Effects of Soil Insecticides on Apple Trees and Resulting Effect on Mite Nutrition1

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Abstract
Apple trees were grown in semifield conditions in cans of soil treated with several levels of DDT, dieldrin, and BHC. Fertilizer was added to the soil in 1956 but not in 1957. Females of Panonychus ulmi (Koch) and Tetranychus telarius (L.) were cultured on leaves detached from the trees and the resulting progeny were counted. Reactions of the trees were measured. Analyses of foliage were made for total nitrogen, phosphorus, and potassium. Analyses of variance were performed on the tree reactions, the results of the foliage analyses, and the mite counts. Tests for correlation coefficient (r) were made on absorbed elements with populations. Root weight was lowered by all treatments in 1956, significantly by dieldrin. DDT at 100 pounds per acre in 1957 increased root weight, not significantly. BHC and dieldrin depressed twig growth, not significantly. Changes in chemical composition of the slower growing, woody apple trees were less striking than those previously found in beans, soybeans, and cotton. On fertilized soil in 1956 DDT at 100 pounds per acre elevated absorbed phosphorus to 0.32% and depressed populations of both species, and at 1,000 pounds increased both populations over the untreated check and significantly over the 100 pound level. In 1957 DDT at 100 and 4,000 pounds on unfertilized soil significantly increased per cent nitrogen over the untreated check and also increased the populations. That year the absorbed phosphorus generally fell below 0.20% and it was significantly correlated positively with T. telarius populations while the relationship between the two in the previous year was generally negative. These results support previous findings of a similar nature concerning ranges of absorbed phosphorus and T. telarius populations. Dieldrin at 50 pounds per acre increased T. telariusover the check and at 500 pounds depressed both species below the check and 50-pound levels. BHC depressed T. telarius populations, only, at both 20-and 200-pound levels. Thus, DDT at 100 pounds per acre has been shown to be capable of significantly affecting both the chemical composition of the apple tree and the mite populations developing thereon. This amount, and even more, of DDT could easily be applied to a commercial orchard over two seasons in a normal spray schedule. Thus, the buildup of insecticide residues on the orchard floor should be considered a potential factor influencing the mite populations. Also evident in this work was the fact that insecticide effects on both foliage composition and mites may vary with the fertility level of the soil.