Latitudinal variation in responses of a forest herbivore and its egg parasitoids to experimental warming
Disrupted biotic interactions are a predicted consequence of anthropogenic climate change when interactants differ in the magnitude or direction of phenological responses. Here, we examined the responses to artificial warming of northern, southern and central populations of the eastern tent caterpillar and its hymenopteran egg parasitoids. We subjected egg masses from each region to the typical conditions they experience in their source locality or to a warmer temperature regime, to quantify the effects of simulated warming on their relative phenology, survival and neonate starvation endurance. In addition, we characterized spring heat accumulation and cloud cover at each collection site using 30 years of hourly weather station data. As predicted, degree-day accumulation rates decreased with latitude; however, the mid-latitude site experienced what we predict to be the harshest spring conditions for tent caterpillars: slow heat accumulation combined with thick cloud cover. Remarkably, caterpillars from this site exhibited the largest phenological plasticity, hatching a month earlier under warmer than under typical conditions and doubling caterpillar survival. Survival of caterpillars from all regions was enhanced at warmer temperatures, whereas parasitoid survival was unaffected. The starvation endurance of hatchlings increased under warmer conditions in the central and southern populations only. We show that phenological responses to warming differed between hosts and parasitoids, resulting in a 5-day reduction in the relative phenology of wasps and caterpillars in the northern population. Our findings caution that responses to global warming are likely to be population or region specific and cannot be readily generalized, particularly for wide-ranging organisms.
KeywordsBaryscapus Chalcidoidea Climate change Malacosoma americanum Phenology Synchrony Tent caterpillars
This research was generously funded by The Washington Biologist’s Field Club, The Harlan Trust Fund, a PhD fellowship from CONACyT (Consejo Nacional de Ciencia y Tecnología, Mexico), and a George Melendez Wright Climate Change Fellowship, National Park Service (USA) to M. Abarca. We thank R. Oppenheimer, C. Indech D. Salehi, and C. Block for their research assistance. The picture for Fig. 1a was kindly provided by J.A. Ballesteros. M. Gates provided invaluable assistance in identifying the egg parasitoids. We are grateful to G. Heimpel, an anonymous reviewer, and all members of DC-PIG and GW Eco-Evo discussion group for constructive comments on previous versions of this manuscript.
Author contribution statement
MA and JTL conceived and designed the experiments. MA performed the experiments, analyzed the data, and wrote the manuscript. PF compiled and analyzed the climate data. All authors participated in editing the manuscript.
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