Intraspecific variation in feeding preference and performance of Galerucella nymphaeae (Chrysomelidae: Coleoptera) on aquatic macrophytes

The feeding preferences and performance of a freshwater macrophyte-feeding chrysomelid beetle (Galerucella nymphaeae) were assessed in laboratory experiments. Populations of Galerucella had a relatively narrow diet breadth, preferring species of Nuphar, Polygonum, and in 1 case Brasenia, while largely ignoring the remaining macrophytes offered in assays. However,because of interpopulation variation in host preferences, the species G. nymphaeae should be considered polyphagous. Distant populations from the Upper Peninsula of Michigan, Indiana, and North Carolina collected from Nuphar spp all readily consumed Nuphar spp. and Polygonum spp, but beetles from a site in south Michigan collected from P. amphibium or from Brasenia schreberi treated Nuphar as a low-preference host. The performance of Galerucella in no-choice assays was clearly related to the behavioral preferences of the larvae: larvae performed well on 3 macrophyte species that they willingly consumed, but performed poorly when they refused to consume alternative host plants and presumably starved to death. It is unknown if the starved beetles could have performed well physiologically if they had eaten alternative hosts. Performance was also related to the quality of host plant, given that beetles collected from Nuphar luteum or N. advena reached a larger adult mass on N. luteum than on N. advena. The feeding preferences of Galerucella were largely non-plastic within a single generation, although there were sometimes behavioral differences among clutchmates raised on different host plants. Morphometric and preliminary allozyme data suggest the North American populations used in this paper are conspecific, but they probably represent a different species than European G. nymphaeae. The intraspecific variation of our North American G. nymphaeae in host preferences and performance suggests that at least 2 different ecotypes occur in North America.