.F"K 
 
 Division of Agricultural Sciences 
 
 *'*ft&? > 
 
 UNIVERSITY OF CALIFORNIA 
 
 
 . : M^Mm : M^m : M0t 
 
 THE 
 
 first obvious sign of attack 
 an almond shoot by a newly 
 cited larva of the twig borer 
 
 
 F . M . S U MMERS 
 
 CALIFORNIA AGRICULTURAL 
 ~nt Station 
 i Service 
 
 CIRCULAR 449 
 
The adult (or moth) phase of the peach twig borer on the left, and the mature larva. The moth is 
 about 1 1 times natural size; the larva about 8 times. 
 
 This circular: tells you how it grows, and how to fight it. 
 
 Control is discussed from the standpoint of the best methods to use in 
 different localities and at different times. 
 
 The peach twig borer (commonly called just "twig borer") is the chief 
 cause of wormy peaches, nectarines, and almonds in California. In apri- 
 cot, plum, and prune orchards it is one of several species of moths, any 
 one of which can blemish or injure an appreciable amount of ripening 
 fruit. 
 
 Replaces Bulletin 708 
 
 THE AUTHOR: 
 
 Francis M. Summers is Associate Professor of 
 Entomology and Associate Entomologist in the 
 Experiment Station, Davis. 
 
 NOVEMBER 1955 
 
The PEACH TWIG BORER 
 
 F. M. SUMMERS 
 
 What it is • where it is found • the 
 
 type and extent of damage 
 
 it does to various California crops 
 
 Ihe peach TWIG borer is a small moth. 
 The larvae produced by the moth attack 
 both twigs and fruit. Although only the 
 larvae are truly twig borers, the name 
 "peach twig borer" (or sometimes simply 
 "twig borer") is used in reference to all 
 stages of this species {Anarsia lineatella 
 Zell.). 
 
 Disastrous outbreaks of the peach twig 
 borer have occurred in past years, and 
 there are few peach growers who are not 
 acquainted with this cause of wormy 
 fruit. The possibilities of fruit loss are 
 so serious that protective sprays are regu- 
 larly applied to peaches and nectarines. 
 Economic losses of apricots, almonds, 
 plums, and prunes occur less often; but, 
 where local situations require, these hosts 
 are also sprayed for twig borer. 
 
 Not a purely local pest 
 
 The peach twig borer is widely dis- 
 tributed throughout the United States. It 
 is also known in Europe and Asia. Al- 
 though all of the peach- and almond- 
 producing areas of California are in- 
 fested, this insect is particularly preva- 
 lent in the San Joaquin and Sacramento 
 Valleys. 
 
 The principal host plants are the al- 
 mond and peach. Other hosts are the 
 apricot, nectarine, plum, and prune. The 
 twig borer has never been found to attack 
 
 any other crop or native plant in Cali- 
 fornia. 
 
 The tiny larvae hatching in late sum- 
 mer hide themselves in cells — hiber- 
 nacula — beneath the surface layers of 
 bark. They are readily distributed in 
 this condition on nursery stock, cuttings, 
 and budwood. Commercial or orna- 
 mental varieties planted in city yards 
 nearly always harbor their quota of twig 
 borers. 
 
 Damage is extensive . . • 
 
 There are two kinds of damage: (1) 
 destruction of buds and young shoots, 
 and (2) worm damage on fruits. 
 
 Larvae hatched in late summer tunnel 
 into the bark, especially in the crotches 
 of older twigs, where they remain in- 
 active during winter. In the spring, 
 shortly before bloom, they become active 
 and enlarge their tunnels or make new 
 ones. Hatching larvae of the summer 
 broods also may live temporarily in bark 
 tunnels before moving onto green foliage. 
 The mining of the bark by immature 
 larvae does not appear to hurt the host 
 plants. 
 
 The small overwintered larvae leave 
 their tunnels during the blooming period. 
 The. first ones to emerge attack opening 
 buds and starting shoots. The larvae 
 grow as the foliage develops, and they 
 usually kill the terminal shoots before 
 
 3] 
 
.*>. .'; 
 
 This overwintering cell (hibernaculum) with a 
 well-developed chimney was found in the crotch 
 between one- and two-year-old peach wood on 
 November 1. It is magnified 5 times. 
 
 A partly dissected hibernaculum on three- 
 year almond wood shows the shallow excava- 
 tion in the fold of tender bark. A short chimney 
 overlies this burrow. Arrow points to the head 
 of the small larva. Magnified about 7 times. 
 
 
 V,:y^ 
 
 they finish feeding. All of the hosts are 
 affected in this manner. But the amount 
 of twig die-back on mature trees, even 
 in severe infestations, is not itself eco- 
 nomically important. The real problem 
 arises when some of the larvae leave the 
 foliage and attack the fruit. The shoots of 
 slower-growing varieties or non-irrigated 
 trees harden-off early. Such hardening or 
 woody twigs are less attractive to the 
 larvae, and the fruits are increasingly 
 liable to attack. 
 
