Evolutionary history predicts plant defense against an invasive pest

It has long been hypothesized that invasive pests may be facilitated by the evolutionary naïveté of their new hosts, but this prediction has never been examined in a phylogenetic framework. To address the hypothesis, we have been studying the invasive viburnum leaf beetle (Pyrrhalta viburni), which is decimating North American native species of Viburnum, a clade of worldwide importance as understory shrubs and ornamentals. In a phylogenetic field experiment using 16 species of Viburnum, we show that old-world Viburnum species that evolved in the presence of Pyrrhalta beetles mount a massive defensive wound response that crushes eggs of the pest insect; in contrast, naïve North American species that share no evolutionary history with Pyrrhalta beetles show a markedly lower response. This convergent continental difference in the defensive response of Viburnum spp. against insect oviposition contrasts with little difference in the quality of leaves for beetle larvae. Females show strong oviposition preferences that correspond with larval performance regardless of continental origin, which has facilitated colonization of susceptible North American species. Thus, although much attention has been paid to escape from enemies as a factor in the establishment and spread of nonnative organisms, the colonization of undefended resources seems to play a major role in the success of invasive species such as the viburnum leaf beetle.