Persistence and residual efficacy of abamectin and spinosad for control of Callosobruchus maculatus and Callosobruchus chinensis on stored cowpea seeds

Abstract

Residual toxic effect of insecticides mixed with or sprayed on stored commodities is a key element of an efficient protection of stored products. The residual efficacy of chlorprifos-methyl, deltamethrin, abamectin and spinosad at application rates of 5.0, 2.0, 2.0 and 1.0 mg/kg, respectively, was tested against Callosobruchus maculatus and C. chinensis on cowpea seeds for five consecutive months (6 bioassays). In each bioassay, the mortality of adults was counted after 7 days, and progeny reduction and cowpea weight loss percentages were recorded after 45 days. Our results revealed that the insecticides caused remarkable adult mortality of two insects for five months storage. Abamectin (2.0 mg/kg) was the most effective insecticide with 96.4–100.0% and 84.4–100.0% adult mortality of C. maculatus and C. chinensis, respectively, during 2 to 5-month period of storage. A gradual decline of mortality was noted over the 3-month period of storage in all insecticide treatments except for abamectin. Complete progeny suppression (100%) of the two insects was observed in abamectin treatments particularly at the first 3 months of storage. While in the case of chlorprifos-methyl, the maximum inhibition of progeny of C. maculatus (9.4 ± 3.5) and C. chinensis (7.2 ± 2.2) was achieved after 45 days of exposure in the first bioassay (0 month), then the progeny of two insects was increased with increasing the storage periods (1–5 months). Moreover, deltamethrin and spinosad did not induce a complete reduction of progeny of two insects during all storage periods. High protection of cowpea seeds against damage caused by two bruchid insects after 45 days of exposure was achieved in all abamectin treatments for five months and chlorprifos-methyl treatments at 0 month. Overall, abamectin demonstrates better residual efficacy than other insecticides over 5-month storage period against C. maculatus and C. chinensis and it could be a useful candidate for protection of stored cowpea seeds.