CONTROL OF SITOPHILUS-ORYZAE (L) (COLEOPTERA, CURCULIONIDAE) IN PADDYRICE USING CHLORPYRIFOS-METHYL OR FENITROTHION IN COMBINATION WITH SEVERAL OTHER PROTECTANTS

Combinations of organophosphorus insecticides (chlorpyrifos-methyl or fenitrothion) with pyrethroids (deltamethrin or bioresmethrin) or an i nsect growth regulator (methoprene) were applied to paddy rice and tes ted against a multi-resistant strain of Sitophilus oryzae (L). The aim s of the study were to determine whether there were significant intera ctions between the grain protectants and whether such interactions wou ld lead to substantial changes in field application rates. In bioassay s of freshly treated paddy, there were significant positive interactio ns in the following combinations: chlorpyrifos-methyl + deltamethrin, chlorpyrifos-methyl + synergised deltamethrin, chlorpyrifos-methyl + s ynergised bioresmethrin, fenitrothion + deltamethrin, and fenitrothion + synergised deltamethrin. This synergism was evident in terms of num ber of adult F-1 produced as web as mortality of parents. There was no interaction between chlorpyrifos-methyl and methoprene. The treatment s chlorpyrifos-methyl 0.625 mg kg(-1), deltamethrin 2 mg kg(-1) and de ltamethrin 0.25 mg kg(-1) + piperonyl butoxide 8 mg kg(-1) each produc ed negligible mortality, but the combinations chlorpyrifos-methyl 0.62 5 mg kg(-1) + deltamethrin 2 mg kg(-1) and chlorpyrifos-methyl 0.625 m g kg(-1) + deltamethrin 0.25 mg kg(-1) + piperonyl butoxide 8 mg kg(-1 ) produced > 60% mortality. Similarly, mortality was 0% for fenitrothi on 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 fenitrothio n 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) produ ced > 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 degrees C, 70% r. h.) following treatment. Strong synergism was evident in tests of fres hly treated paddy, but deltamethrin or deltamethrin + piperonyl butoxi de had little or no effect on the minimum effective application rates of chorpyrifos-methyl for control of progeny in paddy stored for 6 wee ks or longer. The study shows that, despite some significant positive interactions in freshly treated paddy, significant reductions in field application rates are unlikely. Copyright (C) 1996 Elsevier Science L td.