Control of Sitophilus oryzae (L.) (Coleoptera: Curculionidae) in paddy rice using chlorpyrifos-methyl or fenitrothion in combination with several other protectants

Abstract

Combinations of organophosphorus insecticides (chlorpyrifos-methyl or fenitrothion) with pyrethroids (deltamethrin or bioresmethrin) or an insect growth regulator (methoprene) were applied to paddy rice and tested against a multi-resistant strain of Sitophilus oryzae (L). The aims of the study were to determine whether there were significant interactions between the grain protectants and whether such interactions would lead to substantial changes in field application rates. In bioassays of freshly treated paddy, there were significant positive interactions in the following combinations: chlorpyrifos-methyl + deltamethrin, chlorpyrifos-methyl + synergised deltamethrin, chlorpyrifos-methyl + synergised bioresmethrin, fenitrothion + deltamethrin, and fenitrothion + synergised deltamethrin. This synergism was evident in terms of number of adult F1 produced as well as mortality of parents. There was no interaction between chlorpyrifos-methyl and methoprene. The treatments chlorpyrifos-methyl 0.625 mg kg−1, deltamethrin 2 mg kg−1 and deltamethrin 0.25 mg kg−1 + piperonyl butoxide 8 mg kg−1 each produced negligible mortality, but the combinations chlorpyrifos-methyl 0.625 mg kg−1 + deltamethrin 2 mg kg−1 and chlorpyrifos-methyl 0.625 mg kg−1 + deltamethrin 0.25 mg kg−1 + piperonyl butoxide 8 mg kg−1 produced > 60% mortality. Similarly, mortality was 0% for fenitrothion 10 mg kg−1, deltamethrin 2 mg kg−1 and deltamethrin 0.25 mg kg−1 + piperonyl butoxide 8 mg kg−1, but the combinations fenitrothion 10 mg kg−1 + deltamethrin 2 mg kg−1 and fenitrothion 10 mg kg−1 + deltamethrin 0.25 mg kg−1 + piperonyl butoxide 8 mg kg−1 produced > 80% mortality. In long-term experiments mixtures of chlorpyrifos-methyl and deltamethrin or deltamethrin + piperonyl butoxide 8 mg kg−1 were evaluated during up to 24-weeks storage (30 °C, 70% r.h.) following treatment. Strong synergism was evident in tests of freshly treated paddy, but deltamethrin or deltamethrin + piperonyl butoxide had little or no effect on the minimum effective application rates of chorpyrifos-methyl for control of progeny in paddy stored for 6 weeks or longer. The study shows that, despite some significant positive interactions in freshly treated paddy, significant reductions in field application rates are unlikely.