POPULATION ECOLOGY

Abstract We tested the hypothesis that larvae of the pipevine swallowtail butterfly, Battus philenor, employ behavioral and phenotypic plasticity as thermoregula-tory strategies. These larvae are phenotypically varied across their range with predominantly black larvae (southeastern USA and California) and red larvae (western Texas, Arizona) occurring in different regions. Two years of field observations in south Texas indicate that the proportion of red larvae increases with increasing daily temperatures as the growing season progresses. Larvae were also observed to shift their mi-crohabitats by climbing on non-host vegetation and avoided excessive heat in their feeding microhabitat. Larvae of ten half-sib families from populations in south Texas and California, reared under different tempera-ture regimes in common garden experiments, exhibited plasticity in larval phenotype, with larvae from both populations producing the red phenotype at tempera-tures greater than 30C and maintaining the black phenotype at cooler temperatures. However, larvae from Texas were more tolerant of higher temperatures, showing no decrease in growth rate in the highest tem-perature (maximum seasonal temperature) treatment, compared to the California population. In a field experiment, black larvae were found to have higher body temperatures when exposed to sunlight compared to red larvae. These results suggest that microhabitat shifts and the color polyphenism observed in pipevine swallowtail larvae may be the adaptive strategies that enable larvae to avoid critical thermal maximum temperatures