Switched after Birth: Performance of the Viburnum Leaf Beetle [Pyrrhalta viburni (Paykull)] after Transfer to a Suboptimal Host Plant

Host plant switching is common among phytophagous insects. Once optimal food sources have been depleted, immature insects may resort to use of suboptimal hosts in order to complete their development. Such host switching may have dramatic consequences for insect fitness. Here we investigate the effects of host switching in larvae of the viburnum leaf beetle, Pyrrhalta viburni, an invasive landscape pest in North America. Specifically, we examine how transfer of 3rd instar larvae from the optimal host Viburnum dentatum to three suboptimal hosts (V. lentago, V. carlesii, and V sieboldii) affects larval development and survivorship to the adult stage. Larval survivorship, pupal weight, and adult weight were overall lower for P. viburni larvae that switched hosts, independently of the suboptimal host tested. This decrease in performance corresponds to a decreased feeding rate on suboptimal hosts. Subsequent choice tests showed that 3rd instar larvae become less choosy as they approach pupation, and discriminate less between optimal and suboptimal hosts past a certain weight threshold. In conclusion, P. viburni larvae are able to complete their development on suboptimal hosts, but host switching negatively impacts several fitness correlates. Mixed ornamental gardens containing both optimal and suboptimal Viburnum species may provide to outbreaking P. viburni populations opportunities to survive the depletion of their preferred food sources.