, New Mexico, and probably Mexico. It has been recorded from Boulder, Colorado (Cockerell, 1924) and doubtfully from Colombia, South America (Toro, 1929). Brisley (1925) is convinced that D. 11-punctata does not occur in Arizona and believes that reports to the contrary are incorrect. HOST PLANTS The host plants of Diabrotica 11-punctata can well be divided into two groups. One includes the plants upon which the larvae feed and the other those plants which serve as food for the adult. The list in both cases is very large and only some of the more important ones are given below. As would be expected, in many cases the same plant is utilized by both the larva and the adult. Attacked by larva Attacked by adult Grains : Truck and field crops : corn snap beans wheat field beans barley corn Cucurbits: melons cucumbers artichokes sugar beets lettuce alfalfa S( * uash sunflower gourds Fruit and nut crops , Legumes: apricots alfalfa beans peaches nectarines P eas cherries almond Flowering garden plants: feed on the flowers of most plants found in gardens, especially compositae. 6 University of California — Experiment Station Attacked by adult? — Continued Cucurbits: squash melons cucumbers gourds Uncultivated plants: mayweed (Anthemis cotula) ground buffalo gourd (Cucurbita foetidissima) manroot (Echinocystis) developing heads of both tame and wild grasses, and floral parts of many wild flowers. Among the uncultivated plants there are several that are very attractive to the adults. Elmore and Campbell (1936) found that ground buffalo gourd attracted the beetles in large numbers. The present writers have observed the beetles concentrating and feeding heavily on manroot. This plant appears to be selected from all others and often suffers nearly complete defoliation. In the spring the pest concentrates in large number on mayweed, where it appears to be attracted to the flowers. The beetles feed heavily upon the petals and probably feed upon the pollen which is rich in protein. The adults feed extensively upon all of the cultivated crops listed and, while almost any part of the host may be eaten, it is the floral parts that are usually most seriously attacked. LIFE HISTORY The larval and pupal stages of Diabrotica 11-punctata are spent in the soil. The adults lay their eggs about the bases of the plants just beneath the surface of the soil. The eggs are oval and yellowish but before hatching become dingy in color. The larvae, on hatching, start to feed upon the roots of their host plants. They are tan in color and covered with a scattering of rather long hairs. The head and prothoracic shield, as well as the strongly chitinized posterior end, are dark brown to nearly black. There are three larval stages. The greatest increase in weight occurs during the third larval instar. Upon reaching matu- rity the larvae construct an earthen cell, and after spending some time in a prepupal condition they pupate. After a varying period, according to the temperature, the adult emerges. The wing covers of newly emerged adults are soft, the spots are barely distinguishable, and the base color is gray. After a few days the wing covers harden, the spots turn black, and the base color becomes irrcen. The stages in the life history of the insect are shown in figure 1. Tn the laboratory the beetle was reared under constant-temperature condi- tions. The temperature in the cabinets did not vary more than 1.5 degrees Fahrenheit. Eggs were usually obtained from individuals collected in the field. A female beetle was placed in a cage made in the following manner. An ordi- nary 5-inch pie tin was filled with moist sand and covered with a circular piece of green blotting paper. A celluloid cylinder about 4 inches in diameter and 4Vj> inches high, covered with a piece of fine organdy held in place with masking tape, was placed on the blotter. The insects were fed lettuce, and eggs were readily laid on the blotting paper which was kept moist by adding water Bul. 681] Control of Diabrotica to the sand from time to time. The yellow eggs laid either singly or in batches could be readily detected on the green background. The eggs were transferred to the type of rearing dishes used by Michel- bacher (1938) for rearing the garden centipede. These dishes were made by thoroughly mixing 10 parts of plaster of paris, 3 parts finely ground soil, and 1 part animal charcoal. Water was added to the mixture and the whole stirred until the material had the consistency of rather thick cream. This was then Fig. 1. — Life cycle of Diabrotica 11-punctata Mann. : A, Adult; B, egg ; C, first instar larva ; D, dorsal view of last instar larva ; E, lateral view of last instar larva ; F, ventral view of pupa. (All x 7, except the egg which is x 30.) poured into stender dishes to about the depth of V4 inch and allowed to set. These dishes were found to be very well suited for rearing individual