Geographic variation in thermal tolerance of western corn rootworm

Western corn rootworm, Diabrotica virgifera virgifera, is one of the most economically important crop pests in the world with estimates of damage and control costing over $1 billion USD annually. Yet despite an abundance of research devoted to studying rootworm biology in the central Corn Belt of the United States, information on key aspects of their thermal biology is still lacking. In this study, we quantified thermal metrics of western corn rootworm populations from across their range in the United States: we measured critical thermal limits, knock-down resistance, and chill coma recovery for male and female rootworm from 13 laboratory colonies that were collected across 1985 km at locations that varied by up to 5.7 °C in mean annual temperature. We further use these data to test a model from thermal ecology-the thermal adaptation hypothesis-which posits that (1) thermal limits track environmental temperatures and (2) more thermally variable environments support organisms with broader thermal ranges. In doing so, we found that thermal traits varied across populations. However, only heat tolerance traits (critical thermal maximum and knock-down resistance) tracked historical averages of mean annual temperature. Rootworm originating from more thermally variable environments did not exhibit broader thermal ranges. While theory often predicts cold tolerance should track environmental temperatures, our results suggest this pattern may disappear if organisms are reared in the laboratory for multiple generations and instead a legacy effect may exist for heat tolerance that is rarely reported.