Do plant defenses predict damage by an invasive herbivore? A comparative study of the viburnum leaf beetle.

The impact of plant defenses on insect herbivores is widely accepted, but their relative effects on oviposition choice, survival, and larval growth in preventing pest damage, especially for invasive insects, is not fully understood. Here, we examined the potential for plant defenses to reduce the economic and environmental impacts of an invasive herbivore, the viburnum leaf beetle, VLB (Pyrrhalta viburni), on Viburnum species in North America. We used a common garden with 15 host Viburnum species of North American, European, and Asian origin and evaluated oviposition preferences, twig defense against oviposition (a reaction that crushes VLB eggs), larval performance in the lab and field, and foliar damage to mature shrubs in two consecutive years. VLB oviposition preference was the strongest predictor of plant damage, with twig defense and larval performance explaining little of the defoliation patterns. In particular, we showed that VLB females evade key defenses by choosing poorly defended twigs for oviposition; assays on the 15 Viburnum species revealed that adults laid over four times more eggs on dead (undefended) twigs than on living twigs. We additionally tested the hypothesis that shrubs with a higher proportion of dead twigs are preferentially chosen for oviposition, leading to more defoliation by larvae and increased dieback in the following year. We term this the infestation feedback hypothesis. Indeed, we report consistent positive correlations between percentage dieback, oviposition, and percentage defoliation across Viburnum species, and among individuals within two species tested separately (V. dentatum and V. opulus). Our results demonstrate that oviposition preference plays a major role in the susceptibility of Viburnum shrubs to the invasive VLB through adults choosing high-quality species for their larvae (a strong preference-performance correlation) and avoiding well-defended twigs among preferred species. More generally, where invasive insects can avoid plant defenses and when preference and performance are positively correlated, an infestation feedback loop can lead to persistent pest problems. Because dieback weakens Viburnum defenses by providing optimal oviposition sites, we recommend that Viburnum growers mechanically remove dead twigs from susceptible shrubs at the end of the growing season, especially in the early stages of VLB colonization.