 Young trees, replants, and grafts are 
 especially attractive to twig borers. The 
 terminal shoots in new plantings of 
 peaches and almonds may be so severely 
 injured that the trees are retarded or 
 greatly distorted in shape. Whenever 
 twig borer is prevalent, it is wise to spray 
 non-bearing orchards to get well-shaped 
 trees. 
 
 The damage to the fruits of all hosts 
 except almonds is essentially the same. 
 Attacks on fruit begin with "worms" of 
 the May brood and are usually marked by 
 "bleeding," or the presence of sap drop- 
 lets on green almonds, plums, prunes, 
 nectarines, peaches, and to a lesser extent 
 apricots. The larvae often make several 
 attempts to enter young, green fruits but 
 are rarely able to do so at this stage of 
 growth. Early-season injuries occur more 
 often on double fruits and split-pits. The 
 injury to fruits by worms of the May 
 I first) generation is small in comparison 
 with the damage caused by worms of the 
 second or third generations during July 
 and August. However, a bad situation 
 can develop in apricots if the hatching of 
 eggs of the May brood is delayed. In this 
 case the rise of the worm population co- 
 incides with the ripening of early varie- 
 ties. 
 
 Growers commonly speak of twig 
 borer injury to ripening stone fruits as 
 being caused by "stem-worms" or "skin- 
 worms." The worms attack the majority 
 of fruits near the stem end or at points of 
 contact between two fruits. When they 
 crawl about over trees seeking ripening 
 
 [4 
 
fruits, they usually migrate down the 
 stems and usually enter the fruit at that 
 point. Large larvae occasionally pene- 
 trate deeply into the flesh or kernel. 
 
 An increasingly great proportion of 
 the small worms attack later-ripening 
 varieties. They perforate the skin of the 
 fruit, feed in shallow excavations, and 
 are seldom more than one-third to one- 
 half grown when the fruit is picked. As 
 early varieties of fruit ripen and are har- 
 vested, the attack is intensified on later 
 varieties. Destruction increases rapidly 
 at this critical time, when applications of 
 residual insecticides are not permissible 
 because of the poison left on the fruit. 
 Infested fruits dropped by pickers be- 
 come more numerous as picking pro- 
 gresses. 
 
 The feeding of twig borer larvae on 
 green almond hulls is of no concern, ex- 
 cept of course that these individuals add 
 to the density of the general infestation. 
 
 . . . and expensive to growers 
 
 Peaches. Since the peach twig borer 
 was first discovered in California about 
 60 years ago, it has been an annual pest 
 of importance. As early as 1887, a 50 
 per cent loss of peaches was experienced 
 in some districts. The heaviest known loss 
 from its attack occurred in 1931. Officials 
 of the State Department of Agriculture 
 reported an average of 10 per cent in- 
 festation in peaches harvested throughout 
 the peach belt. Individual orchards 
 around Yuba City ranged as high as 70 
 per cent wormy peaches. In more recent 
 years there have been serious outbreaks 
 in peaches, but these have been spotty 
 and irregular in distribution. 
 
 Plums. The so-called stem-worms, or 
 small larvae of the peach twig borer that 
 attack plums at harvest time, usually ap- 
 pear when the early varieties begin to 
 ripen. However, in delayed seasons, dam- 
 age also occurs on late-maturing varie- 
 ties, such as Kelsey, Grand Duke, and 
 President. Even in the seasons of heavy 
 infestation, the damage to the plum crop 
 
 This almond shoot was attacked during May by 
 a twig-feeding larva. 
 
 rarely exceeds 20 per cent. The amount 
 of injury varies greatly in degree between 
 areas and is difficult to predict. 
 
 Apricots. The amount of twig borer 
 injury to ripening apricots follows the 
 pattern of the situation described for 
 plums. Worm damage to apricots is usu- 
 ally more severe in years of light crops. 
 
 Sugar prunes injured by twig borer. 
 
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Almonds. The losses of almonds to 
 peach twig borer are of greatest signifi- 
 cance for the soft-shell varieties, the ker- 
 nels of which are chewed or scarred dur- 
 ing the interval between husk-split and 
 harvest. The meaning of this pest to the 
 almond industry is shown in the data of 
 table 1. This 20-year record of produc- 
 
 tion and grade-out was compiled for this 
 publication by the California Almond 
 Growers Exchange. The dollar losses are 
 compiled for almonds of the Nonpareil 
 variety only, on the basis of shelling 
 operations at the Exchange plant. Ne 
 Plus, IXL, and other susceptible varieties 
 are not included. 
 
 Growth stages and habits of the 
 
 peach twig borer • area problems • 
 
 discussion of the oriental fruit moth 
 
 Good management of twig borer in- 
 festations requires some knowledge of 
 the nature of the pest. The sprays and the 
 periods for spraying described in the 
 next section are based on the principle of 
 treating the pest population before its 
 mid-season build-up. There are still other 
 possibilities, but unless the controls are 
 applied with due regard to growth stages 
 and peculiar habits of the pest, the results 
 are apt to be poor. 
 