larvae, and made accurate observations possible. The outside dimensions of the two sizes of dishes used were as follows : depth, 22 mm, and diameter, 38 mm ; and depth, 30 mm, and diameter, 50 mm. Upon hatching the larvae were separated, placed into individual dishes, and fed soaked wheat grains. It was found that grain soaked overnight provided an excellent food, and that the larvae fed readily even though the wheat had not germinated. Large larvae consumed the internal contents of several ker- nels in 24 hours. There was not much tendency for the larvae to bore into the grain and for this reason they could be readily observed during any stage in s University of California — Experiment Station their development. Fresh food was given the larvae every day, or at least every other day, and moisture was added as needed. It was found necessary to construct cells in which the full-grown larvae could pupate. When this was not done the larvae failed to pupate and after a Fig. 2. — Lid constructed over the cell by diabrotica larva, (x 3.) time died. The circular cells were dug in the "muck plate" with a stiff dissect- ing needle. They were made about ^ inch in diameter and about as deep. The larva entered the cell and constructed a lid over it (fig. 2). The covering was made by scraping material from the sides of the cell and cementing it together with a secretion, apparently obtained from the caudal end of the larva. The larva gathered this secretion in its mouth parts and forelegs and built the lid TABLE 1 Number of Days Kequired by Diabrotica 11 -punctata to Complete Its Development at Various Temperatures Stage 60° F 68° F 75° F 80° F 85° F Range Mean Range Mean Range Mean Range Mean Range Mean Egg days 23-30 10-28 9-14 11-24 8-17 24-38 15-24 66-97 days 26.1 15 6 12 15 4 12 4 28.1 19.6 75 3 days 6-20 6-24 5-10 5-18 4-9 10-25 8-15 34-53 days 13.2 10.8 7.1 8.4 6.9 15 2 11 2 43.1 days 7-9 4-10 3-7 4-9 3-7 9-24 7-9 27-33 days 8.3 6.1 4.6 5 9 4.8 10.7 8.0 29 4 days 6-9 3-7 2-5 3-8 2-5 6-11 5-9 20-27 days 7.1 4 5 3 6 4.8 3.8 8.5 6.6 23 4 days 5-7 2-8 2-5 1-9 1-4 5-12 3-6 18-23 days 6 4 3 4 Second instar Third instar: Feeding period Prepupal period Total * 3 5 5 3 3 8.2 5 2 Total days hatch to 20 3 Number of In run com- pleting nil stages H go H'J 33 26 Discrepancies in tolals are due to differences in numbers of insects used. Bul. 681] Control of Diabrotica 300 «/> a. Ul Ul * vt § Sj 200 Q. 8 UJ UJ CD U. y n \ A /^ i 1 z z < 3 • ; ^ A J \ APR MAY JUNE JULY AUG- SEPT. OCT. NOV- Fig. 3. — Seasonal trend of adult diabrotica population in alfalfa fields in the northwest portion of the San Joaquin Valley in 1941. with baek-and-forward movements of the head. After finishing the lid the larva passed into a prepupal condition and then pupated. Upon emerging, the beetle spent a day or two in its cell before breaking its way out. The insect was reared at several constant temperatures and a summary of the results is given in table 1. Note that development was most rapid at 85° F. At this temperature the mean length of time for the individual to complete its entire development from egg to adult was 27 days as contrasted with 101 days at 60°; at 68° the mean period was 56 days. In late winter and spring it is possible that the soil temperature in the strata in which the larvae live aver- ages close to 60° and for that reason it is probably safe to assume that around 100 days or more are necessary for the first spring brood to develop. In work conducted in bare soil at Davis, California, Smith (1929) found the soil tem- perature to average close to 60° at this depth. The soil temperature later in the year in the Brentwood area probably ranges somewhere between 68° and 60Q a. *500 f=300 200 100 A A r K 1 \ l\ ; K ^L J 1 J ^ -** APR MAY JUNE JULY AUO SEPT. OCT. NOV. Fig. 4. — Seasonal trend of adult diabrotica population in alfalfa fields in the agricultural region adjacent to San Francisco Bay in 1941. 10 University of California — Experiment Station 75°, and if so the developmental period would be expected to range between 37 and 58 days excluding' the period necessary for the females to mature their eggs. As far as temperature is concerned there is probably sufficient time for at least three generations to occur in a year. In the northwest portion of the San Joaquin Valley the seasonal population trend of the adult beetles in alfalfa fields was followed during 1941. The num- ber of beetles collected in 100 sweeps of an insect net was recorded, and on most surveys 10 to 15 fields were examined. The results are shown in figure 3. The points plotted represent the average number of beetles collected per 100 sweeps on any particular survey. The graph shows that there were three dis- tinct peaks; these probably represent broods although there must be consider- -SPRINO BROOD SUMMER BROOD- OVERWINTERING BROOD ADULTS JAN- FEB. MAR. APR. MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC