Comparison of the Toxicity to Insects of Certain Insecticides Applied by Contact and in the Soil1

Abstract

The toxicity of 9 insecticides relative to a standard insecticide (dieldrin = 1.00) was determined: a) by direct application of the insecticides to the test insects using a Potter spray tower; and b) by exposing the test insects to insecticidal residues incorporated into a fieldmoist Beverly fine sandy loam. First-instar nymphs of the common field cricket, Gryllus pennsylvanicus (Burmeister) , were used as test insects. The relative toxicity of the insecticides when applied as contact sprays was parathion 2.72, mevinphos (Phosdrin^®) 2.70, Zectran^® (4- dimethylamino-3,5-xylyl methylcarbamate) 2.32, aldrin 0.50, diazinon 0.44, heptachlor 0.42, DDT 0.19, chlordane 0.10, trichlorfon (Dylox^® ) 0.09. When applied to the soil the relative toxicities were aldrin 2.13, heptachlor 2.00, diazinon 0.64, parathion 0.50, chlordane 0.41, DDT 0.10, Zectran 0.04, trichlorfon 0.017, and mevinphos 0.0084. Aldrin, heptachlor, and chlordane were approximately 4 times as toxic in soil as compared with the direct contact application, diazinon 1.5 times as toxic, DDT 0.5 as toxic, parathion and trichlorfon approximately 5 times less toxic, Zectran 58 times less toxic, and mevinphos 320 times less toxic. Experiments with fumigant toxicity indicated that, at the concentrations used, all the materials with the exception of DDT volatilized from a metal surface to an extent sufficient to cause 95% to 100% fumigant mortality over a 24-hr period. When equivalent amounts of insecticide were incorporated into the soil aldrin, heptachlor, chlordane, trichlorfon, and mevinphos were moderately volatile. Diazinon, parathion, and dieldrin were less volatile, while Zectran and DDT were virtually nonvolatile. It was suggested that the effectiveness of the cyclodiene insecticides aldrin and heptachlor in soils is due to the fact that they act as both contact and fumigant poisons; that diazinon and parathion were more strongly adsorbed by soil than the cyclodiene insecticides, thus rendering them less effective as both contact and fumigant poisons; and that the low toxicity of mevinphos and trichlorfon could be attributed to the influence of soil moisture.