The consequences of interspecific competition between Dinarmus basalis (Rond) (Hymenoptera: Pteromalidae) and Eupelmus vuilleti (CRW) (Hymenoptera: Eupelmidae) on the development of their host populations.
In the dry tropical region of Niger two species of Bruchidae, Bruchidius atrolineatus and Callosobruchus maculatus, were sympatric and developed at the expense of cowpea (Vigna unguiculata). Two species of parasitoids, Eupelmus vuilleti and Dinarmus basalis, were present in the fields and then in the stores after harvest of the seeds. When two two bruchid species were present in the stores, the density of B. atrolineatus remained low and C. maculatus became progressively dominant. From February, the density of the E. vuilleti population increased while that of D. basalis remained low. The interspecific competition between the two parasitoid species explains these differences. Experimental studies in small storage systems showed that the parasitoids perceived the presence of the parasitized hosts, but their behaviour was different. E. vuilleti multiparasitized the hosts carrying the eggs or larvae of D. basalis, while D. basalis avoided the hosts parasitized by E. vuilleti. Under natural conditions in the stores and in the absence of interspecific competition, D. basalis had significant parasitic capacity and caused the strongest regression of the density of the C. maculatus population. In contrast, when both parasitoid species were introduced in the stores, D. basalis density was low and did not vary while that of E. vuilleti progressively increased throughout the study. In the presence of the two parasitoids, the multiplication rate of C. maculatus remained high.