Heatwave ontogenetic timing impacts on potato crop-pest dynamics

Heat waves – brief periods of unusually high temperatures­­ – are damaging to agroecosystems and are increasing in frequency and intensity due to climate change. Despite growing appreciation for the threat that heat waves pose pose to agricultural sustainability, we have a poor understanding of what determines their impact on agroecological interactions in the field. Here we report a field experiment that examined how heat waves and their timing interact with crop pest resistance to influence the interactions between potato (Solanum tuberosum) and its most damaging pest, the Colorado potato beetle (CPB; Leptinotarsa decemlineata). We used open-top chambers and ceramic heaters to generate heat wave conditions in field plots with pest-resistant (Atlantic) and -susceptible (King Harry) potato varieties at four CPB developmental stages (neonate, late-larval, pupa, and adult stages). We then assessed CPB performance at larval and adult stages, leaf herbivory, and tuber yield.

Methods
CPB larval performance: After each of the first two heat waves (the neonate- and larval-stages heat waves), we recorded larval count, length, and instar stage to assess larval survival, growth, and development. 

CPB adult performance: When the first adults began to emerge, which corresponded with the end of the 3rd heat wave, we surveyed plots every other day for six consecutive weeks, and collected and recorded the number of adult beetles emerged. Adult beetles were frozen immediately after collection, oven-dried, weighed, and sexed. 

Leaf herbivory: We estimated leaf herbivory after the 2nd heat wave event to assess the effects of heat wave timing and plant resistance on leaf damage. Percent herbivory across the whole plant was estimated visually following the Primary HerbVar Survey Protocol. At this time, most larvae had pupated, thus allowing us to estimate total herbivory by larvae, when most plant tissue is consumed. 

Potato yield: In the fall, when plants began senescing aboveground, we dug tubers from each plant, sorted them by plot into size and quality classes, and measured the mass of marketable tubers.