 Each generation of the twig borer in- 
 volves four developmental stages: egg, 
 larva (worm stage), pupa, and winged 
 moth. There are as many as four genera- 
 tions in a season. Two complete genera- 
 
 tions occur on all hosts, and there is a 
 substantial third generation on hosts 
 having fruits that ripen late. On very late 
 varieties of peaches, a small part of the 
 population completes a fourth genera- 
 tion. 
 
 Wintered larvae, which complete de- 
 velopment shortly after bloom, transform 
 into moths during April and May. The 
 eggs laid by these moths represent the 
 first period of reproduction. Worms 
 hatched from eggs during May are those 
 of the first or "May" generation. This 
 brood is fairly distinct. The later ones 
 overlap so that, in August and Septem- 
 ber, worms of various ages are present 
 in late-bearing orchards. 
 
 Characteristic injury to kernels of Nonpareil almonds, and, right, Simms peaches injured by twig 
 
 borers during August. 
 
 » 
 
 
 [7] 
 
Differences in the working period of 
 the twig borer are illustrated below, in 
 the extreme cases of apricots and late 
 cling peaches (Levi, Phillips, Gomes, 
 Stuart). 
 
 On apricots 
 
 Larvae of the first generation usually 
 appear in the foliage of apricots in mid- 
 May, although stragglers of this brood 
 can sometimes be found a month later. 
 When larvae of the second generation 
 hatch in peak numbers during July, the 
 apricots have usually been picked and 
 the larvae are obliged to forage on twigs 
 or bark. Many of these immediately dig 
 into the bark and stop growing for the re- 
 mainder of the season. Others finally 
 complete development on twigs. 
 
 It is hard to find any larvae on new 
 growth during August. Wilted apricot 
 shoots are rarely seen at this time of year. 
 The onset of the inactive, bark-inhabiting 
 phase accounts for an abrupt decline of 
 twig borer populations after late July. 
 Since there is therefore little reproduc- 
 tion after mid-season, the populations of 
 twig borers in apricot orchards are gen- 
 erally not heavy. 
 
 On peaches 
 
 In the case of peaches, worms of the 
 second generation appear when varieties 
 
 These eggs were laid on immature leaves of a 
 peach shoot. Enlarged about 6 times. 
 
 such as Shasta and Fortuna are being 
 harvested. However, the infestations con- 
 tinue to build up because the moths move 
 into other, later varieties. In districts 
 having extensive plantings of late 
 peaches, a large fraction of the twig 
 borer population completes a third gen- 
 eration. Maturing larvae of a fourth gen- 
 eration are occasionally found in twigs 
 during September. 
 
 Nectarines and almonds 
 
 The seasonal cycle on nectarines and 
 almonds includes two complete genera- 
 tions — May and July — and a partial 
 third. That is, not all of the larvae of the 
 third (August) generation mature and 
 produce moths. Large numbers of young 
 third-brood larvae penetrate the bark and 
 stop growing. The maturing individuals 
 forage on almond trees until the hulls of 
 the last hardshell varieties begin to dry. 
 
 The proportion of each generation of 
 larvae that bore into bark increases as 
 the season advances until, in late sum- 
 mer, when twigs are hardened and fruits 
 are gone, all are established in cells. 
 
 Inclusive dates on which initial phases 
 of the cyclical activities have been ob- 
 served are given in table 2. 
 
 Area problems 
 
 When orchards are planted close to- 
 gether, the outside rows of the less se- 
 verely infested orchards tend to yield 
 more damaged fruit than the inside rows. 
 This happens because moths flying in 
 from more heavily infested orchards first 
 alight on the marginal trees and deposit 
 their eggs near-by. 
 
 Although the most serious losses are 
 sustained by peach growers, the densest 
 populations of the twig borer occur in 
 almond orchards. The latter can support 
 a relatively large population of twig bor- 
 ers, often without its affecting the salabil- 
 ity of the nut crop. Much of the damage 
 to almond fruits is confined to the flesh 
 of the hulls and is therefore unimportant. 
 Almond producers sustain losses only 
 
when the larvae damage the kernels of 
 the softshell varieties. 
 
 The majority of larvae that hatch from 
 eggs laid when nuts are dry enough to 
 knock go directly into hibernacula (win- 
 ter cells) and do not forage. Nuts left to 
 dry on trees in preparation for ground 
 harvesting with pick-up machines are 
 more or less immune to further damage 
 by the twig borer. 
 
 On the other hand, the flesh of stone 
 fruits is wholly vulnerable, and a lesser 
 population of moths may produce critical 
 losses. It often happens that almond or- 
 chards that do not need spraying for 
 wormy meats serve as sources of infesta- 
 tion for near-by peaches or apricots. If 
 the latter require spraying, then at least 
 the bordering parts of adjacent almond 
 and prune orchards should also be treated 
 if possible. 
 
