Fitness consequences of host plant choice : a fi eld experiment 542

Determining the relative contributions of diff erent ecological factors for herbivore fi tness is one key to understanding the ecology and evolution of host plant choice by herbivores. Natural enemies are increasingly being recognized as an important factor: host plants of inferior quality for development may still be used by herbivores if they provide enemy-free space (EFS). Here we used the tobacco hornworm, Manduca sexta, to experimentally disentangle the eff ects of natural enemies from the potentially confounding factors of host plant quality, competition and microhabitat. We explored the consequences for both individual components of fi tness and total fi tness of M. sexta feeding on a typical high quality host plant, tobacco Nicotiana tabacum and a novel, low quality host plant, devil’s claw Proboscidea louisianica in an experimental fi eld environment in the presence of a parasitoid natural enemy, Cotesia congregata. Although early larval survival, development and growth rates, fi nal body size and fecundity were all reduced for M. sexta feeding on devil’s claw, a high rate of parasitism on tobacco and an absence of parasitism on devil’s claw contributed to similar total fi tness (net reproductive rate, R0) across the two host plant species. Our results suggest M. sexta has adopted a novel host plant (devil’s claw) outside its typical host range because this host plant provides enemy free space. In addition, oviposition behavior of adult female M. sexta appears to be well suited to exploiting the enemy-free space on devil’s claw; oviposition by M. sexta on devil’s claw appears to correspond with seasonal variation in parasitoid abundance.