Combined Roles of Glandular-haired Alfalfa and Natural Enemies in Alfalfa Pest Managment in Virginia

Both alfalfa weevil, Hypera postica (Gyllenhal), (Coleoptera: Curculionidae), and potato leafhopper, Empoasca fabae (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. Bathyplectes anurus (Thompson) (Hymenoptera: Ichneumonidae) was the dominant parasitoid attacking weevil larvae at both locations. Parasitization of weevil larvae by Bathyplectes 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 Zoophthora phytonomi as well, but this was not unexpected.

The genetic variation of B. anurus 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.