 Oriental fruit moth 
 
 The Oriental fruit moth (OFM), 
 
 Grapholitha molesta (Busck), is a pest 
 very similar to peach twig borer, for 
 which growers must be alert. Known to 
 be present in California, it may yet prove 
 an important pest of stone fruits, espe- 
 cially peaches. The larvae of this recently 
 introduced species behave so like the 
 larvae of the true peach twig borer that 
 the two pests are apt to be confused. Both 
 species bore out the terminals of tender 
 shoots in like manner. 
 
 Carefully examine the larvae from 
 twigs or fruits whenever possible. The 
 more mature larvae of the peach twig 
 borer are brown to chocolate, with the 
 dark color disposed in rings encircling 
 each segment. Larvae of the OFM tend 
 to be milky white when very small; those 
 about one-half to five-eights inch long 
 may be cream, flesh, or pink in color. All 
 larvae found in mined twigs during 
 March will be peach twig borers, but 
 thereafter the twigs may be mined by 
 either of the two pests. 
 
 Table 2. Initial Stages of Peach Twig Borer Activities Have Been 
 Observed in the Sacramento Valley, California, on These Dates 
 
 
 Peach 
 
 Almond 
 
 pm w 
 
 Stages 
 
 Inclusive 
 dates 
 
 Average 
 dates 
 
 Inclusive 
 dates 
 
 Average 
 dates 
 
 First larvae resume feeding 
 
 
 
 
 
 under bark (winter gener- 
 
 
 
 
 
 ation) 
 
 Feb. 20-Mar. 16 
 
 Mar. 4 
 
 Jan. 20-Mar. 6 
 Mar. 7-Mar. 20 
 Mar. 22-Apr. 18 
 
 Feb. 14 
 
 First wilted shoots 
 
 Mar 14 
 
 First pupa (winter gen.) 
 
 Mar. 29-Apr. 6 
 
 Apr. 2 
 
 Apr. 3 
 
 First moths (winter gen.) . . . 
 
 Apr. 15-Apr. 18 
 
 Apr. 11 
 
 Apr. 14-May 11 
 
 Apr. 29 
 
 First larva of May brood 
 
 
 
 
 
 (first generation) 
 
 May 4-May 19 
 
 May 12 
 
 May 2-June 7 
 
 May 11 
 
 First pupa of May brood 
 
 
 
 
 
 (first generation) 
 
 June 3-June 7 
 
 June 5 
 
 May 22-June 22 
 
 June 6 
 
 First moths of May brood 
 
 
 
 
 
 (first generation) 
 
 June 15 
 
 
 May31-July6 
 
 June 20 
 
 
 First larva of midsummer 
 
 
 
 
 
 brood (second generation) 
 
 June 24 
 
 
 July 1-July 16 
 
 July 6 
 
 
 Larvae feeding on nuts (sec- 
 
 
 
 
 
 ond and third generations) 
 
 
 
 
 July 2-Aug. 27 
 
 
 
 
 Seasonal activity completed . 
 
 Oct. 18 
 
 
 Sept. 1-Sept. 30 
 
 Sept. 12 
 
 
 Data recorded by Wymore (1932), Jones and Smith (1933), S. F. Bailey (1938-47). 
 
 [9] 
 
Larvae tunnelling into the deep-lying 
 tissues or pits of stone fruits should be 
 regarded with suspicion. The twig borer 
 most commonly makes superficial tunnels 
 
 in ripening fruits, whereas the OFM is 
 more apt to attack green fruits and to 
 burrow around the area of the pit or 
 kernel. 
 
 What to do about the twig borer • 
 
 choice of sprays • why spraying 
 
 at different times is necessary 
 
 For best control of twig borer infesta- 
 tions apply insecticides early in the 
 season. Since there are several periods 
 during which it is most practical to use 
 these, and since there are a number of 
 available chemicals, you may select al- 
 ternatives that best fit with the spray 
 operations required for control of dis- 
 eases, mites, or other insect pests. 
 
 One spray should be applied before or 
 immediately after bloom, and/or one 
 spray should be applied in May. 
 
 How much worm damage occurred in 
 last year's crop? The answer to this ques- 
 tion should be the basis for deciding 
 whether to use one or two sprays during 
 the season. Experience indicates that one 
 well-applied spray is enough for very 
 light, or trace, infestations — whenever 
 crop losses have been trivial for several 
 seasons in succession. Low-grade infesta- 
 tions of this kind are apt to become 
 troublesome if controls are omitted or 
 poorly applied. The single treatment can 
 be directed against worms of the over- 
 wintered generation (delayed-dormant or 
 pink bud or popcorn spray) or against 
 those of the first generation (May spray) . 
 The May spray is not desirable for early- 
 ripening fruits on which residues may per- 
 sist. 
 
 Two sprays are advisable for peach, 
 almond, or prune orchards in distress, as 
 judged by appreciable crop damage in 
 the preceding year. One of the sprays is 
 applied either before or immediately 
 after bloom and the second in May. 
 
