Synergism of insecticides by herbicides

The herbicides atrazine, simazine, monuron, and 2,4-D (2,4-dichlorophenoxyacetic acid) enhanced the toxicity of selected insecticides to Drosophila melanogaster Meigen, Musca domestica L., and larvae of Aedes aegypti L. The insecticides-nine organophosphorus compounds, two chlorinated hydrocarbons, and one carbamate-were used at dosages that resulted in low insect mortalities, while the herbicides by themselves were nontoxic. Atrazine was most effective. With increasing amounts of this herbicide and constant amounts of some insecticides, increasing mortalities of fruit flies were observed. Exposure of the insects for 24 hours to carbofuran (0.5 microgram), p,p'-DDT [1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane] (4 micrograms), parathion (0.35 microgram), and diazinon (0.2 microgram) alone resulted in mortalities of 7.5, 9.5, 8, and 10.5 percent, respectively. Based on dosage mortality curves obtained with increasing amounts of atrazine, mortalities of 50 percent of the insect populations would have been achieved with 23, 40, 6, and 10 micrograms of atrazine added to the abovementioned dosages of carbofuran, DDT, parathion, and diazinon, respectively.