Fig. 5. — Seasonal life history of diabrotica in the lower San Joaquin Valley. able overlapping. Similar surveys were conducted in the agricultural region adjacent to the San Francisco Bay; on the average many more beetles were collected per field and instead of three peaks only two occurred. The results are shown in figure 4. The seasonal life history as shown in figure 5 has been constructed from our knowledge concerning the insect, and is what might be expected to occur in a climate such as is found in the lower San Joaquin Valley. Copulation occurs frequently; and the beetles live for a period of several months. Egg laying may also occur over a long period ; one female was recorded as laying 1,784 eggs over a period of 19 weeks in the laboratory. Some of the laboratory-reared females have laid more than 600 eggs, and the average length of life of 20 individuals was 165 days. The insect passes the winter in the adult stage, although under some condi- tions it is possible that all stages of the insect might be found during the colder months. Larvae have been taken late in the fall. The only clearly defined brood is probably that which occurs in the spring. Because the beetles may lay eggs over several months the distinguishing of later broods is somewhat diffi- cult. Bul. 681] Control of Diabrotica EXPERIMENTAL METHODS USED 11 In determining populations in an orchard, five to ten trees were selected at random. Large sheets varying in size from 18 x 18 feet to 24 x 24 feet were placed under the trees, as shown in figure 6. To facilitate placing them under a tree a slit was made half way down the middle. To determine the beetle population the trees were dusted with an insecticide containing pyrethrum. This knocked the beetles from the trees and they could be collected from the -Nectarine tree with a sheet spread under it for catching beetles. sheets and counted. Numerous tests were conducted to determine the efficiency of this method, and it was found that, with proper precautions, practically all the beetles in a tree could be removed. If an effective pyrethrum dust is applied when the air temperature exceeds 60° F, some of the beetles will fly from the tree. For most accurate counts, a temperature of 55° or lower is desirable. In determining mortality, beetles were picked from the sheets and placed with undusted foliage in paper bags. At the end of 24 hours the beetles were examined. They were divided into three groups : (1) those that were definitely dead ; (2) those that showed signs of life but were unable to fly ; and (3) those that appeared normal. The last group was counted as fully recovered, the second group was held for further examination, while the first group was used in determining mortality at the end of 24 hours. At the end of 48 hours, or 12 University of California — Experiment Station sometimes later, the beetles of the second group were again examined and placed in dead and living groups. The dead were added to the number dead at the end of 24 hours, and the mortality for the 48-hour period determined. The bags containing the beetles were kept in a room where the temperature never became excessive. This means of determining mortality proved to be the most satisfactory of several methods tried. Experiments were run in which the col- lected insects were caged, together with a branch, in a tree ; or placed with foliage in screen cages supported in a tree. With screen cages in the orchard the mortality was slightly higher than where the beetles were placed in bags in the laboratory. The higher mortality may have been due to the direct, hot, sunlight on the cages. There was no satisfactory way to avoid this, and it was found that the most accurate method was to place the beetles in bags and hold them in a room as described above. Caged insects in a tree cannot move about freely and if conditions, where they are caged, became very unfavorable they have no means of escape. The method used probably gave a conservative esti- mate of the actual mortality in the field. Insects were always picked up by hand from the sheets for it was found that if they were shaken into the center of the sheet and then poured into a sack, the mortality was increased. This was unquestionably due to the fact that as they were being rolled into a pile they were covered with dust, which resulted in the death of most of the beetles. Hand-picking the beetles from the sheets apparently did not injure them. This was well demonstrated where the dust drift knocked large numbers of the beetles from trees onto the sheets. These were picked up by hand and after 48 hours only 24 were dead out of 1,910 collected. Natural mortality certainly would account for the death of these few beetles. In making field counts. either a hand rotary duster or a power duster was used. In making population counts in uncultivated