Susceptibility ofCallosobruchus maculatus(Coleoptera: Bruchidae) and its ParasitoidDinarmus basalis(Hymenoptera: Pteromalidae) to Sulphur-Containing Compounds: Consequences on Biological Control

Abstract

In West Africa Callosobruchus maculatus F. causes major losses during storage of cowpea (Vigna unguiculata Walper). The larvae of C. maculatus are parasitized inside the seeds by Dinarmus basalis (Rondani). African farmers introduce aromatic plants into storage systems at the beginning of the storage period that release toxic volatile compounds into the headspace of the stores. The susceptibility of C. maculatus and D. basalis to two plant sulphur-containing compounds, methylisothiocyanate (MITC) and dimethyl disulfide (DMDS), was analyzed under laboratory conditions. The adults of C. maculatus and D. basalis had the same susceptibility to MITC but the parasitoid was more susceptible to DMDS than its host. The larvae were less affected by the treatments than the adults; the seed coat acts as a barrier for volatile penetration. Susceptibility to DMDS was high when the C. maculatus larvae were extracted from the seeds. DMDS was more toxic than MITC and differences in penetration inside the seeds could explain these results. The higher susceptibility of D. basalis to the treatments could have consequences on biological control in storage systems. The presence or absence of leaves of the shrub Boscia senegalensis Lamarck (Capparaceae), which release MITC into the jar atmosphere could have an impact on numbers of C. maculatus and D. basalis. When D. basalis adults were put into the storage systems in the absence of B. senegalensis leaves, successive generations of the parasitoids maintained the C. maculatus population at a low density. In the presence of B. senegalensis leaves, the D. basalis population was more affected by the treatment than its host C. maculatus and was incapable of preventing the increase in bruchid population. This traditional method appears to increase seed weight losses by limiting the efficiency of the biological control.