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 (Pi c), and one endemic pteromalid, Dinarmus basalis (Rondani), are presen t in the Bobo Dioulasso region of Burkina Faso where this study was ca rried out. B. atrolineatus adults emerging in the stores from December to January are in reproductive diapause and the C. maculatus populati on is small. Numbers of C. maculatus increase substantially from Febru ary onward when temperatures and humidity rise. The number of D. basal is is low at the beginning of cowpea storage in December and the paras itoid cannot control the C. maculatus population, which therefore caus es important seed weight losses. When adults of D. basalis are introdu ced into the stores at regular intervals, either during the first 2 mo of storage or during the entire storage period, the parasitoids reduc e 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 th e fields before harvest or were infested artificially. The introductio n of high numbers early in the storage period limited the buildup of t he bruchid population even when the climatic conditions become favorab le for the rapid multiplication of C. maculatus. Studies in the labora tory under controlled conditions confirmed the results obtained under natural conditions in Burkina Faso. A high ratio of parasitoids to hos t larvae and pupae was critical to the successful biological control. It is only when this ratio is high that C, maculatus populations can b e 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 o r 7 mo.