Conquering a defense-free space: History and ecological context of viburnum leaf beetle invasion in North America

Background/Question/Methods

The viburnum leaf beetle Pyrrhalta viburni (Payukull) is a chrysomelid native to Eurasia and invasive in the Northeastern U.S. and Southern Canada.  It attacks plants belonging to the genus Viburnum, a clade of small trees and shrubs of worldwide importance as ornamentals. In addition to its impact in gardens and managed landscapes, P. viburni devastates populations of several native Viburnum spp. in natural habitats. The disappearance of Viburnum shrubs may have dramatic consequences for resident songbirds, which rely on Viburnum fruits as a major food source. To explain the success of P. viburni as an invader, several studies were conducted to examine the selection pressures from top-down and bottom-up forces in P. viburni native and introduced range.

Results/Conclusions

On one hand, specialized natural enemies (i.e. parasitoids) were only found in Eurasia, but their impact on P. viburni native populations was minimal (8% parasitism rate). On the other hand, plant defenses, specifically twig defense against oviposition (a wound response crushing the eggs, which are laid within the twigs), were found to be much lower in North American species than in their Eurasian counterparts in a phylogenetic field experiment using 16 species of Viburnum. This convergent continental difference in defenses contrasts with little difference in the quality of leaves for beetle larvae.  Females show strong oviposition preferences that correspond to larval performance, regardless of continental origin, which has facilitated colonization of susceptible North American species.

In North America and in Eurasia, oviposition preferences of P. viburni were found to be well correlated to the defensive capacities of their hosts; P. viburni females showed aggregative oviposition on hosts whose twig wound response could be overcome, but not on others. These results illustrate the importance of twig wound response for P. viburni fitness, and the behavioral adaptations displayed to avoid these defenses.

The combination of these two factors, absence of specialized natural enemies and a generalized trend of reduced plant defenses in native hosts, coupled to the effectiveness of an “offensive” behavior (aggregative oviposition), provided a coevolutionary mismatch facilitating P. viburni  devastating invasion.