Abstract
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.
Highlights
Dietary breadth is a key component of the ecology, evolution, and diversification of phytophagous insects [1,2]
Insect herbivores are grouped into three categories depending on the range of foods they can consume: monophagous insects only feed and develop in nature on one plant species, oligophagous insects feed on a number of species belonging to one families, and polyphagous insects feed on a wide range of plant species belonging to many families [3]
Our study shows that P. viburni 3rd instar larvae are negatively affected by the transfer from an optimal host (V. dentatum) to a suboptimal host; at least a small percentage of larvae managed to reach the adult stage after transfer on each of the suboptimal hosts tested (Figure 1), pupal and adult weight were consistently lower after transfer compared to larvae that were kept on
Summary
Dietary breadth is a key component of the ecology, evolution, and diversification of phytophagous insects [1,2]. Most phytophagous insects, even among polyphagous species, complete their development on a single plant, the one where eggs were deposited [4]. Some insect herbivores have been shown to extend their normal dietary range during a population outbreak after depletion of their usual food sources [12]. Such circumstantial host switching may be detrimental to the fitness of the herbivore if the new host is a suboptimal species that would normally be avoided [13], but is obviously beneficial if the alternative is starvation and death. In the case of biological invasions of insect herbivores, dietary flexibility is often regarded as a trait facilitating both the establishment and spread of the exotic herbivore into new environments [14]
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