Persistence and residual activity of an organophosphate, pirimiphos‐methyl, and three IGRs, hexaflumuron, teflubenzuron and pyriproxyfen, against the cowpea weevil, Callosobruchus maculatus (Coleoptera: Bruchidae)

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Abstract
Three insect growth regulators (IGR), the chitin synthesis inhibitors (CSI) teflubenzuron and hexaflumuron and the juvenile hormone mimic (JHM) pyriproxyfen, as well as the organophosphate (OP) pirimiphos‐methyl, were evaluated for their activity against the cowpea weevil, Callosobruchus maculatus (F), in cowpea seeds stored for up to 8 months post‐treatment. The initial activity data showed that, based on LC50 level, teflubenzuron had strong ovicidal activity (LC50 = 0.056 mg kg−1) followed by pirimiphos‐methyl (1.82 mg kg−1) and pyriproxyfen (91.9 mg kg−1). The residual activity data showed that none of the IGRs tested had strong activity when applied at 200 mg kg−1 in reducing the oviposition rates of C maculatus at various storage intervals up to 8 months post‐treatment. However, teflubenzuron reduced adult emergence (F1 progeny), achieving control ranging from 96.2% at 1 month to 94.3% at 8 months. Hexaflumuron showed a similar trend in its residual activity, ranging between 93.8% control at 1 month to 88.2% control at 8 months post‐treatment. However, pyriproxyfen was more active than the CSIs tested and caused complete suppression (100% control) of adult emergence at all storage intervals. Unlike the IGRs tested, pirimiphos‐methyl applied at 25 mg kg−1 was more effective in reducing oviposition rates of C maculatus up to 8 months post‐treatment. A strong reduction of adult emergence was also observed at various bimonthly intervals (98.6% control at 1 month to 91.6% control at 8 months post‐treatment). The persistence of hexaflumuron and pirimiphos‐methyl in cowpea seeds was also studied over a period of 8 months. The loss of hexaflumuron residue in treated cowpeas (200 mg kg−1) was very slow during the first month post‐treatment (4.43%). At the end of 8 months, the residue level had declined significantly to 46.4% of the initial applied rate. The loss of pirimiphos‐methyl residue in treated cowpeas (25 mg kg−1) was relatively high during the first month post‐treatment (36.7%) and increased to 81.6% after 8 months. Copyright © 2003 Society of Chemical Industry