Dealing with variable spring conditions: The strategy of eastern tent caterpillars (Malacosoma americanum)

Eastern tent caterpillars (Malacosoma americanum) have a wide distribution, including the entire east coast of the United States. The larvae are gregarious spring feeders and their hatching is synchronized with the bud burst of their main host plant, Prunus serotina.  Larval mortality can result from asynchrony between herbivore and host plant phenology and from failure to thermoregulate during cold spring periods. Variation in hatching time within an egg mass increases the likelihood of synchrony with the host plant, but results in a small initial colony size, which restricts caterpillar thermoregulation ability. To investigate the environmental pressures involved in shaping the egg hatching strategy of tent caterpillars, we evaluated the variability of spring conditions across a latitudinal range, and the hatching patterns of egg masses under controlled conditions. Regression and clustering techniques were employed to evaluate the spatial and temporal variation of two spring variables: degree-day accumulation required for caterpillar hatching and incidence of larvae-threatening cold periods. We included 30 years of data from each of 181 meteorological stations within the range of M. americanum. We found a latitudinal gradient in variability of spring conditions, with extreme latitudes (both north and south) being less variable than mid-latitudes.  Hatching data revealed that individual caterpillars within an egg mass vary in the degree days required to hatch and therefore egg masses can exhibit extended hatching periods, particularly during relatively cold springs. Understanding the mechanisms that allow for herbivore-host plant synchronization processes is particularly important to predict and evaluate the impact of climate change on plant-insect interactions