The Catalpa bignonioides food web: Implications of variable interactions among four trophic levels

Catalpa bignonioides (Bignoniaceae) employs predators and parasitoids as a defense against Ceratomia catalpae (Lepidoptera: Sphingidae). Ants and wasps are attracted to sugar produced at foliar extrafloral nectaries. Sugar production rates increase after Ceratomia caterpillar herbivory, and ants (Forelius pruinosus) are found at greater abundance on leaves with more productive nectaries. Ant attendance increases after damage to leaves and plants and mortality rates are greater on plants with more ants. This ant-plant interaction can be characterized as an inducible, biotic defense against caterpillars. Caterpillar populations are also depressed by Cotesia congregata larval parasitoids, although these parasitoids are attacked by a diverse suite of secondary parasitoids (hyperparasitoids). Rates of hyperparasitism rates and species richness of hyperparasitoids per parasitoid cocoon mass increased from summer to fall, and were hyperparasitism rates were greater in the lower canopy of Catalpa trees. Host density, host location within the tree, and hyperparasitoid diversity all emerged as statistically significant predictors of hyperparasitism rate, but no single variable emerged as the uniquely accurate or useful predictor. The native range of C. bignonioides is currently being invaded by the red imported fire ant, Solenopsis invicta, and I contrasted the capacity of F. pruinosus versus S. invicta as plant mutualists. The two species were responsible for similar rates of caterpillar mortality, although field observations suggest that S. invicta encountered fewer caterpillar aggregations and was not attracted by increases in extrafloral sugar production at damaged leaves. Solenopsis rarely collected extrafloral nectar and competitively excluded or killed the traditional mutualist partners of C. bignonioides (ants and Cotesia parasitoids). Recruitment of Ceratomia moths was greatly decreased in Solenopsis-dominated habitats as a result of increased rates of pupal predation, thereby diminishing the plant's need for protection. It is widely recognized that S. invicta simplifies invaded habitats by replacing native arthropods; my research demonstrates that interactions within invaded communities are simplified as well.