regions or in alfalfa fields an insect sweeping net was used. The amount of injury done was determined by field counts usually for each of the different fruit pickings, and was based on what was found in the field boxes. Some counts were made in packing sheds. Injured fruit was divided into two categories: (1) primary injury, which included the fruit that had been attacked by the beetle only; and (2) fruit that had been injured by other agencies, although fed upon by diabrotica. INJURY TO DECIDUOUS FRUITS, AND ECONOMIC IMPORTANCE Although this beetle feeds on both the foliage and fruit, cases where serious damage is done to foliage are not numerous. This type of injury is likely to be most severe on recently planted trees, and on such trees defoliation may reach a point where control measures become necessary. During the 1941 season at Brentwood the foliage on newly planted almond trees was so heavily attacked that dusting was necessary. It is doubtful whether foliage feeding ever reaches a point where control measures would be justified in an established orchard. Diabrotica is principally a pest of the fruit, but the very green fruit is not attacked. Usually the fruit is Left untouched until it has at least reached the mature-green stage and in mosl cases it is not attacked until it starts to turn color and ripen. As ripening progresses feeding increases, and in cases of severe outbreaks nearly all the fruit may be damaged by the time the crop is Bul. 681] Control of Diabrotica 13 Fig. 7. — Various degrees of primary injury by diabrotica on apricots. An uninjured fruit is shown at the upper left. Fig. 8. — Primary injury to nectarines by diabrotica. 14 University of California — Experiment Station tree ripe. In 1940 very little of the fruit in the Brentwood area was attacked before it started to turn color. In 1941 feeding started earlier, when much of the fruit was reaching the mature green stage. Apparently this beetle would rather feed upon the foliage than the very green fruit. This is very fortunate for if the pest attacked the fruit in all stages of its development, control would be nearly impossible, or at least the expense would be prohibitive. Control is . ■ .• '■:.■' '...., Fig. 9. — Primary injury to freestone peaches by diabrotica. practical only because the pest confines its attack to the nearly mature fruit. This means that in general the period during which the crop needs protection is hardly ever more than 15 days. This is a relatively short period when it is considered that very large beetle populations may be present in orchards for a period exceeding 2 months. Diabrotica is very important in spreading brown rot. The beetles feed on in t <§*> <§i> <§b> (§k <§k <§£> <§£> <££> Fig. 16. — Diagram of an orchard showing the patli that the duster should follow in relation to the direction of the dust drift. tion of the drift changes so that it goes through the undusted part of an orchard. This must be done even though the dusting operation has just been started. The amount of dust in t he air from dusting one row if drifted through the orchard is sufficient to knock most of the beetles from the trees. Because the direction of drift is so important, considerable time should be spent in determining this be lore beginning operations. In early morning the drift may be veiy variable and within 5 minutes it may shift to any point of the compass. However, on most mornings the drift finally stabilizes in one direction. The direction of drill can be accurately determined by taking a small amount of dust in the palm of the hand and then clapping the hands to create a cloud of dust. Or the drift of tobacco smoke can be noted. If, after a few minutes, it is found that the drift remains constant, dusting can be started. The farmer Bul. 681 Control of Diabrotioa 25 should have other work planned so that the crew can be kept occupied if it is necessary to stop dusting. An ideal dusting arrangement is shown in figure 16. The time that dusting can be started in the morning is dependent upon temperature. When the temperature drops to 63° F operations can be started, and if the drift is in a constant direction work can be continued until an air TABLE 4 Effectiveness of Ground Pyrethrum Flowers and Lethane 384, in Talc, against Diabrotioa on Deciduous Fruit Trees ; 1940* Date dusted Tempera- ture, degrees F Pounds per acre 66 55 50 45 59 50 62 50 62 80 61 50 58 38 50 50 59 53 58 55 50 60 Average number of beetles per tree Number used in determining mortality Per cent mortality after 24 hours Per cent mortality after 48 hours June 13 June 13 June 18 June 20 June 20 June 29 July 5. . July 18 July 19 July 30 July 31 292 196 228 171 217 180 238 177 434 149 371 1,444 1,387 1,140 684 435 1,266 1,192 1,238 2,174 1,044 1,855 73.9 37.2 55.5 89.0 100.0 66.5 86.9 70.7 90.6 84.7 89.1 65 84 * The dust contained 0.15 per cent pyrethrins, except