Evaluation of mechanical defense provided by pericarps of three different Crotalaria species to their seeds against a specialist herbivore, Utetheisa ornatrix: a case for a possible host-herbivore evolutionary arms race

Past studies that explored the evolutionary arms race between toxic Crotalaria plants and their herbivores have mostly focused on chemical co-evolution. In this highly speciose genus of plants, we hypothesize that other defenses, such as mechanical protection of the seeds, which are exploited by specialist herbivores for their nutrients and alkaloids, have also been evolving, together with the herbivores’ ability to overcome them. To test this hypothesis, we assessed the cost of penetration imposed on a specialist herbivore, the larvae of the Ornate Bella Moth Utetheisa ornatrix, by the pericarps of three Crotalaria species. We tested the Florida native C. pumila, which has supposedly been in co-existence with U. ornatrix in the New World for thousands of years, the introduced species C. spectabilis that is native to Asia, and another introduced species, C. pallida, which is native to Africa, where multiple tiger moth species, including ones ancestral to the genus Utetheisa such as the genus Amphicallia, attack Crotalaria. Our evaluation was based on the ability of larvae to penetrate pods, their mortality, the rate of development of 4th through ultimate instar larvae when reared on open vs. closed pods as diet, and the wing span of resultant adult moths as a measurement of fitness. The cost associated with larval penetration of pericarps was greatest when feeding on C. pallida, followed by C. pumila, with the cost of penetration of C. spectabilis barely detectable in our study. We found that there are important structural and mechanical differences in the cellulose fibers forming the inner layer of the pericarps, which could help explain these differences in cost of penetration. Similar research on other Crotalaria species and associated tiger moths is required to test the evolutionary arms race hypothesis.