Enemy-free space is important in driving the host expansion of a generalist herbivore to an inferior exotic plant in a wetland of Yangtze Estuary

Abstract

Novel interactions between exotic plants and native herbivores can be driven by multiple ecological and evolutionary processes, but the underlying mechanisms have rarely been explored from a multitrophic perspective. In this study, we examined the effects of host plant quality and natural enemies on the host plant use of a native generalist Laelia coenosa, which was recently known to expand its host range from indigenous Phragmites australis to invasive exotic Spartina alterniflora in the wetlands of Yangtze Estuary, China. A field population survey showed that larval density of L. coenosa was significantly higher in S. alterniflora monocultures (SM) than in P. australis monocultures (PM) and patches in S. alterniflora-dominated communities (PIS). Nonetheless, an indoor rearing experiment suggested feeding on S. alterniflora significantly reduced larval growth rate, cocoon weight, adult lifespan and female lifetime fecundity of L. coenosa in comparison with P. australis. Despite an induced feeding preference in 5th instar larvae, both 1st and 3rd instar larvae of L. coenosa significantly preferred leaves of P. australis over S. alterniflora. These results contradicted the field population survey. Subsequent field experiments showed larval mortality of L. coenosa significantly increased in habitats with SM < PIS < PM. The same trend was observed in the egg parasitism rate of L. coenosa by a specialized parasitoid Telenomus laelia. Generally, our study suggests that the enemy-free space provided by the invasive S. alterniflora is important in driving the host expansion of L. coenosa to this inferior exotic plant.