 Insecticides 
 
 Those especially serviceable for con- 
 trolling twig borer are basic lead arsenate 
 and DDT. Parathion and dinitrophenates 
 are advantageous in some cases where 
 twig borer is not the only consideration. 
 These materials are listed below as op- 
 tions for appropriate periods. The dos- 
 ages are given as amounts of formulated 
 material needed for 100 gallons applied 
 in conventional or bulk sprays. These 
 concentrations have been used success- 
 fully in commercial bulk sprays with vol- 
 umes varying from 450 to 600 gallons 
 per acre. 
 
 The gallonage and quantities of mate- 
 rials may be adjusted for semi-concen- 
 trate methods of application according 
 to data published by Borden (California 
 Agriculture, January, 1952). 
 
 Sprays applied during the delayed- 
 dormant, pink bud, popcorn, or petal fall 
 periods are essentially treatments applied 
 at different growth periods to larvae of 
 the same (overwintered) generation. 
 
 Reasons for spraying 
 at different periods 
 
 Delayed-dormant sprays. Spray- 
 ing in the late-dormant period has several 
 disadvantages in the control of twig 
 borer. Larvae inactive in their cells are 
 apparently less susceptible to toxicants 
 than when they resume feeding activities 
 while still in the burrows. Coverage must 
 be very thorough to wet the larvae hidden 
 in bark cells hard to spray. Residual in- 
 
 [10 
 
secticides applied very early weather-off fecting winter eggs of mites and aphids 
 
 to some extent before the larvae desert as well as twig borer. The dinitrophenol 
 
 the winter cells and contact or chew on compounds kill wintering larvae within 
 
 spray-covered parts of the host. In spite their cells. The sprays should wet all sur- 
 
 of these limitations, early pre-bloom faces of the main limbs and trunks as well 
 
 sprays fit into some control programs as brushwood. We have as yet no infor- 
 
 and, with reasonable care, can be made mation covering possible uses of dinitro 
 
 to give good results. compounds for twig borer on apricots. 
 
 The dinitrophenol compound, tri- In the San Joaquin Valley and in parts 
 
 ethanolamine dinitro-O-secondary-butyl- of the Sacramento Valley having com- 
 
 phenate (DN-289, Elgetol 318) , is appro- parable winter temperatures and rainfall, 
 
 priate for use on almonds and prunes basic lead arsenate can be applied with 
 
 (but not on peaches or nectarines) good effect somewhat earlier than in the 
 
 as a general-utility cleanup spray af- pink bud stage. This is sometimes done 
 
 CHOICE OF SPRAYS 
 
 Amounts for 
 100 gallons 
 I. Near-bloom sprays (Use only one) 
 Delayed-dormant 
 
 DN-289 or Elgetol 318 (almonds, prunes) 2 quarts 
 
 or 
 
 DDT 50 per cent wettable 2 pounds 
 
 or 
 
 Basic lead arsenate 4 pounds 
 
 or 
 
 Parathion 25 per cent wettable plus dormant old emulsion 2 pounds plus 
 
 3 gallons 
 Pink bud or popcorn 
 
 DDT 50 per cent wettable 2 pounds 
 
 or 
 
 Basic lead arsenate 4 pounds 
 
 Petal fall 
 
 DDT 50 per cent wettable 2 pounds 
 
 or 
 
 Basic lead arsenate 4 pounds 
 
 or 
 Parathion 25 per cent wettable 2 pounds 
 
 II. May spray (Use only one) 
 
 Basic lead arsenate 4 pounds 
 
 or 
 DDT 50 per cent wettable 2 pounds 
 
 or 
 Parathion 25 per cent wettable 2 pounds 
 
 III. Supplemental or preharvest treatments (Use only one) 
 
 DDT 5 per cent dust, with or without sulphur 30-40 pounds per acre 
 
 or 
 30-70 dust (30 per cent basic lead arsenate 
 
 plus 70 per cent sulphur) 40-50 pounds per acre 
 
 [11] 
 
when a dormant oil spray for mites or 
 scale insects is used as the vehicle. The 
 timing must be appropriate for the ap- 
 plication of winter oil. 
 
 Experimental work shows that DDT 
 can also be used effectively as early as 
 the delayed-dormant period (table 3) . 
 
 Parathion and dormant-oil-spray 
 mixtures are not suggested primarily 
 for controlling twig borer. This potent 
 combination has a special use in the 
 hands of competent spray operators for 
 combatting infestations of olive (Parla- 
 toria) scale on stone-fruit trees. How- 
 ever, it is very effective in destroying 
 hibernating twig borer larvae and win- 
 tering mite eggs. If this scale treatment 
 is used, no other twig borer spray need 
 be applied in pink bud or petal fall 
 periods. 
 
 Pink bud or popcorn sprays. Grow- 
 ers who rely on pre-bloom applications 
 of fungicides to peaches and almonds, 
 and who do not put on a petal fall spray 
 for diseases, mites, or other insects, find 
 it convenient and economical to incorpo- 
 
 rate basic lead arsenate or DDT in the 
 fungicide spray. Both of these materials 
 are compatible with Bordeaux, neutral or 
 basic coppers, and iron or zinc dimethyl 
 dithiocarbamate fungicides. Recent in- 
 vestigations on miticides have shown that 
 it is also possible to include one of sev- 
 eral sulphonate miticides (Ovotran, 
 Genite 923) in pink bud or popcorn 
 sprays in order to control brown almond 
 mites on almonds and peaches. 
 
