Temperature effects on the overwintering survival and date of emergence of the bean leaf beetle (Cerotoma trifurcata)

Background/Question/Methods

With winter temperatures projected to increase by up to 6˚C in southern Ontario by the 2080s, the success of invasive agricultural pests, at the northern limits of their current ranges, could be affected as a result of changes in overwintering survival. The bean leaf beetle (Cerotoma trifurcata), which has become a major pest of soybean in the Midwestern states and has spread into Canada, overwinters in the leaf litter of soybean fields or adjacent forests. While an increase in overwintering survival under higher temperatures has been observed in some insects, it is also possible that reduced snow cover from warmer air temperatures could make the litter layer colder and more vulnerable to freeze-thaw cycles, resulting in decreased survival. For three years, we examined the effects of temperature by placing adult beetles outside in buried pots covered in leaf litter and exposing them to one of three treatment levels over the winter: heated to 4-6˚C above ambient using ceramic heat lamps; unheated, with snow cover left intact; and, unheated with snow manually removed, to determine the effect of the lack of snow cover versus heat. In the spring, date of first emergence and proportion of beetles surviving the winter were determined.

Results/Conclusions

Bean leaf beetles in the heated pots were found to emerge approximately two weeks earlier in the spring. The ability of these beetles to emerge earlier when temperatures permit, combined with the increased number of degree-days accumulated over the growing season, may allow for additional generations under future climate change scenarios, complicating management programs for this important pest. Surprisingly, there was no significant difference in overwintering survival between treatment levels. This lack of treatment effect could be the result of temperatures under the leaf litter never reaching the supercooling point of this beetle in any of the pots. To supplement these results, a survivorship function was fitted for the bean leaf beetle using survival data obtained from growth cabinets that were set at a more controlled range of temperatures.