Synergism of insecticides by herbicides: effect of environmental factors.

The synergism of parathion and p,p'-DDT [1,1,1-trichloro-2,2-bis-(p-chlorophenyl)ethane] by atrazine was investigated as a function of soil type, age of pesticide soil residues, and the presence of soils in quiet or turbulent water. Compared to previous tests in which the pesticides were applied on glass surfaces, a significant reduction of the toxicity of the insecticides to fruit flies and of the synergistic effects of atrazine was observed with soils, particularly a silt loam. The effects of atrazine as a synergist in soil declined rapidly within 4 days. The toxicity of parathion in water and its synergism by atrazine were significantly reduced by soil sediments, depending on the type and amount of soil present. Soils were highly effective in turbulent water: in water containing the relatively high parathion concentration of 0.3 part per million, 93 percent of the mosquito larvae present died within 24 hours, yet this solution was rendered nontoxic by being mixed with 5 grams of a loam soil. With atrazine present in the latter system, however, 38 percent of the mosquito larvae died. Thus, insecticides can be more or less toxic, depending on their concentrations, the presence of synergists, and the environmenetal conditions.