Plant-mediated interactions between an invasive oak gall wasp and a native butterfly

Background/Question/Methods Many species that expand their ranges occur at high abundances and have large impacts in their new locales. In some cases, these outbreaking species affect native species through interspecific competition. Competition among insect herbivores has had a controversial history, but recent studies suggest that inducible plant responses in the form of nutrient loss and increased secondary compounds are a primary driver of competitive interactions. Few of these studies have examined indirect plant-mediated interactions between gall producing insects and other insect herbivores. Gall wasps are conspicuous and manipulate nutrients and secondary compounds in their host. Our study investigates the effect of an invasive oak gall wasp that recently expanded its range on a native oak feeding butterfly through induced plant responses. This wasp occurs at higher abundances in its invaded range than in its native range, and it causes foliar damage to its host plant. We predicted that the invasive wasp affects butterfly performance by reducing foliar nutrients and/or increasing tannins. To determine if gall wasps negatively affect butterflies we placed caterpillars in enclosures on trees categorized as having low, moderate or high wasp damage. To measure the effect of wasp damage on leaf nutrients, we repeatedly sampled 30 trees at multiple sites with varying levels of damage. We quantified wasp damage using photo-imaging analysis of selected leaves and measured the amount of pooled foliar C:N. Results/Conclusions We found no difference in caterpillar survivorship among trees or treatment levels. We did find differences in body size among treatments, however, such that caterpillars living on high damage trees were smaller than those on medium or low wasp damage trees. We found a strong relationship between damage and C:N such that high damage was positively related to C:N. This pattern was mostly driven by a loss of nitrogen. In addition, we found that caterpillar body size was negatively related to C:N. We conclude that the invasive wasp reduces the growth of a native butterfly by reducing foliar nitrogen. We also are performing protein-binding assays to detect the effect of wasp damage on foliar tannins and caterpillar growth.