Synergism of Carbamate and Organophosphate Insecticides by Noninsecticidal Carbamates2

Nearly 200 materials, primarily noninsecticidal carbamates and related chemicals, were evaluated as synergists for carbamate and organophosphate insecticides against several strains of the house fly, Musca domestica L. About half of the compounds, many of them new, were especially prepared for this study. Others were obtained from commercial sources or from the chemical files of the Entomology Research Division, Agricultural Research Service, USDA. Many materials were active as synergists. The most effective reduced resistance to Isolan® (I-isopropyl-3-methyl-5-pyrazolyl dimethylcarbamate) and to a mixture of carbaryl:piperonyl butoxide in carbamate-resistant strains from 10-fold to 2-fold or less. With the organophosphate malathion, the most effective synergists reduced resistance from 300-fold to 5-fold or less. Synergism of carbaryl against a susceptible fly strain was demonstrated in limited tests. The synergists compared favorably with piperonyl butoxide as carbamate synergists and favorably with DEF® (S,S,S-tributyl phosphorotrithioate) as malathion synergists. The results support the hypothesis that different alleles of the same major gene confer resistance to carbamates and organophosphates in the house fly.