Abstract
The mortality caused by combined topical application of carbamate and organophosphorus insecticides was not more than expected from additive effect against a susceptible leafhopper strain and two strains which were resistant to organophosphorus but susceptible to carbamate insecticides. However, against the Nakagawara strain (N), which was resistant to both organophosphorus and carbamate insecticides, remarkable synergism was detected between a carbamate insecticide, propoxur, and organophosphorus insecticides such as malathion, diazinon, demethoate and phenthoate. When any one of seven carbamate insecticides were combined with malathion and applied against the N strain, the LD50 were almost the same, ranging from 25.0 to 42.5μg/g of body weight of the insect. The LD50 values of these insecticides, when applied singly, ranged from 18.8 to 263μg/g. Synergism was also detected at the similar degree between malathion and propoxur against the other leafhopper populations which were resistant to both organophosphorus and carbamate insecticides.
