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.

Both alfalfa weevil, <i>Hypera postica</i> (Gyllenhal), (Coleoptera: Curculionidae), and potato leafhopper, <i>Empoasca fabae</i> (Harris), (Homoptera: Cicadellidae), remain key pests of alfalfa in Virginia. Commercial varieties of potato leafhopper-resistant (or glandular-haired) alfalfa were released in the mid-1990s, but the impact of alfalfa weevil on these varieties has not been well documented. In 1999, two large-scale field experiments were initiated to compare the performance of a glandular-haired alfalfa variety against a standard, non-glandular-haired variety under both alfalfa weevil and potato leafhopper pest pressures in the southwestern and Piedmont regions of Virginia over a 3 year period. Results indicated that alfalfa weevil must be managed in potato leafhopper-resistant alfalfa to limit crop loss. Surprisingly, similar densities of potato leafhoppers were found in both the glandular-haired and standard varieties. Both varieties frequently had similar yields and forage quality. In general, the glandular-haired variety did not outperform the standard variety. Results also indicated that insecticide application did not always provide the expected benefits of higher yields and forage quality, despite reducing pest densities for 2-3 weeks after application. These data suggest that the economic thresholds for one or both of these pests in Virginia may require adjustment. The potential impact of glandular-haired alfalfa on the natural enemies of alfalfa weevil was examined as well. <i>Bathyplectes anurus</i> (Thompson) (Hymenoptera: Ichneumonidae) was the dominant parasitoid attacking weevil larvae at both locations. Parasitization of weevil larvae by <i>Bathyplectes</i> spp. did not appear to be adversely affected by the presence of glandular trichomes on the potato leafhopper-resistant variety. Glandular trichomes had little impact on the infection of weevil larvae by the fungus <i>Zoophthora phytonomi</i> as well, but this was not unexpected. The genetic variation of <i>B. anurus</i> was surveyed at both study sites using RAPD-PCR to establish or eliminate the possibility that differences in parasitization levels between the Piedmont and southwestern regions could be attributed to the presence of different parasitoid strains. Most of the detected phenotypic variation was attributed to within population variation, with very little variation occurring between the two populations. However, the between population variation was statistically significant in 2000, but not in 2001. / Ph. D.