Dietary and multitrophic effects of the furanocoumarin xanthotoxin differ between two confamilial generalist herbivores, <i>Chrysodeixis includens</i> and <i>Trichoplusia ni</i> (Lepidoptera: Noctuidae)

Different generalist herbivore species within a lineage can differ in their responses to a single plant defensive trait. Herbivore responses to plant defensive traits can also influence their interactions with their own natural enemies. Here, we rear 2 generalist herbivores, Trichoplusia ni Hübner (Lepidoptera: Noctuidae) and Chrysodeixis includens (Walker) (Lepidoptera: Noctuidae), on artificial diets containing 0%, 0.01%, 0.05%, and 0.10% xanthotoxin. We measured and compared growth rate, body size, and mortality among these 4 dietary xanthotoxin concentrations. C. includens grew more slowly and was more likely to die before adulthood as dietary xanthotoxin increased and also varied in body size. Trichoplusia ni had a higher mortality rate overall, which increased as dietary xanthotoxin increased; however, there were no effects on growth rate and body size for this species. In a second experiment, the same 4 diets were given to larvae of both species parasitized by the polyembryonic parasitoid Copidosoma floridanum Ashmead (Hymenoptera: Encyrtidae), and growth rate, mummy size, and mortality were measured and compared. There were no sublethal effects on growth rate or mummy size, and mortality increased in both herbivore species but was higher in C. includens. These results show that different generalist herbivore species can respond differently to plant defensive traits, possibly due to different levels of adaptations to a specific trait, and these responses can affect higher trophic levels. Further research can compare the multitrophic impacts of variation in responses to plant defensive traits among more generalist herbivores.