Effect of Soils upon the Uptake of Systemic Insecticides by Plants1

Abstract

The introduction of organic systemic insecticides has led to a number of new methods of application for control of foliage-feeding insect pests, among which, various types of soil treatment are of importance. In evaluating the efficiency of soil treatments in the field it has been noted that the effectiveness of these applications often depends upon soil conditions. Therefore, experiments were conducted to determine 1) the effect of various soil types upon systemic insecticide absorption by plants from the soil, and 2) the characteristics of soil which bind the insecticides against the leaching action of water. The absorption of some systemic insecticides by peas from four soils and two sands was measured by aphid bioassay and anticholincstcrase analysis. Greater residues and longer periods of insect kill resulted from soil drench applications to sands and sandy soil than from applications to other soils (silt loam, clay loam, and muck). Pea aphid (Macrosiphum pisi (Harris)) control from soil treatments with Phosdrin varied from 2 days on plants grown in loam and muck to 19 days on plants in sand. Likewise, demeton and phorate, formerly designated Thimet® (0,0-diethyl S-(ethylthio) methyl phosphorodithioate), were most effective in the sands and least effective in the muck, with insect kill from 10 to 100% occurring on peas in all soils and sands at 41 days after treatment. Schradan was also more effective in treated sands and sandy soils. Because of its slow toxicological action schradan provided low insect mortality when counts were taken 24 hours after the insects were subjected to treated plants. Unlike the other insecticides, Isolan® (dimethyl 5-(1-isopropyl-3-methyl-pyrazolyl) carbamate) was absorbed in greater amounts by peas from the muck than from either of the loam soils. Anticholinesterase analysis of pea plants from Isolan and Phosdrin® (1-rnethoxycarbonyl-1-propen-2-yl dimethyl phosphate, 60% technical) treated soils gave results comparable with those obtained by insect bioassay. The binding of an insecticide in soil was studied by leaching radioactive Phosdrin through columns of 12 soils. The largest amounts of Phosdrin were bound by peat (127 micrograms per 100 grams of soil), with less being bound by heavy loam soils (10 to 27 micrograms per 100 grams of soil) and the least by sandy soils (3 to 8 micrograms per 100 grams of soil). The amount of Phosdrin bound by the various soils correlated in a positive manner with the base exchange capacity, organic matter content and nitrogen content, but it was concluded that the organic matter content was primarily responsible for insecticide binding. Other chemical and physical properties of the soil did not correlate with the binding of Phosdrin.