Side‐effects of cowpea treatment with botanical insecticides on two parasitoids of Callosobruchus maculatus

Studies on the protective effect of botanical products against pest insects have infrequently been extended to side-effects on natural enemies. Indirect effects of botanicals on the storability of seeds could occur through their possible negative impact on biological control agents. Four plant powders and six plant oils with a known effect on the cowpea beetle Callosobruchus maculatus (Fabr.) (Coleoptera: Bruchidae) were investigated for their effect on the beetles' egg parasitoid Uscana lariophaga (Steffan) (Hymenoptera: Trichogrammatidae) and the larval parasitoid Dinarmus basalis (Rondani) (Hymenoptera: Pteromalidae). All treatments caused a decrease in parasitisation by U. lariophaga, and developing parasitoids of this species were affected by powders of Nicotiana tabacum (L.) and Tephrosia vogelii (Hook. f). In a two-choice situation using a linear olfactometer, U. lariophaga was repelled by most of the oils. In a no-choice situation, parasitisation by D. basalis was hampered by treatment with plant powders, but eggs that were laid developed normally. In a Y-tube olfactometer, this parasitoid did not discriminate between odours of untreated and plant-powder-treated beans. The powders of N. tabacum and T. vogelii had stronger negative effects on the two parasitoids than the powders of Azadirachta indica (Juss.) or Blumea aurita (DC). In samples collected from untreated traditional storage facilities, subsequently treated with plant powders in the laboratory, none of the treatments could prevent the increase in beetle numbers. At 24 days after treatment, most beetles had emerged from beans treated with powders of N. tabacum and T. vogelii. Parasitoids were affected by the botanical insecticides tested here, but the powders of A. indica and B. aurita may be compatible with biological control by D. basalis.