DDT has also shown promise in limited tests against many other fruit insects, among them the tarnished plant bug, which causes distortion of peach fruit, the oriental fruit moth, the rose chafer, the pear thrips, several species of leafhoppers, and the Japanese beetle and its grubs. Tests against the grape berry moth, the apple maggot, the cherry fruitflies, the peach-tree borer, and some other insects affecting fruit trees have given inconclusive or conflicting results.

DDT seems to have little or no practical value against such important orchard insects as the plum curculio, the San Jose scale, other scale insects, the pear psylla, and several species of orchard mites.

DDT seems to be safe enough for use on fruit trees. Some injury has resulted from the application of mixtures with oil emulsion, but the part played by DDT in this injury has not been entirely clear.

One factor that is important with such a powerful insecticide as DDT is its unfavorable effect on beneficial insects that normally keep many of our insect pests within bounds. Probably the most serious problem of this kind has been the tremendous increase in the populations of mites of various species that has often followed the use of DDT. At ordinary strengths, DDT has little effect on the mites, but it does kill off many of the ladybird beetles and other enemies of the mites. If the use of DDT in orchards becomes general, mite control is certain to become a major problem, even in orchards in which growers have never previously realized that mites were present.

Another problem is that of spray residues. On the basis of studies made thus far, the Food and Drug Administration has announced that no action would be taken on apples or pears containing residues of DDT not in excess of 7 parts per million ( about 0.05 grain per pound).

Remarkable as DDT seems, it may prove to be only a stepping stone to even better insecticides. Certain compounds closely related to DDT are being tested; some material in this group may be found that has the effectiveness of DDT without its disadvantages. Many other complex organic compounds are also receiving attention. Prominent among them is benzene hexachloride. Benzene hexachloride has given promising results against the plum curculio and several other insects not affected by DDT, without stimulating increases in mite populations. Unfortunately, benzene hexachloride has an offensive, musty odor that could be imparted to sprayed or dusted fruit.

Several new insecticides are available for spraying when the trees are dormant. Neither lime-sulfur nor the petroleum oils, used during the first 25 years of the century, had much value in the control of aphids, which pass the winter as tiny black eggs on the twigs and smaller branches. Several materials have recently become available for use against aphid eggs during the dormant period.

For a number of years tar-distillate oils, byproducts from the manufacture of gas, have been used effectively for the control of aphids in the winter egg stage. More recently there have been introduced several materials referred to as dinitro compounds. Two of the more common of these are dinitro-o-cyclohexylphenol and dinitro-o-cresol. These compounds are effective against the eggs of aphids and may be used with oil sprays for the combined control of aphids, European red mites (in the egg stage), and the San Jose and other scale insects. So, instead of one or two materials for dormant spraying, the grower now has a choice of a half dozen.

The control of the plum curculio in southern peach orchards has been approached from a new angle in the chemical treatment of the ground late in the spring. Two compounds, dichloroethyl ether and dichloroethyl formal, have been successfully used for this purpose on an experimental basis. If the soil treatment is found feasible, it will reduce or eliminate the need for lead arsenate, which causes serious damage to peach foliage and is only partially effective in curculio control.

New materials have been found for the control of the peach-tree borer, to replace paradichlorobenzene, which sometimes injures young peach trees and has other disadvantages. Emulsions of ethylene dichloride have been found effective over a wider range of conditions than paradichlorobenzene, and, when properly used, are less likely to cause injury to the trees. More recently propylene dichloride has been found even more effective in borer control than ethylene dichloride.

Many of the new insecticide materials have been tested over too short a period to permit any evaluation of their probable ultimate place in the orchard insect control program. Despite the increasing complexity of the list of available insecticides and the problems occasioned by their use, rapid progress is being made.

THE AUTHOR

B. A. Porter is in charge of the Division of Fruit Insect Investigations in the bureau of Entomology and Plant Quarantine. He joined the Department in 1917, and for many years conducted field investigations of various orchard-insect problems in Connecticut, Indiana, and elsewhere.