Temperature-Dependent Development of OverwinteringAphthona lacertosaandA. nigriscutis(Coleoptera: Chrysomelidae): Two Flea Beetles Introduced for the Biological Control of Leafy Spurge,Euphorbia esula

Two flea beetles, Aphthona lacertosa Rosenheim and A. nigriscutis Foudras, have been introduced to control leafy spurge, Euphorbia esula L., in North America. We determined the effect of temperature on developmental rates of overwintered third instars to adult for each species. Developmental rates were determined by field collecting soil cores containing overwintering larvae and holding soil cores at constant temperatures of 12, 15, 18, 21, 22.5, and 26degreesC with 24-h light. Linear regression indicated that the lower developmental thresholds for A. lacertosa and A. nigriscutis were 7.5 and 9.3degreesC, respectively, and the median-required cumulative degree-days for emergence was 461 DD for A. lacertosa and 433 DD for A. nigriscutis. For each species, a three-parameter Weibull function was fit to a cumulative probability distribution of emergence. First emergence estimates for each species were calculated by multiplying the gamma (gamma) parameter from the fit of the Weibull function with median-required degree-days from the linear model. First emergence for A. lacertosa and A. nigriscutis were predicted at 326 and 322 accumulated DD, respectively. Emergence models were validated with springtime field captures in Minnesota, Montana, and North Dakota. Results overall indicate a lack of bias in first emergence predictions for both species. However, there was a possible bias for predicting first emergence of A. nigirscutis in Montana, which was on average 15 d late. We speculate that different climatic and environmental conditions may have played a role.