The effects of chilling and forcing temperatures on spring synchrony between larch casebearer and tamarack

Spring phenological synchrony can be important for tree-insect interactions. Depending on the magnitude and direction of phenological shifts, overwintering insects could be affected in many ways, for example, facing starvation or having to contend with increased chemical or physical defences of host trees. If temperature has different influences on the phenology of trees and insects, climate change can alter spring phenological synchrony. In this experiment, we exposed tamarack seedlings and larch casebearer larvae from Minnesota, USA, to a variety of chilling and forcing temperatures and measured spring phenology (twig bud break and larval activation). We additionally measured casebearer performance on seedlings that were exposed to different forcing × chilling levels, tracking larval survivorship to adulthood. Warmer forcing enhanced larval activation and bud break, but larval development slowed down past 21°C. Higher chilling temperatures accelerated bud break, but the effect was inconclusive for larvae. There was no chilling × forcing interaction for either species. Spring activity accelerated more quickly with increases in temperature for larvae than for seedlings, resulting in increased phenological synchrony at warmer temperatures. Activation rates for overwintering larvae were highest at 27°C, while survivorship to adulthood following spring activation was highest at 21°C. At temperatures at or beyond 27°C, no larvae reached adulthood. Warmer winters and springs will likely initially increase spring synchrony between tamarack and larch casebearer, exposing larvae to younger, potentially more nutritious foliage, but extremely warm spring temperatures may decrease survivorship of larvae to adulthood.