Alfalfa weevil (Coleoptera: Curculionidae) pest status and incidence of Bathyplectes spp. (Hymenoptera: Ichneumonidae) and Zoophthora phytonomi (Zygomycetes: Entomophthorales) in Virginia

We surveyed 187 alfalfa fields in Virginia from 1996 to 1998 to determine the current pest status of the alfalfa weevil, Hypera postica (Gyllenhal) and incidence of the principal natural enemies of its larval stage. Fields located in the Piedmont region of the state had significantly higher levels of alfalfa weevil (1.6-2.8 larvae per stem) compared with the Shenandoah Valley (0.7-1.6 larvae per stem) and southwestern region (0.6-1.4 larvae per stem). In addition, 94.2% of the fields surveyed in the Piedmont were above the economic threshold for alfalfa weevil compared with 25.3% in the Shenandoah Valley and 17.9% in the southwestern region. A significantly lower percentage of alfalfa weevil larvae were parasitized in the Piedmont (16-32%) compared with the Shenandoah Valley (45-73%) and southwestern region (46-71%). The dominant larval parasitoid found throughout our survey was Bathyplectes anurus, which accounted for approximately 95% of all parasitoids that emerged from alfalfa weevil larvae. The entomopathogenic fungus, Zoophthora phytonomi, infected an average of 19.6% of alfalfa weevil larvae across all regions and years of our survey. In the Piedmont in 1997, however, fungal infection of alfalfa weevil larvae exceeded 50% when optimal conditions for an epizootic occurred.