Exploring Tradeoffs of Alternative Life History Strategies in Two Texas Butterfly Species
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The evolution of gregarious feeding is an intriguing problem in ecology. It occurs in many phytophagous insects and typically coincides with eggs lain in large clutches. Despite many benefits to gregarious feeding, including accelerated larval growth rates, not all species feed gregariously suggesting disadvantages to gregariousness. To investigate the advantages and disadvantages, I studied a system of two sympatric, congeneric butterfly taxa that employ drastically different oviposition and larval feeding strategies. The Emperor butterflies both lay eggs on Hackberry trees (Celtis laevigata, C. reticulata); the Hackberry Emperor (Asterocampa celtis) lays a single egg and caterpillars feed alone, while the Tawny Emperor (A. clyton) lays a large clutch and caterpillars feed gregariously. To explore whether gregarious feeding drives clutch size differentiation while simultaneously filling in natural history information on Asterocampa butterflies this research addressed the following questions: 1) Is there a difference in emergence in terms of relative abundance of Asterocampa between sexes or species? 2) Do Asterocampa species have an oviposition preference between Celtis (Hackberry) host plants? 3) Do Asterocampa larvae experience enhanced performance in gregarious feeding groups? I predicted that the advantage of gregarious feeding would be realized for A. clyton, where females lay large clusters of eggs. Fruit baited traps were used to capture females. Phenological variation in relative abundance was analyzed with partial correlation coefficients. Female oviposition preference was assessed with a choice experiment with leaves of both host trees. Caterpillar group sizes were manipulated across the two host plant species. The group sizes approximated ranges from nature. Caterpillar performance was measured by average weight gained and proportion of caterpillars reaching their second instar. Oviposition preference was quantified using a Bayesian hierarchical model. Caterpillar performance was analyzed using an ANOVA in R. There were no differences in relative abundance across time. Females of both butterfly species preferred to oviposit on C. laevigata. I found significant differences in weight gained between group size treatments and caterpillar species but not between host species. I also found a significant difference in proportion reaching second instar between group size treatments, caterpillar species and host species. My results indicate a disadvantage to being gregarious and demonstrate that solitary feeding can enhance caterpillar performance; thus, the predicted benefits of gregariousness for caterpillar performance were not observed. The advantages of gregarious feeding were not realized in terms of caterpillar performance but might be related to defense against natural enemies.