Geographic variation in the seasonal responses of eastern tent caterpillars and their egg parasitoids
Species with broad distributions may exhibit population-level variation in their responses to climate change. Here, we examined geographical variation in the phenological responses of a forest herbivore and its egg parasitoids (Hymenoptera). Eastern tent caterpillars (Malacosoma americanum, ETC hereafter) lay egg masses on the twigs of their main host, black cherry (Prunus serotina). Caterpillars hatch early in the spring, while parasitic wasps remain inside the eggs during caterpillar development and emerge ~2 months later, on time to attack recently laid egg masses. We collected ETC egg masses from northern, southern, and central U.S. populations to 1) investigate the effect of warming on the phenology and performance of caterpillars and their parasitoids, and 2) to determine if there is local adaptation in caterpillar and wasp traits related to seasonality. We performed paired warming simulations, splitting egg masses in half and then assigning each to either the typical conditions in their home range or a +4°C temperature regime. Aditionally, we performed a reciprocal transplant experiment subjecting whole egg masses from all three localities to either southern or northern conditions. In both assays we used growth chambers to manipulate temperature and we assessed caterpillar and parasitoid hatching/emergence time, survival, and caterpillar starvation endurance.
Warming simulations showed that caterpillars from all populations advanced their hatching time under warm conditions from 1 to 4 weeks, with caterpillars from the central populations exhibiting the largest difference. Similarly, wasp emergence also advanced ~ 2 weeks under warmer conditions, affecting the relative phenology of wasps and caterpillars most strongly in the northern population. Hatching success and starvation endurance increased under warmer conditions in two out of three populations. The reciprocal transplant experiment showed a reduction in both hatching success and parasitoid emergence under northern (cold) conditions for all populations. Consistent with local adaptation, hatching time was delayed under unfamiliar conditions and the highest levels of starvation endurance were recorded under familiar conditions (southern regime for southern caterpillars and northern regime for central and northern populations). The relative timing of caterpillar hatching and parasitoid emergence in the southern population was reduced from two to one months under northern conditions. We show that ETC populations exhibit local adaptation in traits related to seasonality and that both caterpillar and parasitoid survival might be enhanced under warmer spring conditions. We found that consistently, the central population exhibited the largest phenological plasticity and could therefore be better equipped to cope with environmental unpredictability.