Analysis of the insecticidal activity of methylisothiocyanate on Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) and its parasitoid Dinarmus basalis (Rondani) (Hymenoptera: Pteromalidae)

The bruchid Callosobruchus maculatus causes major losses during the storage of the seeds of Vigna unguiculata in West Africa. An endemic parasitoid, the pteromalid Dinarmus basalis is present in the stores and can reduce the increase in bruchid populations. African farmers often place in the stores the leaves of a shrub, Boscia senegalensis Lam.(Capparaceae), which release methylisothiocyanate (MITC). This compound is toxic to adult bruchids and could reduce seed losses but its influence on the bruchid natural enemy D. basalis was hitherto unknown. The susceptibility of C. maculatus and D. basalis to MITC was assessed under laboratory conditions. The susceptibility of adults of both species towards MITC were similar. After a 24 h exposure, the LC₅₀ was respectively 0.48 and 0.54 mg/l for C. maculatus and D. basalis. The ovicidal activity of this compound was high; the LC₅₀ of C. maculatus eggs was 0.04 mg/l after a 24 h exposure to MITC. High MITC concentrations only slightly affected the survival of C. maculatus during its post-embryonic development in the seeds of V. unguiculata. Gas chromatographic analysis demonstrated that 25-35% of the MITC present in the atmosphere of the experimental jars was absorbed by the seeds but concentrations inside the cotyledons were too low to influence the survival of the C. maculatus larvae. The D. basalis larvae developing at the expense of their host inside the larval galleries, were more affected by the treatment. The introduction of B. senegalensis releasing MITC in the storage systems could reduce the density of the parasitoid population and so increase the seed losses by permitting the development of the bruchid population.