Headlee was particularly interested in fruit insects and in his work on their control he was assisted by Alvah Peterson, Louis A. Stearns, and others. Since 1927 Byrley Driggers, too, has conducted investigations on these insects, and since 1925 Ginsburg has handled the biochemical pleases of the work. The interests of these men has centered particularly on the codling moth, apple aphids, European red mite, leopard moth, Oriental fruit moth, peach borers, strawberry weevil, blackberry psyllids, and raspberry crown borer. From time to time other fruit insects of lesser importance have also been studied and control measures developed, for example, the Comstock mealy bug, woolly aphids, scale insects, and insects causing "cat-facing."
Although Smith's spray schedule had given moderately satisfactory control of the codling moth and other insects destructive to apples, by 1919 in some parts of the state more than half of the apple crop was turning out to be wormy because of this pest. Headlee then undertook a detailed study of the insect. He found that in a normal year in the southern section of the state there were two broods of this pest. For six years he observed the time of emergence of moths and took regular temperature records, from which data a standard relation between accumulated temperature units and the emergence of both the overwintering and the summer broods of the moth was determined. Thus it became possible to develop a system by which growers could be advised of the best time for application of codling moth cover sprays. Commercial apple growers of the state have depended on this information from the Experiment Station for many years in timing spray applications.
Headlee was one of the men in the country who first realized that the tolerance placed on the amount of lead arsenate on the fruit at harvest, and the damage inflicted by insecticides on trees in foliage, and their poisoning of the soil necessitated finding satisfactory substitutes for lead arsenate. In 1928, Headlee and his associates had found that since tannic acid would fix nicotine, nicotine remained longer on foliage and fruit. The effectiveness of lead arsenate was improved, first, by the addition of summer oil which killed eggs and held the lead arsenate more firmly to the fruit, and second, by the addition of nicotine sulfate to the lead arsenate-oil which killed adult moths. Although this so-called triple spray gave improved codling moth control, it also had disadvantages, for removal of tile lead arsenate at harvest became more difficult, and injury to foliage increased. In the '30s and '40s, with intensive orcharding and restriction of chemical residues on fruit, and with the increased age of the orchards, trouble with codling moths increased. There was also evidence of the development of insect resistance to lead arsenate.
When in 1933 Driggers began comparisons of lead arsenate and nicotine tannate and more or less accidentally discovered the superiority of bentonite to tannic acid in fixing nicotine, a commercial nicotine-bentonite spray was developed as a result. After a few years, under a program of lead arsenate early in the season and nicotine-bentonite or oil-nicotine later on, the control of codling moth improved. This spray schedule lessened the residue problem, avoided washing of fruit, and decreased soil poisoning and foliage injury.
In 1943, when USDA investigators were testing DDT as a wartime substitute for rotenone in controlling Japanese beetle, it was observed that this insecticide also controlled the fruit insects in the same areas. The following year, tests revealed the effectiveness of DDT against codling moth, and this insecticide soon replaced all those previously in use. However, as early as 1943 it was noted that where DDT was used, mites, red-banded leaf roller, and some other insect pests increased.
By 1916 rosy apple aphids had become increasingly troublesome and Peterson began a study of its control which continued for several years. The three principal species of apple plant lice passed the winter in the egg form on the twigs and branches of apple trees, so a method was sought to destroy the aphids in the egg stage. Peterson found that carbolic acid would penetrate the eggs, that lime-sulfur apparently so hardened the shells that hatching could not occur, and that nicotine was effective only after the soft tissues were exposed. It was noted, however, that the hard, shiny shell split some time before the egg hatched. Field experiments showed that a combination of lime sulfur and nicotine applied after the eggs had split and 'just before the leaves began to emerge from the fruit buds gave satisfactory control as a delayed dormant spray.
When, however, the European red mite became established in New Jersey orchards, it was evident that the mite could only be controlled in its egg stage with oil treatment. It thus became necessary to revamp the delayed dormant spray so that it would destroy not only the eggs of apple plant lice but also overwintering eggs of mites. After a number of years of study which took note of Peterson's early findings on the effect of carbolic acid, it was found that a combination of lubricating oil emulsion with cresylic acid at the delayed dormant period would destroy the overwintering eggs of both pests. This treatment, recommended for the first time in 1930, was at once widely adopted.
