Bathyplectes anurus (Thomson) and B. curculionis (Thomson) (Hymenoptera: Ichneumonidae) are specialist parasitoids introduced to the United States for classical biological control of alfalfa weevil, Hypera postica (Coleoptera: Curculionidae). Adults of both species are morphologically similar, generally described as wasps ≈3-4 mm long with black, robust bodies. The Bathyplectes spp. occupy sympatric ranges throughout much of the United States and often coexist within alfalfa fields. In Virginia, B. anurus is the primary biological control agent, accounting for more than 90% of alfalfa weevil parasitization. A previous study, however, found that the levels of control by B. anurus varied among the three alfalfa-growing regions of Virginia, and noted that little is known about the mechanisms or factors responsible for the differential levels of parasitization. Of the factors that may affect host parasitization, three were addressed in this study: population density and spatial distribution of host and parasitoid, the effectiveness of parasitoids in response to host density as measured by the population functional response, and genetic variation among and between populations of parasitoids. GPS-referenced quadrat sampling was conducted in alfalfa fields in the Piedmont and southwestern region to assess alfalfa weevil population levels and spatial distribution, and the level and distribution of parasitization by B. anurus and B. curculionis. Results showed that there was no significant difference in alfalfa weevil densities among alfalfa fields in the Piedmont and southwestern region, and also no significant difference between regions in the level of parasitization. Mean alfalfa weevil density per 0.2-m2 quadrat per sampling date was 11.0 in the Piedmont and 8.0 in the southwestern region; mean percent parasitization per 0.2-m2 quadrat per sampling date was 9.1% in the Piedmont and 9.9% in the southwestern region. The spatial distributions of alfalfa weevil larvae and parasitization were found to be aggregated in all alfalfa fields sampled. The population functional response of B. anurus attack on alfalfa weevil was determined qualitatively to be Type II in all alfalfa fields sampled, except the field located at Blacksburg in the southwestern region. The Type II functional response suggests that, in general, the parasitoid has an inverse density dependent attack response to the density of alfalfa weevil larvae in the two regions of Virginia. Molecular analyses were conducted to 1) derive genetic sequences for B. anurus and B. curculionis, which could be used to identify and differentiate the two species, and 2) test for genetic variation within and between regional populations of Bathyplectes. Sequences for each species were obtained for the D2–D3 region of the 28S ribosomal DNA (28S rDNA) gene; sequence identity between B. anurus and B. curculionis was 96.6%. The 28S rDNA sequences were also obtained for another Bathyplectes species, possibly B. stenostigma, and for four taxa of Hymenoptera that hyperparasitize B. anurus and B. curculionis cocoons. Sequences were obtained for B. anurus and B. curculionis for a portion of the mitochondrial gene cytochrome oxidase subunit 1 (COI); sequence identity between B. anurus and B. curculionis was 89.7%. Genetic variation in the COI gene was observed for both species. One variation of the COI sequence expressed by 25% of B. anurus specimens (Haplotype 2) might indicate regional genetic variation correlated to temperature, in particular the average number of days per year that a region experiences temperatures above 32°C. Two major COI sequence haplotypes found in B. curculionis specimens might be indicative of incipient speciation. / Ph. D.