| Understanding what determines host range---the number and type of different resources used by an individual, population, or species---is a fundamental question in biology. I explore ecological and evolutionary determinants of host plant range in the tobacco hornworm, Manduca sexta. I have taken advantage of both domesticated laboratory populations of M. sexta, and a recent host plant shift in wild M. sexta in the southern US, to examine how the recent evolutionary history, host plant quality, natural enemies and environmental temperatures impact the performance and fitness of M. sexta. Using laboratory experiments, I demonstrated severe reductions in performance and fitness associated with feeding on an evolutionarily novel host plant, devil's claw ( Proboscidea louisianica): survival, growth and development rates, immune function, final body size, fecundity and total fitness were all reduced for M. sexta reared on devil's claw compared to their typical host plant, tobacco (Nicotiana tabacum). I found that these costs can be ameliorated under warmer thermal conditions, as the typical negative relationship between body size and rearing temperature was reversed on devil's claw. However, one of the greatest drivers of M. sexta's adoption of devil's claw appears to be escape from an important braconid parasitoid natural enemy, Cotesia congregata. A field experiment demonstrated that the intrinsic costs of using devil's claw were offset by enemy release, resulting in comparable total fitness of M. sexta feeding on devil's claw and tobacco. In general, domesticated laboratory populations of M. sexta exhibited qualitatively similar responses as wild M. sexta however, there were a few key differences, e.g., domesticated M. sexta exhibited relatively greater reductions in survival and fecundity on devil's claw. My research shows that the selective environment, and abiotic and biotic ecological factors are important components of host range, sometimes interacting in surprising ways to alter overall herbivore fitness and host plant us... |
