Factors determining host plant range in two lycaenid butterflies.

Abstract

Components of host plant affiliation for two, small, blue butterflies were examined and compared. The two butterflies, Glaucopsyche lygdamus and Plebijus icariodes (Lycaenidae), are superficially quite similar but differ in host range. Oviposition preferences were determined for each butterfly species by following individual butterflies in the field and recording butterfly behavior, host plant availability and host plant characteristics. Where preferences for one host species or one plant part over another were found, potential explanations were pursued by assessing and comparing larval performance on those plant species and parts in question. Larval performances were measured in terms of survival, growth, and ant attendance in the field, and survival, growth, and pupal mass in the laboratory. All foods were also analyzed for alkaloid content, and larvae were raised on plants known to differ in alkaloid content. Individuals of both butterfly species preferred to oviposit on those host species with which they had had recent experience; nevertheless, individual butterflies of both species frequently oviposited on multiple host species during the course of a single follow bout. For G. lygdamus the availability of flower buds was critical for ovipositing adults and feeding larvae. Flower buds of any one host species were unpredictable, however, and G. lygdamus consequently utilized different host species at different times. This observation predicted a positive relationship between butterfly population density and host species diversity. This prediction was tested and supported by two large-scale surveys of hostplant patches. P. icariodes differed from G. lygdamus in that both ovipositing adults and feeding larvae preferred old leaves to flower buds. The two species also differed in diapause stage, growth rates and reaction to alkaloids. Results predicted a relationship between diapause stage, oviposition site on the plant, and host range. The prediction was tested and upheld by a general survey of temperate lycaenid butterflies. The relationship was significant even when phylogenetic relationships were included in the analysis, and diapause stage was suggested as the characteristic evolutionarily most constrained.