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 and Bruchidius atrolineatus, and one endemic pteromalid parasitoid, Dinarmus basalis, are present in the Bobo Dioulasso region of Burkina Faso. B. atrolineatus adults emerging in stores from December to January were in reproductive diapause and the C. maculatus population was small. Numbers of C. maculatus increased substantially from February onward when temperatures and humidity increased. Numbers of D. basalis were low at the beginning of cowpea storage in December and the parasitoid was unable to control the C. maculatus population, which caused significant seed weight losses. When adults of D. basalis were introduced into stores at regular intervals, either during the first 2 months of storage or during the entire storage period, the parasitoids reduced the increase in C. maculatus and the seed weight losses were <100 g kg-1 seed. 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 became favourable 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 for 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 storage, is possible and limits weight losses of stored seeds to <10% after 6 or 7 months.