Latitudinal variation in the phenological responses of eastern tent caterpillars and their egg parasitoids

Abstract

1. Eastern tent caterpillars (Malacosoma americanum) are broadly distributed within North America, with populations spanning a wide range of environmental conditions. Their egg masses are consistently attacked by a variety of wasps in the superfamily Chalcidoidea. We performed a reciprocal transplant‐type experiment to assess the performance of three populations spanning 15° of latitude when subjected to temperature regimes resembling southern or northern conditions.2. Exposure to warm temperatures and short overwintering periods (southern conditions) resulted in the increased survival of both caterpillars and parasitoids from all populations. By contrast, the ability of caterpillars to withstand starvation was maximised when exposed to conditions similar to their native region.3. Caterpillar and wasp phenology differed among populations even when exposed to the same temperature regime. Individuals exposed to novel conditions hatched 2–6 weeks later than those experiencing native conditions. Under typical conditions, the relative phenology of wasps and their hosts exhibited a latitudinal gradient consistent with growing season length, with southern, central, and northern wasps, emerging 50, 45, and 36 days, respectively, after their hosts.3. We identified four genera of primary parasitoids, which emerged within a narrow 2‐week span, and one hyperparasitoid, which emerged in distinct pulses over an approximately 5‐week span, possibly indicating the presence of a second generation.4. Caterpillars and wasps exhibited distinct phenological responses according to population of origin, indicating that not only pre‐hatching winter and spring conditions, but also historical factors, which may include local adaptation, maternal effects, and oviposition time, influence their phenological responses.