Alfalfa Weevil (Coleoptera: Curculionidae) Population Dynamics and Mortality Factors in Virginia

Abstract

The alfalfa weevil, Hypera postica (Gyllenhal), is a major pest of alfalfa in Virginia despite being under complete biological control in the northeastern states. In 1997 we initiated a 2-yr study of alfalfa weevil populations at three different geographic regions of Virginia to better understand the ecological mechanisms underlying the differences in pest pressure among the major alfalfa-growing regions of the state. Alfalfa weevil populations from the low-lying Piedmont plateau, Shenandoah Valley, and southwestern mountain-valley regions were sampled regularly from No- vember until first harvest in each season. Fields located in the Piedmont had significantly higher alfalfa weevil numbers than those in the Shenandoah Valley and southwestern region. Warmer winter temperatures in the Piedmont contributed to a much higher rate of alfalfa weevil oviposition and concomitant higher infestations of larvae in the spring compared with the higher elevations. Parasitization of adult weevils by Microctonus aethiopoides (Loan), which plays a key role in reducing alfalfa weevil reproduction in the northeastern states, was low in all regions of Virginia, particularly the Piedmont. Mortality of alfalfa weevil larvae was comparable to that reported in more northerly states. The dominant natural enemy of weevil larvae was the parasitoid, Bathyplectes anurus (Thom- son), which parasitized 36—92% of larvae. Analysis of alfalfa weevil life tables showed that different mortality factors were important in explaining variations in total alfalfa weevil mortality.