for applications of July 5 and July 19, which contained 0.20 per cent. All the mixtures contained 2.00 per cent Lethane. The dusts were applied by power duster except for the application of July 18, which was by airplane. TABLE 5 Effectiveness of Ground Pyrethrum Flowers and Lethane 384, in Talc, against DlABROTICA on Deciduous Fruit Trees ; 1941* Date dusted Tempera- ture, degrees F Pounds per acre Average number of beetles per tree Number used in determining mortality Per cent mortality after 24 hours Per cent mortality after 48 hours June24| July 16 54 61 61 57 54 57 64 45 45 50 50 55.5 613 1,279 1,326 839 1,250 1,014 3,675 6,398 4,040 4,197 6,252 4,058 31.4 97.5 78.0 97.5 91.5 87.0 34.6 99.0 July 16 84.0 July 18 98.5 July 29 95.5 July 29 91.0 * The dust contained 0.15 per cent pyrethrins, except for the application of July 16, which contained 0.20 per cent. All the mixtures contained 2.00 per cent Lethane. t The trees were very wet from a rain during the night. temperature of about 65° is reached. Minimum temperatures occur just about daybreak so there is a considerable period before and after sunrise that is suitable for dusting. The morning temperatures at Brentwood where most of the investigations were conducted were usually favorable. During periods of very hot weather, however, the temperature sometimes failed to fall as low as 63°. Other factors that may result in relatively high early morning tem- peratures are cloudiness and wind. If rain has fallen during the night and the foliage is not dry by morning, dusting should be delayed a day. Some evidence was obtained that seemed to 26 University of California — Experiment Station indicate that an excess of moisture reduced the effectiveness of a contact in- secticide containing pyrethrum. Just what action occurs is not known, but it is possible that the excess moisture keeps the insecticide from making maxi- mum contact with the insect. The two insecticides that were thoroughly investigated were pyrethrum- Lethane 384 in talc dust, and a 5 per cent Pyrocide dust. The former had 0.15 1400 £IJ200 S ijoooi 5 800 o cr < fe 6001 UJ CO 400 UJ O <200| UJ — 5 6~I0 DAYS 11-20 21-30 Fig. 17. — Reduction in diabrotica population in four orchards fol- lowing dusting with ground pyrethrum flowers and Lethane 384 in talc. The four tall bars at the left indicate the populations at the time of dusting; the shorter bars indicate later counts in the respective orchards. The key to the bars, the composition, and rate of applica- tion are as follows: The black bar, 0.20 per cent pyrethrins and 2.00 per cent Lethane 384, applied at 45 pounds per acre; the vertically hatched bar, 0.15 per cent pyrethrins and 2.00 per cent Lethane 384, at 50 pounds per acre; the cross-hatched bar, 0.15 per cent pyrethrins and 2.00 per cent Lethane 384, at 50 pounds per acre; and the hori- zontally hatched bar, 0.15 per cent pyrethrins and 2.00 per cent Lethane 384, applied at 55 pounds per acre. or 0.2 per cent pyrethrins and 2.0 per cent Lethane. The value of this dust in controlling this pest was thoroughly demonstrated during the 1940 season. The work in 1941 substantiated the results obtained the previous season. When applied under favorable conditions mortalities reach nearly 100 per cent. Some of the results obtained with this dust mixture in 1940 are shown in table 4, and for 1941 in table 5. It should be noted that in most cases very good kills were obtained. In all cases a close 1 correlation was found to exist between mor- tality counts at time of dusting and reduction in the population as determined Bul. 681] Control of Diabrotica 27 by later counts made in the orchards ; this is clearly shown in table 6 for five of the orchards dusted in 1941. In 1941 this type of dust was commercially applied on an experimental basis in four orchards. The results of these dustings are graphically shown in figure 17. All four of the orchards were treated under favorable conditions and very good control was obtained. Both peach and nectarine orchards were dusted and in each case the dust was applied at approximately 50 pounds to TABLE 6 Comparison of Actual Number of Beetles Found per Tree with the Expected Number as Calculated from Mortality Counts at the End of 48 Hours, Following the Application of a Pyrethrum- Lethane 384 Dust Initial population per tree at time ot dusting (from table 5) Per cent mortality at end of 48 hours (from table 5) Beetles per tree calcu- lated from 48-hour mor- tality count Actual num- ber found per tree 72 hours after dusting 613* 34.6 98.5 95.5 91.0 99.0 401 97 56 91 13 435 839 27 1,250 136 1,014 76 l,279f 96 * The trees were very wet from a rain during the night, t Pyrethrin? 