Institut de Recherche sur la Biologie de l'Insecte (ESA CNRS 6035), Avenue Monge-Parc Grandmont, 37200 Tours, France.
The Coleoptera Bruchidae Callosobruchus maculatus causes important damages during the storage of the seeds of the leguminosae Vigna unguiculata in West Africa. The females oviposited on the pods and larval development is entirely within the seeds.The Hymenoptera Dinarmus basalis is also present in the granaries and parasitize the bruchid larvae. It is an efficient natural enemy which could be used for biological control. In order to reduce the bruchid damages, the african farmers introduce in the granaries the leaves of a Capparidacae; Boscia senegalensis which produce volatile sulphur compounds and particularly the methylisothiocyanate. This compound was not transformed during the seed storage. However, 30 to 40% of the methylisothiocyanate was absorbed by the seeds and was not released after the treatment. The influence of this compound on the survival rates of adults and larvae of C. maculatus and D. basalis was tested. The LD50 of C. maculatus ( 0.54µg/l) and D. basalis adults (0.48 µg/l) were not very different. The eggs of bruchids were also very sensitive to methylisothiocyanate but this compound caused a limited mortality during the larval growth of the bruchid and its parasitoid. During the storage of V. unguiculata seeds, the use of B. senegalensis leaves or methylisothiocyanate reduced the density of the hymenoptera adults but had a limited influence on the bruchid larvae which are the hosts of D. basalis. In these conditions, these treatments with methylisothiocyanate could so limit the efficiency of the biological control against C. maculatus.