A stage-based population model for alfalfa weevil in Virginia

Alfalfa weevil, Hypera postica, is an important pest of alfalfa in many areas of the U.S. including the three alfalfa-growing regions in Virginia: the Shenandoah, Piedmont, and Southwest. A survey of alfalfa fields in Virginia revealed that alfalfa weevil populations in the Piedmont are significantly higher with lower levels of parasitism compared with populations in the Shenandoah and Southwest regions. We developed a Lefkovitch stage class matrix model to better understand the dynamics of alfalfa weevil populations among the three alfalfa-growing regions. The population in each region was divided into five life stages (Egg, L1 larvae, L2 larvae, Pupa, Adult), and data from partial life tables developed by Kuhar et al. (2000) and others were used to estimate the life cycle parameters (survival, development, fecundity) in the matrix model. The projection interval for the model was one degree-day. We used the model for each region to determine the intrinsic rate of increase, stable age distribution, and reproductive value of each life stage for the alfalfa weevil population. Elasticity analyses were also used to determine the proportional sensitivity of the population growth rates to changes in life cycle parameters. Overall, alfalfa weevil population growth rate was higher in the Piedmont compared with the other two regions, and the growth rate in each region was highly sensitive to changes in the parameters related to the egg and pupal stages. The model can be useful for improving our knowledge of alfalfa weevil population dynamics and for effective management of the insect.