The role of host-plant association on the phenotypic and genetic differentiation of sympatric populations of hymenopteran parasitoids.

Research on hymenopteran parasitoids has shown that strong associations exist between parasitoids and the plants on which their hosts occur. Chemical and morphological cues from host plants can be used by parasitoids to locate and oviposit in their hosts. Such dependency suggests that host-plant association might mediate parasitoid specialization. If that specialization occurs, I believe that phenotypic and genotypic differences among sympatric populations of parasitoids searching for hosts in different host plant species might exist. Parasitoid specialization by host-plant can be enhanced if gene flow is restricted. Further, the life-histories and host species utilization patterns of parasitoids also may influence the nature and degree of phenotypic and genotypic differentiation. Generalist and specialist parasitoids might present differences in gene flow and thus in genetic differentiation. Since generalist parasitoids can find a greater variety of hosts in a greater variety of habitats than can specialist parasitoids, generalist parasitoid individuals are assumed to move less to find hosts and consequently exhibit less gene flow than specialist parasitoid individuals. Thus, I propose that phenotypic and genotypic differences between parasitoids ovipositing in different host-plants should be more pronounced in generalist than in specialist parasitoids.

To test my hypotheses I am using the specialist parasitoid Aleiodes nolophanae (Ashmead) (Hymenoptera: Braconidae) and the generalist parasitoid Cotesia marginiventris (Cresson)(Hymenoptera: Braconidae). Both are the most abundant parasitoids of the green clover worm (Plathypena scabra (F.) (Lepidoptera Noctuidae)) at my study sites in Maryland, USA. Phenotypic differences in weight, developmental time, sex ratio, percent parasitism, and search behavior, as well as genotypic differences (as reflected in AFLP patterns) are been analyzed. Samples are being taken from sympatric populations of the generalist and specialist parasitoids, ovipositing on the green clover worm, in alfalfa and in soybean at various sites throughout the state of Maryland.

So far, phenotypic differences between parasitoids attacking the green clover worm in alfalfa and soybean have been found in A. nolophanae. No genotypic differences in A. nolophanae attacking the green clover worm on this two plants have been detected. Current research is analyzing phenotypes and genotypes from the generalist Cotesia marginiventris to see if the degree of phenotypic and genotypic differentiation is more pronounced than in the specialist A. nolophanae.