Colorado potato beetle (Coleoptera: Leptinotarsa decemlineata) diapause phenology and neonicotinoid resistance in Wisconsin

Observed temporal changes of Colorado potato beetle (Coleoptera: Leptinotarsa decemlineata) colonization of commercial potato in Wisconsin is thought to be associated with emerging neonicotinoid resistance. Changes in Colorado potato beetle colonization time produced the hypothesis that adult emergence patterns and fitness are linked to resistance. This study measures emergence patterns and fecundity of field collected populations with varying levels of neonicotinoid susceptibility. Our objectives in 2010 and 2011 were twofold: 1) assess diapause phenology for several populations with known levels of neonicotinoid resistance and 2) quantify post-diapause fitness of emerged adults. In September of each season, four field populations were allowed to diapause naturally in randomized complete block design of replicated screen cages (n = 2,000 individuals/population). In the spring, emerging adults were collected, sexed, weighed and measured every other day until no adults were recorded for a period of four consecutive observations. Accumulated soil degree days (base 10C) were recorded for the duration of each sample season as an explanatory variable. Collected beetles were staged into two week cohort groups, allowed to reassortively mate and lay eggs. Egg masses were collected and counted daily for a two week interval. Cumulative adult emergence counts were modeled with logistic regression (nlme, R-Core 2008). Fecundity was modeled with Poisson regression (GLM, R-core 2008). Peak emergence differed significantly among populations and years (P<0.001). Highly resistant populations emerged earlier and were more fecund (P<0.001). Model estimates indicate highly resistant populations demonstrate limited fitness costs and emerge over a shorter interval earlier in the season.