Mechanisms Maintaining Two Feeding Strategies in the Moth Symmetrischema lavernella

Traditional explanations for specialized host use patterns by herbivores include evolving to use high quality plant hosts, avoid natural enemies and make efficient use of the most common food resource in the environment. While several studies have combined two of these hypotheses, we are still in need of understanding how an increasing number of factors interact to determine herbivores’ diets. Symmetrischema lavernella is a phytophagous gelechiid moth whose hosts appear to be limited to the genus Physalis (Solanaceae). Flower buds and fruits serve as larval substrates producing budworms and frugivores, respectively, and both strategies are used in each of the 3+ generations of the moth. However, each larva can adopt only one strategy, leading to the prediction that the most profitable strategy should persist over time. Alternatively, frugivores and budworms would coexist if selective pressures render both strategies similar in costs and benefits or show enough variation to impede fixation on the most profitable strategy. This study was aimed at explaining the occurrence of these two feeding strategies in S. lavernella by assessing the effects of larval substrates on growth performance and survival, and resource availability through field observations and experiments. Frugivore pupal weights were found to be 33% greater than in budworms, with females being heavier for both feeding strategies. Moreover, frugivores showed greater survival than budworms in natural patches of their host plant. Parasitoid wasps and a larger frugivore acting as a predator on S. lavernella were rare, suggesting natural enemies are not important agents of mortality–at least in late summer, when this study was conducted. On the other hand, lab experiments showed that the feeding strategy chosen corresponds to the size of the floral bud entered: caterpillars that enter small floral buds (<4.2mm) adopt the budworm strategy, eating plant reproductive tissue and preventing the flower from opening; caterpillars that enter larger floral buds or open flowers adopt the frugivore strategy, burrowing into the ovary and eating ovules as the fruit develops around it. Together these findings indicate greater body size and survival in frugivores favor frugivory but its benefits are limited by the availability of floral buds large enough to support frugivores. Given that each fruit supports only one frugivore, S. lavernella faces high intraspecific competition and thus, a high opportunity cost in avoiding small buds, favoring maintenance of the budworm and frugivore strategies.