Orchard Insecticides

by B. A. PORTER

RAPID PROGRESS is being made in the effective and economical control of the important orchard pests. Government agencies and chemical companies are developing a growing number of new and effective insecticides. The situation is quite different from that of the first 25 years of the century, when fruit growers had a limited and rather rigid list of insecticides. By that time lead arsenate had largely replaced other arsenicals for the control of the codling moth, plum curculio, and other chewing insects. Lime-sulfur and mineral-oil sprays were used during the winter for scale insects. Kerosene emulsion, nicotine sulfate, or soap was used to check aphids and other soft-bodied insects. Citrus growers used sulfur, mineral-oil sprays, and hydrocyanic acid fumigation under tents. Paradichlorobenzene was effective in controlling the peach-tree borer. Those insecticides were almost the entire list of materials available for general use in the orchard.

The second 25 years of the century is witnessing the introduction of many new insecticides for use in orchards. From 1925 to 1935 perhaps the chief addition was cryolite, several million pounds of which are now used annually in the Northwest for codling moth control. In 1925, growers began to use the highly refined "white" or "summer" oils for spraying fruit trees in foliage. These oils are less injurious to plant tissue than oils of less refinement. They are used as contact sprays for scale and certain other insects, and as stickers or deposit builders for lead arsenate and other stomach poisons.

Many of the new materials that are now appearing are the reward of long and extensive efforts to find a suitable material to replace lead arsenate to control the codling moth. Lead arsenate is still standard in Most areas but it does not always give adequate control. It often injures the trees, and results in excessive undesirable accumulations in the soil. In many localities a full schedule of lead arsenate applications leaves excessive spray residues on fruit at harvesttime.

Literally thousands of materials have been tested as possible replacements for lead arsenate, but few have survived initial tests.

New ways of using nicotine have been devised. Originally employed chiefly as a contact insecticide against soft-bodied sucking insects, nicotine was used in such a form that it volatilized readily, and was washed off quickly by rains. Several compounds of nicotine have now beer, developed that remain effective on the fruit or foliage over periods of days or weeks. The leading mixture of this type is nicotine bentonite, which has been used effectively in thousands of acres of apple orchards in Indiana, Illinois, and elsewhere.

Phenothiazine, first tested in 1934, is very poisonous to the codling moth, but has given uncertain results. Much of this irregularity was overcome in the drier parts of the Northwest by the use of a very finely divided material. Elsewhere the finely divided material is still not entirely dependable. Unfortunately phenothiazine causes certain susceptible individuals to suffer a serious skin condition similar to severe sunburn. Spray men, pickers, and others who work in sprayed trees may suffer from it. Phenothiazine causes some interference with proper sizing and coloring of fruit; it is also rather expensive. Despite these difficulties, a few growers use phenothiazine, usually mixed with lead arsenate.

Xanthone also has shown promise for control of the codling moth, especially in the Northwest, but the results have been irregular and its acceptance by growers has been limited. Xanthone exhibits one unexpected quality: Its continued use seems to prevent outbreaks of orchard mites and the woolly apple aphid. On this account there is now some interest in the use of xanthone with DDT.

Tartar emetic, formerly used chiefly as a poison in ant baits, came into use for the control of the citrus thrips in California about 1939. Within a few years, however, it seemed to have much less value for the purpose in certain areas. Apparently the susceptible thrips had been killed off, leaving a more resistant race of them to carry on.

The outstanding new material. is DDT. From the very first laboratory tests, it has given marked control of the codling moth. In many cases a spray containing one-half pound of DDT, in a water-dispersible powder, per 100 gallons, has given much better control than 3 pounds of lead arsenate. The use of DDT at reduced strengths (4 to 6 ounces per 100 gallons), with about half the usual strength of lead arsenate or nicotine bentonite in all apple sections and with cryolite or xanthone in the Northwest, has also given good control. Besides the usual small-scale experiments, tests have also been carried on in large blocks in commercial plantings, in cooperation with growers, with similar outstanding results.