Ecological trade-offs in development and defence in a specialist moth when feeding on four congeneric host plants

Herbivore diet breadth has been hypothesised to be driven by plant quality (physiological efficiency hypothesis) or natural enemies (enemy-free space hypothesis). These hypotheses on their own are now known to be insufficient explanations for the range of herbivore diet breadths and more integrative approaches consider how trade-offs and ecological contingencies shape host plant use by herbivores. I carried out laboratory experiments to test whether physiological efficiency and defence against natural enemies trade off when larvae of the ornate bella moth, Utetheisa ornatrix (L.) (Lepidoptera: Erebidae), a genus-specialist pod-boring herbivore, feeds on leaves and pods of four Crotalaria species (Fabaceae). Different components of U. ornatrix physiological efficiency traded off amongst each other and with defence against adult predation by the spider Lycosa erythrognatha Lucas (Araneae: Lycosidae). Crotalaria micans Link provided the highest survival and fast development but low pupal weight and defence. Crotalaria ochroleuca G. Don provided poor survival, slow development, and low defence, but high pupal weight. Crotalaria pallida Aiton provided slow development and intermediate survival and pupal weight, but the best defence. Crotalaria vitellina Ker Gawl. provided high survival, fast development, and efficient defence, but low pupal weight. Compared to leaves, feeding on green seeds provided greater defence against predation, and developmental benefits, independent of host plant species, but there was a large cost in boring into the pod to reach the seeds. I therefore provide evidence that the trade-offs among physiological efficiency components and between physiological efficiency and defence could maintain multiple host plant use in specialist herbivores. I also show that feeding on green seeds incurs a cost through pod-boring but gives the moth benefits in adult defence. Therefore, the evolution of host plant use and pod-boring in insect herbivores could be driven by defence as much as by physiological efficiency.