In the next ten years the use of tar oils (developed by the British) and later a number of di-nitro compounds was substituted for previous control measures against red mite and aphids. Oil emulsion was found effective in the case of infestations of mites in the summer, but with the development of DDT and the increase in mites which followed its use, the mite problem became critical. A number of materials developed commercially and specifically for mite control were tested. But the use of the phosphate materials, notably parathion, resulted in populations of insecticide resistant mites in many orchards. If this problem has been less serious in New Jersey than in some other states, it is because growers were warned against the danger in overuse of new materials.
Another pest which inflicted serious damage on apple trees in the 30's was the leopard moth. Although it was found that borers could be killed by squirting nicotine-bentonite into their burrows or by inserting wires by hand, the use of DDT is believed responsible for the virtual disappearance of this pest from commercial orchards today.
By 1923 the Oriental fruit moth had become in some respects the most important enemy of peaches in New Jersey. An emergency appropriation in 1924 enabled Alvah Peterson to initiate studies of this insect, which were continued by Louis A. Stearns and Driggers. It was found that considerable percentage of the insects wintered on the ground in peach mummies and debris which could be destroyed by early spring plowing or discing of the orchard. It was also noted that fruit infestation could be slightly reduced by the use of nicotine sprays. But these measures did not prove to be sufficiently effective. Driggers' further study showed that a high percentage of the larvae could be destroyed by applications of finely divided talc before they entered the fruit, and that effectiveness was increased by use of 2 per cent oil, and further increased by the addition of pyrethrum.
All of these investigators demonstrated that parasitic insects were capable of effecting a high degree of natural control. In the period from 1927 to 1946 it was shown that the important larval parasite, Macrocentrus ancylivorus, not present naturally in northern New Jersey, could be introduced into these orchards in sufficient numbers to materially reduce the fruit moth infestation, and beginning in 1940 for four or five years Driggers worked cooperatively with the N.J. Department of Agriculture on a project of rearing large numbers of these parasites in the laboratory for release in orchards. When DDT and parathion became available, however, they were found to be so effective that the rearing project was discontinued. Studies of the parasite led to recommendations, however, especially for control of the moth in the southern part of the state, where growers were advised to rely on parasites for control of early broods with applications of insecticides later in the season to prevent larval entry into the fruit. More recently when parathion was found to be effective against the plum curculio and the first brood of Oriental fruit moth, spraying of the first brood is considered sufficient control of Oriental fruit moth for the entire season. Although DDT was also found to be an effective control when used only against the first brood, its use is responsible for an increase in peach tree borer and mites which, in turn, must also be controlled.
For many years peach trees were greatly injured by the peach borer, which doubtless reduced the life of the trees by almost 50 per cent. From 1917 to 1924 a thorough investigation of the borer was conducted by Alvah Peterson. Studies of the borer's life history and habits were made and the use of paper collars and other means of control were tested. From 1924 until 1943 tests of materials developed in various parts of the country, including paradichlorobenzene crystals and ethylene dichloride emulsion, were made and their use recommended to New Jersey growers. Then in 1943, tests showed the effectiveness of spraying trunks of trees with DDT once in the middle of July and again about the middle of August. Further tests, demonstrated that young borers could be destroyed by BHC before they penetrate deeply into the tree. As a result the present practice is to apply DDT in the middle of July to kill the early hatching larvae and then BHC in late September to kill the late-hatching borers.
During 1915 and 1916 a study was made of the strawberry weevil, which had become a limiting factor in strawberry production in the southern half of the state. It was determined that a I to 5 mixture of arsenate of lead and sulfur applied on foliage and buds from the time the weevils began to feed would give satisfactory protection. Peterson found that blackberry psyllids could be controlled by spraying blackberry bushes in late May and early June with nicotine and soap. About 1924 Headlee found that another fruit insect, the raspberry crown borer, could be controlled by applications of tobacco dust which destroyed the larvae that overwintered on the bases of the canes just below the surface of the ground.