Chemical defense across three trophic levels: Catalpa bignonioides, the caterpillar Ceratomia catalpae, and its endoparasitoid Cotesia congregata.

Plant secondary chemistry can vary among plant tissues, individuals, and populations, and this variation has population-level consequences for upper trophic levels. In this study, we examined the multi-trophic consequences of variation in iridoid glycosides, which are a component of plant defense against generalist herbivores and also contribute to the unpalatability of sequestering herbivores to both vertebrate and invertebrate predators. Several populations of Catalpa bignonioides were located and examined for the presence of the Catalpa Sphinx, Ceratomia catalpae, a specialist herbivore of Catalpa. We quantified iridoid glycoside content in Catalpa Sphinx caterpillars and in damaged and undamaged C. bignonioides leaves. Overall, leaves of C. bignonioides that were damaged by Catalpa Sphinx caterpillars contained lower concentrations of two major iridoid glycosides, catalpol and catalposide, than leaves of undamaged trees from naturally occurring populations. Catalpa Sphinx caterpillars sequester only catalpol, and increasing catalpol and catalposide concentrations in leaves were associated with increased catalpol sequestration by caterpillars. The parasitoid Cotesia congregata develops successfully inside catalpol-sequestering Catalpa Sphinx caterpillars, and we examined parasitoid larvae for the presence of catalpol. Parasitoid larvae dissected from caterpillars contained catalpol, but at lower concentrations than their host caterpillars. The variation in chemical defense documented here has rarely been documented over multiple trophic levels, but such resolved systems are ideal for examining competing hypotheses about the effects of plant secondary metabolites on higher trophic levels.