0.20 per cent, all others 0.15 per cent. TABLE 7 Effectiveness of 5 Per Cent Pyrocide Dust against Diabrotica on Deciduous Fruit Trees ; 1941 Date dusted Tempera- ture, degrees Pounds per acre Average number of beetles per tree Number used in determining mortality Per cent mortality after 24 hours Per cent mortality after 48 hours June 21 41 62 60 54 40 46 50 50 300 514 1,369 658 2,402 4,114 13,692 3,948 37.5 78.0 74.5 84.4 43 July 5 99 0* July 15 78 5 July 26 95 5 * Sacks containing beetles were placed in a car exposed to the direct rays of the sun. The car became hot and the high mortality indicated is believed to be due to excessive temperature. the acre. Because of the stabilized population a single dusting was sufficient to give protection to the ripening fruit during the entire harvest. A Pyrocide (pyrethrum-impregnated) dust which had a pyrethrins con- tent of 0.1 per cent was also used extensively. This dust was commercially applied on an experimental basis to four orchards. The results of these dust- ings are given in table 7 and graphically shown for three orchards in figure 18. As with the pyrethrum-Lethane 384 dust, a rather close correlation was noted to exist between the mortality counts at time of dusting and the reduction in the population as determined later by counts in the orchard. This relation is shown in table 8. In all but one case the dust was applied under favorable con- ditions. In this orchard there was some drift into the nondusted portion of the orchard and thus the mortality was, in general, less than might have been 28 University of California- 1400 -Experiment Station q: UJ Q_ < »— o cc CO < Q U_ O a: UJ CD 2 lx UJ 1.200 ipoo 800 600 400 200 21-30 Fig. 18. — Reduction in diabrotica population in three orchards fol- lowing dusting with Pyrocide (pyrethrins, 0.1 per cent) in talc. The three tall bars at the left indicate the populations at the time of dusting; the shorter bars indicate later counts in the respective orchards. The orchards represented by the black and the cross-hatched bars received applications of 50 pounds per acre ; that of the horizontally hatched bar, 46 pounds per acre. Ik- lb. JULY AUGUST Fig. 19. — Results of applying dusts containing pyrethrins, at different times. The bars marked "dusted" represent the average beetle populations per tree in 1 he respective orchards at the time of dusting. The other, and shorter, bars rep- resent subsequent counts. Since the orchards were all in the same general area, and the dates of application covered a 2-week interval, it can be assumed that the sharp reduction in population after dusting was due to the effects of the insecticide rather than natural factors. Bul. 681] Control of Diabrotica 29 otherwise expected; however, even where the dust was applied under more favorable conditions the mortality from the Pyrocide dust was hardly equal to that obtained with the pyrethrum-Lethane 384 dust. The control nevertheless was satisfactory, and from visual observations equalled that obtained with the latter mixture. Although relatively large populations were left in two of the treated orchards, feeding by the beetles ceased almost entirely. The dust must have been responsible for this, for in each case the fruit was being seriously attacked up to the time it was applied. The protection afforded lasted until the entire crop was harvested. This was obtained with a single dusting, and the period of control lasted at least 2 weeks or longer. There was a marked reduction in populations in all orchards where experi- mental commercial dusting was carried out. In order to show that these TABLE 8 Comparison of Actual Number of Beetles Found per Tree with the Expected Number as Calculated from Mortality at the End of 48 Hours, Following the Application of a 5 Per Cent Pyrocide Dust Initial population per tree at time of dusting (from table 7) Per cent mortality at end of 48 hours (from table 7) Beetles per tree calcu- lated from 48-hour mor- tality count Actual num- ber found per tree 72 hours after dusting 514 99.0* 78.5 95.5 43 5 295 30 171 31S 1,369 457 658 84 300 214 * Sacks containing beetles were placed in a car exposed to the direct rays of the sun. The car became hot and the high mortality indicated is believed to be due to excessive temperature. reductions were due to dusting and not some other factor, figure 19 has been prepared. Here results for five orchards are graphically shown ; the period covered is from July 15 to August 15. The beetle population at the time each of the orchards was dusted, and the populations as determined by later counts are shown. None of the orchards were more than 2 1 /2 miles apart, and in all cases they consisted of peach or nectarine trees. The area was a rather homo- geneous one and for that reason if the reduction in population was due to some natural factor, it would be expected to occur in all the orchards at about the same time. However, an examination of the graph shows that dusting was done over a period of about 2 weeks and that in every case the reductions in populations did not occur until the dust was