Analyse de l’activite insecticide du methylisothiocyanate sur callosobruchus maculatus (f.) (coleoptera : bruchidae) et son parasitoide dinarmus basalis (rondani) (hymenoptera : pteromalidae)

Le Coleoptere bruchidae Callosobruchus maculatus (Fab.) provoque des pertes importantes au cours du stockage du niebe en Afrique de l’Ouest. Un parasitoide endogene Dinarmus basalis Rondani (Pteromalidae) est present dans les greniers et peut dans certaines conditions reduire la population de la bruche. Certains agriculteurs introduisent dans leurs greniers les feuilles d’un arbuste, Boscia senegalensis (Lam.) (Capparaceae) qui emettent dans l’atmosphere des systemes de stockage des composes soufres volatiles dont le methylisothiocyanate (MITC). Ce compose est toxique envers les adultes de C. maculatus mais son influence sur son ennemi naturel est jusque la inconnue. La sensibilite de la bruche et du parasitoide au MITC a ete analysee en conditions de laboratoire. Les adultes des deux especes ont une sensibilite similaire au MITC. Apres 24 h d’exposition a ce compose, la CL50 etait respectivement de 0,48 et de 0,54 mg/l chez C. maculatus et D. basalis. L’activite ovicide du MITC est tres importante puisque la CL50 des oeufs de C. maculatus est de 0,04 mg/l. Les larves de C. maculatus ne semblent pas etre affectees par les traitements au MITC contrairement a celles du parasitoide D. basalis qui ont montre une plus grande sensibilite a ce compose. L’introduction dans les greniers des feuilles de B. senegalensis qui y liberent le methylisothiocyanate reduit la densite de la population du parasitoide et entraine une augmentation des pertes en poids des graines en favorisant le developpement de la population de la bruche. The bruchid Callosobruchus maculatus F. causes major losses during the storage of the seeds of Vigna unguiculata (Walp.) in West Africa. An endemic parasitoid, the pteromalid Dinarmus basalis Rond. 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 adults susceptibility 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 concentrations only slightly affected the survival of C. maculatus during its postembryonic 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.