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 explore alternative strategies a system of two sympatric butterfly taxa that employ drastically different oviposition and larval feeding strategies were used. These two sympatric butterflies lay eggs on Hackberry trees; one species lays a single egg and caterpillars feed alone, while the other lays a large clutch and caterpillars feed gregariously. This study aimed to understand the evolution of gregariousness through investigating its impacts on caterpillar performance. Fruit baited traps were used to capture females and caterpillar group sizes were manipulated across 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 for each replicate. These are appropriate fitness proxies because faster weight gain and shorter time to molting limits larval exposure to natural enemies. Throughout this experiment 320 larval group replicates were deployed on to hosts. The data were analyzed using an ANOVA in R.
Results/Conclusions
Weight gain ANOVA’s showed a significant difference in weight gained between group size treatments of larvae; larvae in groups of 10 gained significantly less weight than gregarious groups (ANOVA: F=5.94, DF=3, p=0.0006). There was also a significant difference in weight gained between larvae species; Asterocampa celtis larvae gained significantly more weight than A. clyton (ANOVA: F=85.79, DF=1, p<2e-16). However, larvae showed no significant difference in weight gained between leaf species (ANOVA: F=1.65, DF=1, p=0.20). Proportion reaching second instar ANOVA’s showed a significant difference in proportion reaching second instar between group size treatments; more solitary larvae reached their second instar than gregarious larvae (ANOVA: F=6.85, DF=3, p=0.0002). There was a significant difference in the proportion reaching second instar between larvae species; more Asterocampa celtis reached their second instar than A. clyton (ANOVA: F=28.7, DF=1, p=1.67e-07). There was also a significant difference in proportion reaching second instar based on leaf species; more larvae reached second instar on Celtis reticulata leaves (ANOVA: F=8.46, DF=1, p=0.0039). Overall, results indicate a disadvantage to gregariousness and demonstrate that solitary feeding can enhance caterpillar performance; thus, suspected benefits of gregariousness on caterpillar performance are not supported, furthermore there is still no hypothesis explaining gregariousness of these caterpillars.