applied. It is possible that other factors may influence the population of beetles in the orchard during harvest. Picking of the fruit certainly causes some move- ment of the beetles and may at times drive large numbers from an orchard. While this may be possible it is probably not the usual condition. Large migra- tions from an orchard do not occur until harvest is complete or nearly so. It is the belief of some growers that sulfur dust will drive the beetles from an orchard. During the 1941 season much sulfur was applied shortly before the fruit had begun to ripen in an effort to check brown rot which was rather prevalent. In most of these orchards there was no evidence of the beetles being 30 University of California — Experiment Station killed or driven from them. In the region of heavily infested orchards at Brentwood it was necessary to use an insecticidal dust to protect the ripening crop from serious damage. At the time these dusts were applied the number of beetles per tree in some of the orchards exceeded 1,000. Under certain con- ditions a sulfur dust may cause the beetles to leave an orchard but the treat- ment at best is of doubtful value and should not be relied upon to save a crop from serious damage. Under favorable conditions a single application of an effective dust is all that is needed to protect a ripening fruit crop. The conditions under which it should be applied have already been discussed. However, if the beetle 1400 SEPT- Fig. 20. — Reinfestation of an orchard by diabrotica in late summer following the reduction of the population by dusting in midsummer. population has not become stabilized, and there are large numbers still migrat- ing into an orchard, a second application might be necessary. This condition is very likely to occur early in the season in apricot orchards where many beetles may be still moving in from the uncultivated areas after the apricots have begun to ripen. Because of the danger of beetles migrating into orchards late, control measures should be delayed as long as possible so that a maximum number of beetles can be killed. In certain localities, large numbers of beetles may migrate into orchards as late as September. Because of this it is not uncommon to find large populations building up in orchards following the application of effective insecticides. This is graphically shown in figure 20. The orchard was dusted on July 30, and the population was much reduced, but later increased numbers of beetles were found owing to migration into the planting. The present authors have not had opportunity to conduct any extensive control experiments where the insect has caused economic damage to recently planted trees. However, in one ease when there was a serious infestation in small almond trees the farmer used a lead arsenate spray to good advantage. In still another ease which involved almond trees a cryolite dust was used effectively. Bul. 681] Control of Diabrotica 31 NATURAL ENEMIES Probably the most important parasite of Diabrotica 11-punctata is a tach- inid fly, Celatoria diabroticae (Shimer). Shimer (1871) reared this parasite from D. vittata and described it in the genus Melanosphora. Koebele (1890) reported a dipterous larva parasitizing Diabrotica in California and stated that Alexander Craw had noted the parasite as early as 1886. Coquillett (1889) reared a tachinid fly from D. 11-punctata and later (1890) he obtained a number of specimens for which he erected a new genus and species — Cela- toria crawii. In his revision of the Tachinidae, Coquillett (1897) placed C. crawii as a synonym of C. diabroticae. Celatoria diabroticae probably lays its eggs in the abdomen of its host while in flight; the larva develops there and in issuing it breaks away the larger portion of the beetle's abdomen, falls to the ground, and probably pupates in the surface soil. The brown puparia are frequently seen on the bottom of cages which have contained beetles collected in the field. Only one larva to a host has been observed. Although Coquillett (1890) and Shimer (1871) thought the parasite very effective, the present authors have never observed heavy parasitism. Sell (1915) recorded it as rare and it is the opinion of the present authors that this parasite is of little importance in holding Diabrotica 11-punctata in check. Chaetophleps setosa Coq. is another very similar parasite of Diabrotica. However, it is most effective in the East. Bussart (1937) gives an excellent account of the bionomics of this parasite. There are many other natural enemies of Diabrotica beetles. Some of these have been considered by Chittenden (1919) and Houser and Balduf (1925). These include rodents, birds, predaceous beetles, mites, spiders, and nema- todes. The present writers have observed evidences of rodents, probably field mice, feeding on diabrotica, and have on numerous occasions seen them spun up in webbing of