 Pink bud sprays are often used on 
 peaches in the interior valleys, whereas 
 the popcorn sprays are well adapted for 
 almonds. Pre-bloom sprays are not the 
 best choice for twig borer control on 
 apricots, plums, or prunes because the 
 timing is premature for other worms usu- 
 ally associated with these fruits. Further- 
 more, the pink bud spray is not regarded 
 as the most effective against twig borer 
 in the Santa Clara Valley and in cool 
 coastal sections. 
 
 Petal fall spray. A spray applied in 
 the advanced petal fall period is possibly 
 the most profitable single treatment for 
 
 Table 3. Results of Pre-bloom Sprays of DDT Applied to 4-year 
 Almond Trees, Arbuckle, California 
 
 Materials per 100 gallons spray 
 
 Check 
 
 DDT 50 per cent wettable, 1 lb 
 
 DDT 50 per cent wettable, 2 lb 
 
 DDT 50 per cent wettable, 4 lb 
 
 Check 
 
 DDT 25 per cent emulsible concentrate 
 1 qt 
 
 DDT 25 per cent emulsible concentrate 
 2qt 
 
 Check 
 
 DN-289, 2 qt 
 
 Check 
 
 Number of strikes on 10 trees* 
 
 Sprayed 2/2 
 (delayed dormant) 
 
 87 
 3 
 
 
 1 
 47 
 
 5 
 129 
 
 9 
 91 
 
 Sprayed 2/14 
 (pre-pink) 
 
 129 
 3 
 
 
 
 107 
 
 
 
 245 
 
 Sprayed 2/24 
 (pink bud) 
 
 148 
 
 
 
 
 123 
 
 
 123 
 
 * Strikes counted five weeks after full bloom. 
 
 [12 
 
twig borer on stone fruits. Fresh deposits 
 of insecticide are thus provided at the 
 critical time when larvae are beginning 
 to forage on starting shoots and floral 
 parts. The volume of spray needed to 
 cover the small leaves is less than that 
 required for a later, May spray. And — 
 especially important for apricots, plums, 
 and prunes — the petal fall spray serves as 
 the initial treatment for a variety of other 
 insects. DDT sprays, as applied primarily 
 for thrips, or parathion (wettable) ap- 
 plied for bud moth also give excellent 
 results for twig borer. 
 
 Older publications on sprays refer to a 
 "jacket" spray for twig borer. Since the 
 jacket is not shed by young fruits for 
 several weeks after petal fall, this term is 
 confusing about when to begin spraying. 
 No precise data are available to show the 
 relation between delayed timing and re- 
 duced kill, but indications are that sprays 
 delayed until two or three weeks after 
 petal fall are too late for maximum value. 
 In the period before spraying, some of 
 the half-mature larvae are apt to escape 
 exposure to the insecticide — especially 
 lead arsenate — because they bore into 
 and remain within the stems of larger 
 twigs until feeding is concluded. 
 
 To minimize the killing of honeybees 
 and other essential pollinators, postpone 
 starting the spray operation until petal 
 drop is well advanced (90 to 100 per cent 
 petal fall) on the variety having the lat- 
 est flowering date. For example, spray all 
 of the almonds in one planting when 
 blossoms fade on the variety last to 
 bloom. Follow the same procedure for 
 each of the different hosts. 
 
 May spray. Another favorable op- 
 portunity to spray for twig borer arises 
 when the eggs of the first generation 
 begin to hatch during May. In the case 
 of peaches and nectarines, this spray is 
 commonly used to apply sulphur for the 
 control of powdery mildew. 
 
 The time interval during which con- 
 trol measures can be applied for maxi- 
 mum effectiveness against twig borer is 
 
 comparatively short. The timing of this 
 application is the same for different 
 hosts, but the date varies from season to 
 season (table 2, page 9) and cannot be 
 readily established beforehand. 
 
 Questions about the comparative val- 
 ues of single sprays applied in the differ- 
 ent periods of growth — pink bud or petal 
 fall or May — cannot be answered simply. 
 When almond and peach trees were 
 sprayed with lead arsenate during each 
 of these growth periods and the numbers 
 of worm-damaged shoots counted within 
 two to four weeks after each application, 
 the differences due to the timing were 
 not significant. Each application killed 
 about the same number of worms (tables 
 4, 5). Similar results were obtained for 
 DDT and parathion sprays. 
 
