Plasticity in a key life-history trait contributes to population cycles in an insect herbivore

Research Highlight: Myers, J. H., & Cory, J. S. (2025). Long-term population dynamics of western tent caterpillars: History, trends and causes of cycles. Journal of Animal Ecology. https://doi.org/10.1111/1365-2656.70104. For centuries, population cycles have intrigued ecologists and posed challenges for resource managers. These dramatic fluctuations are influenced by strong interactions with natural enemies and/or the climate, yet these external drivers alone are typically insufficient to explain the observed cycles. Cyclic changes in life-history traits (e.g. fecundity) often play a significant role, though the mechanisms underlying these regular phenotypic shifts remain largely undetermined. Here Myers and Cory (2025) convincingly demonstrate the key role of plastic changes in fecundity in driving the 8-11-year population cycles of the western tent caterpillar Malacosoma californicum pluviale. These cycles are partially driven by lethal infections from a specialized baculovirus Malacosoma pluviale nucleopolyhedrovirus. Although tent caterpillars evolve increased resistance to the virus following peak infection periods, this resistance does not incur a fecundity cost, suggesting that eco-evolutionary feedback does not regulate this cycle. Instead, sublethal viral infections induce plastic reductions in fecundity. Declines in food quantity and quality following peak defoliation periods likely further contribute to these plastic changes. While climate variation does influence population growth, future climate change is unlikely to disrupt these cycles. Taken together, this long-term research underscores the importance of phenotypic plasticity in shaping dramatic herbivore population cycles. Future research on eco-evolutionary dynamics should consider, more even-handedly, alternative mechanisms by which the environment can feedback to cause phenotypic change.