Larch casebearer (Coleophora laricella) is an invasive moth that is causing widespread defoliation of tamarack (Larix laricina) in northern MN. Phenological matching between the presence or absence of needles or leaves and insect emergence in spring can play a large role in the success of invasive insects. Because the spring phenology of most trees and insects is dominated by responses to temperature cues (winter chilling and spring forcing), climate change has the potential to change interactions between larch casebearer and tamarack if they show different phenological adaptations. To elucidate potentially mismatching responses of L. laricina and C. laricella to temperature cues, we conducted a chilling/forcing experiment. On November 20, 2015, we placed 48 tamarack seedlings and 200 larch casebearer larvae in 4 chilling chambers (-7°C, 1°C, 4°C, 10°C; no photoperiod). On March 17, 2016, we spread the seedlings and insects over 6 growth chambers (6.5°C, 10°C, 15.5°C, 21°C, 27°C, 32°C; 16 hours photoperiod). Phenological development (needle bud break, active wandering by larvae) was monitored every other day and then compared across chilling and forcing temperatures using multiple linear regression. Within 2-4 weeks after needle flush, five larvae were caged onto each tree and monitored for adult emergence.
Results/Conclusions
We found no evidence that phenological mismatches result from the temperature responses of L. laricina and C. laricella. For both species, phenological development (needle bud break, active wandering) was significantly advanced when they were exposed to warmer forcing temperatures across all warm temperature treatments. Generally, for all treatments, the development of C. laricella was slower than that of L. laricina. There were also significant differences between chilling treatments. For both species, the warmest chilling treatment caused significantly faster phenological development than the other chilling treatments, and development was slowest in the coldest chilling treatment. It is likely that in the warmest chilling treatment, chilling and forcing were accumulating at the same time because it was above the developmental threshold for tamarack. Mortality in seedlings or branches was only found in individuals exposed to the coldest chilling treatment. Temperature was important for survival of larvae to adulthood and intermediate temperatures (15.5, 21 °C) had the highest percent emergence. Because of the similarity in temperature response of both species, we suggest that global warming is not going to contribute to mismatches of needle flush and casebearer emergence in the spring in MN.