Biological control of bruchids in cowpea stores by release of Dinarmus basalis (Hymenoptera: Pteromalidae) adults

Bruchids cause major losses during cowpea storage in West Africa. Two species, Callosobruchus maculatus (F.) and Bruchidius atrolineatus (Pic), and one endemic pteromalid, Dinarmus basalis (Rondani), are present in the Bobo Dioulasso region of Burkina Faso where this study was carried out. B. atrolineatus adults emerging in the stores from December to January are in reproductive diapause and the C. maculatus population is small. Numbers of C. maculatus increase substantially from February onward when temperatures and humidity rise. The number of D. basalis is low at the beginning of cowpea storage in December and the parasitoid cannot control the C. maculatus population, which therefore causes important seed weight losses. When adults of D. basalis are introduced into the stores at regular intervals, either during the first 2 mo of storage or during the entire storage period, the parasitoids reduce the increase in C. maculatus numbers and the seed weight losses are < 100 g/kg of seeds. Releases of D. basalis adults were made in stores containing varieties of cowpea which had been infested naturally in the fields before harvest or were infested artificially. The introduction of high numbers early in the storage period limited the buildup of the bruchid population even when the climatic conditions become favorable for the rapid multiplication of C. maculatus. Studies in the laboratory under controlled conditions confirmed the results obtained under natural conditions in Burkina Faso. A high ratio of parasitoids to host larvae and pupae was critical to the successful biological control. It is only when this ratio is high that C. maculatus populations can be controlled. Biological control of bruchids, using inoculation by D. basalis adults in cowpea stores at the beginning of the storage time, is possible and limits weight losses of stored seeds to <10% after 6 or 7 mo.