spiders. Although the number of beetles destroyed by any one of these natural agencies may not be great, it is probable that in the aggre- gate the natural enemies play at least a minor roll in limiting the beetle. SUMMARY Diabrotica, or western spotted cucumber beetle, Diabrotica 11-punctata Mann., is mainly western in distribution and is most destructive in California and Oregon. The eggs are laid about the bases of host plants, the larvae feed on the roots, and on reaching maturity construct a cell in which they pupate. All stages other than the adult are spent in the soil. Both the larvae and adults feed on many kinds of plants. In most of central California there are probably three generations a year. The insect passes the winter in the adult stage. The first spring generation occurs largely in the uncultivated regions. Because of this the size of the first brood is largely dependent upon rainfall. Unless there is a good growth of vegetation in the winter and early spring, the first brood of beetles is likely to be small. This first brood appears during the latter part of April through May and most of June. In the uncultivated areas the beetles may concentrate in large numbers on the floral parts of such plants as timothy and mayweed (Anthemis Cotula). As these plants dry up the beetles 32 University of California — Experiment Station move into the cultivated areas, and large numbers find their way into decidu- ous fruit orchards. Serious infestations in the fruit orchards are likely to occur only in those years when large beetle populations develop in the unculti- vated regions. Even in these years it is only the orchards adjacent to the uncultivated regions that are likely to suffer damage. The insect attacks both the fruit and foliage of deciduous fruit trees. Serious injury to foliage has never been observed in well-established orchards, al- though newly planted trees may sometimes be heavily attacked. The beetles are principally a pest of the ripening fruit, and if not controlled may injure the fruit by eating holes in it. Besides feeding on the fruit, they are important in spreading the brown-rot organism. Control is possible because the beetles do not attack the fruit until it is nearly ripe. As a result protection is necessary for a period of only about 2 weeks. A dust containing 0.15 or 0.2 per cent of pyrethrins and 2.0 per cent of Lethane 384 in talc, applied at 50 pounds to the acre, was found to be very effective. Mortalities of nearly 100 per cent were obtained. A 5.0 per cent Pyrocide dust (0.1 per cent pyrethrins) was also used to good advantage. The mortality was not so high as that obtained with the former dust, but the fruit protection appeared to be about equal. For satisfactory control it is necessary that the dusts be applied at a tem- perature of less than 65° F, and that the velocity and volume from the duster be adequate to push the cloud of dust through the tree. The full dosage, to be effective, must be applied from one side of the tree ; very small amounts of dust dislodge the beetles from the trees with a sublethal dose, and it is for this reason that the rows of trees cannot be treated from both sides. It is very important also that the drift be into the dusted area ; for if into the nondusted portion of the orchards the beetles will be knocked out of the trees with a sublethal dose, and as the day warms up they will recover and fly back into the trees. Dusting operations should be stopped as soon as the drift changes into the undusted part of the orchard. This precaution cannot be overempha- sized. Life history studies were conducted in the laboratory. The beetles were reared in constant temperature cabinets, that did not vary more than ± l 1 /^ F. At 60° the beetles completed their development from egg to adult in 101 days, while only 27 days were necessary at 85°. It was found that soaked wheat kernels and lettuce leaves served as excellent food for the larvae. Before the larvae would pupate it was necessary to construct cells for them. If this was not done they would die. ACKNOWLEDGMENTS It is with sincere pleasure that the authors express appreciation to Mr. C. B. Weeks of the Balfour Guthrie Company at Brentwood for the splendid help and cooperation extended in conducting this investigation. Thanks are due to many of the frail growers in the Brentwood area for the loan of equipment and materials, and to the various insecticide companies who furnished insecti- cides for experimental purposes. Bul. 681] Control of Diabrotica 33 LITERATURE CITED Besse, R. S. 1936. Effect of agricultural and home economics research on Oregon's agricultural prog- ress. A report of activities and accomplishments for biennium ending June 30, 1936. Oregon Agr. Exp. Sta. Bul. 350: 1-88. Brisley, Harold R. 1925. Notes on the Chrysomelidae (Coleoptera) of Arizona. 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