 Another method was used to determine 
 the effectiveness of differently timed 
 sprays. In this series of experiments, lead 
 arsenate sprays were applied to almond 
 trees in the petal fall and May periods, 
 but the results were tabulated as per- 
 centages of almond kernels damaged at 
 harvest time. Averages for six seasons of 
 plot trials show that the lead arsenate 
 sprays applied in May tend to yield more 
 worm-free nuts than when the sprays are 
 applied at petal fall (table 6). The 
 method of counting damaged shoots a 
 short time after spraying is especially use- 
 ful for comparing insecticides, whereas 
 the method of counting damaged kernels 
 gives a more conclusive measure of ulti- 
 mate effects of spraying on the insect 
 population during the whole period of 
 crop production. 
 
 Timing May sprays. It has been the 
 custom of Farm Advisors and other com- 
 petent observers in the principal peach- 
 producing sections to determine the time 
 for applying May sprays. The date is 
 announced locally as soon as determined. 
 
 Twig borers developing in May feed 
 mostly on succulent shoots. The success 
 of the May spray, therefore, depends 
 upon the application of materials before 
 eggs of the May brood hatch in peak 
 
 [13] 
 
Table 4. Effect of Single Sprays Applied to 3-year Almonds in 
 Post-bloom Periods, Arbuckle, California 
 
 Materials per 100 gallons spray 
 
 Check 
 
 Basic lead arsenate, 4 lb 
 
 DDT 50 per cent, 2 lb 
 
 Parathion 25 per cent, 2 lb 
 
 Number of strikes on 10 trees 
 
 After petal-fall 
 spray 
 
 After 
 May spray 
 
 194 
 
 145 
 
 16 
 
 24 
 
 4 
 
 2 
 
 8 
 
 5 
 
 Petal fall sprays applied March 26, 50 gal. per 20 trees per plot; strikes counted March 13. May sprays 
 applied May 20, 50 gal. per 15 trees per plot; strikes counted May 27. First larvae of "May" generation found 
 on May 14. 
 
 Table 5. Effect of Single Sprays Applied to 3-year Elberta Peaches in 
 Pre- and Post-bloom Periods, Hughson, Stanislaus County, 
 
 California 
 
 
 Number of strikes on 10 trees 
 
 Materials per 100 gallons spray 
 
 After 
 pink-bud spray 
 
 After 
 petal-fall spray 
 
 After 
 May spray 
 
 Check 
 
 Basic lead arsenate, 4 lb. . 
 
 189 
 
 78 
 
 
 
 
 
 2C8 
 
 83 
 
 4 
 
 8 
 
 176 
 67 
 
 DDT 50 per cent, 2 lb 
 
 13 
 
 Parathion, 2 lb 
 
 4 
 
 
 
 Pink bud sprays applied March 17, 100 gal. per 56 trees per plot; strikes counted April 8. Petal fall sprays 
 applied April 1, 100 gal. per 48 trees per plot; strikes counted April 13. May sprays applied May 24, 100 gal. 
 per 30 trees per plot; strikes counted June 6. First "May" larvae appeared May 16. 
 
 Table 6. Average Percentages of Worm-infested Nonpareil Meats Har- 
 vested from Plots of Almonds Sprayed with Basic Lead Arsenate, 
 
 Arbuckle, California* 
 
 
 1937 
 
 1938 
 
 35.0 
 20.5 
 11.4 
 
 9.1 
 
 1939 
 1.5 
 
 1.8 
 1.6 
 
 1.4 
 
 1940 
 
 1941 
 
 1942 
 
 Mean 
 
 of 
 
 averages 
 
 Check 
 
 Petal fall spray only . . 
 
 May spray only 
 
 Petal fall plus May 
 
 spray 
 
 July spray 
 
 7.3 
 5.3 
 
 3.7 
 3.0 
 
 2.1 
 2.7 
 2.2 
 
 20.8 
 18.0 
 17.9 
 
 19.3 
 
 8.4 
 4.2 
 
 4.5 
 
 12.5 
 9.6 
 6.8 
 
 
 * Bulletin 708, Bailey, 1948. 
 
numbers. Hatching normally begins on 
 May 11-12 in the central valleys, and the 
 increase in the number of damaged 
 shoots — new entries — levels off about 
 June 1. Spray materials applied within 
 5 to 10 days after hatching begins appear 
 to give best results. 
 
 The timing of this spray is best accom- 
 plished by observations made on un- 
 sprayed, small almond or peach trees — 
 two to three years of age — that are 
 known to be well infested. At least there 
 should be numerous signs of terminals 
 killed back by worms of the preceding 
 brood. Begin looking for new strikes dur- 
 ing the first week of May and continue 
 at intervals of about three days until you 
 find the first newly hatched larva. If addi- 
 tional strikes are found within the next 
 day or so, you are reasonably sure that 
 the hatching of the oncoming brood is 
 under way but not advanced. 
 
 The object is to find very small larvae 
 in initial stages of development that have 
 not had time to work far down into the 
 core of the twigs. Disregard drooped or 
 dying shoots already deeply mined by 
 larvae of the earlier brood. The new 
 strikes most often seen are those in which 
 entries are made through the petioles of 
 the newest leaves. A single wilted leaf is 
 the clue to the presence of a tiny larva. 
 The smallest larvae are most easily found 
 on almond and peach trees. They are 
 
 The first obvious sign of an attack on an al- 
 mond shoot by a newly hatched larva. Point of 
 entrance is marked by a few granules of excre- 
 ment and webbing between the stem and peti- 
 ole of the first well-developed leaf on the left. 
 
 very hard to notice on apricots, plums, or 
 prunes. 
 
