Latitudinal gradients and the shaping of life-history traits in a gregarious caterpillar

Abstract

The present study aimed to investigate how the impact of several factors linked to geography would shape life-history traits in a gregarious species, using the pine processionary moth (PPM) Thaumetopoea pityocampa as a model system. PPM has a wide geographical distribution over the Mediterranean Basin, and it is a strictly gregarious species throughout larval development, where the total reproductive output of each female forms a colony. We reviewed both published and unpublished data on PPM from all over its distribution in the Mediterranean Basin and extracted data on fecundity, egg size, egg parasitoid mortality, flight period, and development time. These life-history traits were then related to location, expressed as latitude and altitude, local average temperatures, and host tree species. We found that PPM fecundity increaseed with latitude, concomitant with an increase in the length of development and an earlier onset of adult flight. These results are the opposite of that found in other Lepidoptera species with a wide geographical distribution, as well as in insects in general. We propose that a large colony size in PPM is important at higher latitudes because this confers an advantage for thermoregulation and tent building in areas where larvae have to face harsher conditions during the winter, thus shifting the optimal trade-off between the number and size of eggs with latitude. However, host tree species also affected the relationship between egg number and size and the optimal outcome of these traits is likely a compromise between different selection pressures. © 2010 The Linnean Society of London.