Global warming is increasing the discrepancy between green (actual) and thermal (potential) seasons of temperate trees

Feasting on the ordinary or starving for the exceptional in a warming climate: Phenological synchrony between spongy moth (Lymantria dispar) and budburst of six European tree species

Global warming is affecting the phenological cycles of plants and animals, altering the complex synchronization that has co-evolved over thousands of years between interacting species and trophic levels. Here, we examined how warmer winter conditions affect the timing of budburst in six common European trees and the hatching of a generalist leaf-feeding insect, the spongy moth Lymantria dispar, whose fitness depends on the synchrony between egg hatch and leaf emergence of the host tree. We applied four different temperature treatments to L. dispar eggs and twig cuttings, that mimicked warmer winters and reduced chilling temperatures that are necessary for insect diapause and bud dormancy release, using heated open-top chambers (ambient or +3.5°C), and heated greenhouses (maintained at >6°C or >10°C). In addition, we conducted preference and performance tests to determine which tree species the larvae prefer and benefit from the most. Budburst success and twig survival were highest for all tree species at ambient temperature conditions, whereas it declined under elevated winter temperature for Tilia cordata and Acer pseudoplatanus, likely due to a lack of chilling. While L. dispar egg hatch coincided with budburst in most tree species within 10 days under ambient conditions, it coincided with budburst only in Quercus robur, Carpinus betulus, and, to a lesser extent, Ulmus glabra under warmer conditions. With further warming, we, therefore, expect an increasing mismatch in trees with high chilling requirements, such as Fagus sylvatica and A. pseudoplatanus, but still good synchronization with trees having low chilling requirements, such as Q. robur and C. betulus. Surprisingly, first instar larvae preferred and gained weight faster when fed with leaves of F. sylvatica, while Q. robur ranked second. Our results suggest that spongy moth outbreaks are likely to persist in oak and hornbeam forests in western and central Europe.