 There is an identifiable break between 
 the overwintered brood and the May 
 brood of larvae. Stragglers of the over- 
 wintered brood can sometimes be found 
 in twigs during the early part of May. All 
 are nearly mature individuals about to 
 conclude the twig-boring phase. These 
 disappear from the foliage about two 
 weeks before the new hatch begins. Pro- 
 longed periods of below-average tempera- 
 tures during April can delay the maturing 
 
 Photomicrograph of a very young larva partly dissected out of its burrow in the tip of a young 
 
 peach shoot. Enlarged about 6 times. 
 
 [15 
 
period of the brood to follow. For ex- 
 ample, during the spring of 1948, worms 
 of the overwintered generation persisted 
 in the feeding phase until about May 20, 
 while the next brood did not appear until 
 June 7. 
 
 Supplemental treatments 
 
 A difficult problem in control arises 
 when ripening fruits are found to be 
 severely infested with worms — especially 
 fruits of table varieties not subject to 
 washing or processing before going to 
 market. At present, no method can be 
 suggested for disinfesting nectarine, apri- 
 cot, plum, or prune orchards at this late 
 date, because of the probability of adul- 
 teration with poison residues. 
 
 Growers of cling peaches have been 
 able to get partial relief late in the season 
 by dusting infested orchards with basic 
 lead arsenate and sulphur "30-70" dust. 
 Since 1947, 5 per cent DDT dust has been 
 applied in many cling-peach orchards 
 where many worms in the first varieties 
 to ripen indicated more trouble later. 
 Trained observers and growers believe 
 that DDT dust gives better protection 
 than 30-70. 
 
 The usual procedures for peeling 
 peaches in caustic soda solution should 
 eliminate or reduce to trace level in the 
 canned product the residues of the lead 
 arsenate or DDT dusts. The possibility 
 of contaminating the canned fruit in 
 some degree by pre-harvest treatments 
 depends upon time and mode of applica- 
 tion, amounts of toxicant deposited, and 
 method of processing. 
 
 It is best to depend upon earlier sprays 
 for worm control — at least until a pre- 
 harvest treatment can be perfected that 
 is satisfactory in all respects. 
 
 A limited amount of experimental 
 work was done some years ago with 
 shortlived botanicals — rotenone and fixed 
 
 nicotine — applied to plums. Older rec- 
 ommendations made by the University 
 included these botanicals, but growers 
 have used them too rarely to show their 
 value. Whether the results justify the ex- 
 pense is not established. 
 
 Lead arsenate or DDT? 
 
 Bulk sprays of basic lead arsenate, 
 four pounds per 100 gallons, reduce the 
 number of strikes 78 to 92 per cent; 
 while comparable sprays containing two 
 pounds of 50 per cent DDT per 100 gal- 
 lons have yielded values not less than 93 
 per cent reduction of strikes. There is 
 therefore no question about the superi- 
 ority of DDT over basic lead arsenate 
 for reduction of twig borer populations, 
 but unfortunately DDT also kills many 
 of the desirable or beneficial insects, 
 which prey on aphids, scale insects, red 
 spider mites, moths, etc. Lead arsenate 
 does not appreciably affect the parasites 
 and predators in a direct way. 
 
 Possibly there is least harm to bene- 
 ficial insects when DDT is applied early 
 in the growing season, before or imme- 
 diately after bloom. The wisdom of sub- 
 stituting DDT for lead arsenate in sprays 
 regularly applied during May — or later — 
 is still not settled. 
 
 Some processors of peaches for baby 
 foods prefer DDT over lead arsenate for 
 use in sprays applied during May. One 
 reason for this relates to difference in 
 stability of the two spray chemicals. Lead 
 arsenate is the more stable compound, 
 and traces of its toxic elements persist 
 for a much longer period than the useful 
 life of the compound. The preparation of 
 baby foods can offer a problem in this 
 respect because some variations of the 
 peeling process do not insure complete 
 elimination of all traces of the lead 
 arsenate. 
 
 In order that the information in our publications may be more intelligible it is sometimes necessary 
 to use trade names of products or equipment rather than complicated descriptive or chemical iden- 
 tifications. In so doing it is unavoidable in some cases that similar products which are on the market 
 under other trade names may not be cited. No endorsement of named products is intended nor is 
 criticism implied of similar products which are not mentioned. 
 I2m-11,'55(B1657)A.A. »« ^^HH^ 
 
 Co-operative Extension work in Agriculture and Home Economics, College of Agriculture, Universitj »f California, and United Stalea Department of Agriculture 
 CO-operating. Distributed in furtherance of the Arts of Congress of May 8. and June 30, 1914. J. Earl Coke. Director. California Agricultural Extension Service.