Host preference and larval performance suggest host race formation in Galerucella nymphaeae

In this study we investigated the possibilities for host race formation in Galerucella nymphaeae. This is a chrysomelid beetle feeding on four different hosts, belonging to two different plant families, the Nymphaeaceae and Polygonaceae. Previous results showed that beetles living on the two different host families differ in morphology, i.e., body length, mandibular width, and color of the elytra. In the current study, the preference of G. nymphaeae for four hosts was investigated, together with larval performance on these hosts. In a multichoice experiment, both parents and offspring showed a strong feeding preference for their natal host plant family: between 88-98% of the total consumption consisted of the natal host plant family. Females preferred to lay eggs on their natal host family: 81-100% of the egg clutches were laid on the natal host family. Host preference was accompanied by differences in offspring performance. Offspring survival was 1.2-25 times as high on the host family from which their parents originated than on the hosts of the other plant family. Furthermore, larval development tended to progress faster on the natal than on the other host family. Since the beetles use their host plant as a mating place, positive assortative mating is a likely consequence of the beetles' host preference. Together, these results suggest that there are two host races of G. nymphaeae: one living on Nymphaeaceae and the other on Polygonaceae.