Past freeze-thaw events on Pinus seeds increase seedling herbivory.

Seed and seedling survival are key components of plant population persistence. Although freeze–thaw events are experienced by many dispersed seeds in temperate ecosystems, it is unclear whether freeze–thaw stresses experienced by seeds can alter seedling susceptibility to herbivores during the growing season. We evaluated how freezing stress (temperature at −6°C for 6 h) experienced by seeds of two conifer species (Pinus resinosa and Pinus strobus) affects seedling herbivory by a generalist herbivore (Spodoptera exigua). For both Pinus species, herbivores consumed twofold more seedling biomass from freeze–thaw-treated plants than seedlings from the constant temperature treatment. Herbivores grew ~66% faster when feeding on freeze–thaw P. resinosa seedlings relative to controls, but herbivore growth rate did not differ between treatments for P. strobus. Our results show that the thermal environment experienced by a seed can have subsequent effects on plant–herbivore interactions, suggesting that (1) early ontogenetic stress could be a cryptic, yet unappreciated, determinant of future herbivory and (2) increasingly frequent cold events, such as those projected under winter climate change, may amplify seedling herbivory and reduce recruitment in managed and natural conifer forests.