The flea beetle Phyllotreta nemorum L. (Coleoptera: Chrysomelidae) is genetically polymorphic for resistance against the defences of one of its host plants, Barbarea vulgaris R.Br. (Brassicales: Brassicaceae). Whereas resistant flea beetles are able to use B. vulgaris as well as other cruciferous plants as food, non-resistant beetles cannot survive on B. vulgaris. This limitation to host plant use of non-resistant beetles could potentially lead to asymmetric gene flow and some degree of genetic isolation between the different resistance-genotypes. Therefore, we studied the extent of genetic differentiation at neutral allozyme loci between samples of flea beetles that were collected at different locations and first tested for resistance phenotype. Since earlier work has shown a weak, but significant, effect of geographical distance between the samples on their genetic differentiation, in the present study variation at the neutral allozyme loci in P. nemorum was partitioned between geographical distance and resistance-phenotype. Both sources independently contributed statistically significantly to population differentiation. Thus, there appears to be a limitation to genetic exchange between the resistant and non-resistant flea beetles when corrections are made for their geographic differentiation. This is consistent with the presence of some degree of host race formation in this flea beetle.

A central issue in the evolutionary ecology of species interactions is coevolution, which involves the reciprocal selection between individuals of interacting species. Understanding the importance of coevolution in shaping species interactions requires the consideration of spatial variation in their strength. This is exactly what the, recently developed, geographic mosaic theory of coevolution addresses. Another major development in the study of population ecology is the introduction of the population genomics approach in this field of research. This approach addresses spatial processes through molecular methods. It is of particular interest that population genomics is especially applicable to natural populations of non-model species. We describe how population genomics can be used in the context of the geographic mosaic of coevolution, specifically to identify coevolutionary hot-spots, and to attribute genetic variation found at specific loci to processes of selection versus trait remixing. The proposed integration of the population genomics approach with the conceptual framework of the geographic mosaic of coevolution is illustrated with a few selected, particularly demonstrative, examples from the realm of insect-plant interactions.

The interaction between the flea beetle, Phyllotreta nemorum L. (Coleoptera: Chrysomelidae), and its host plants is well suited to study the dynamics of a geographic mosaic of (co)evolution. The flea beetle can either be resistant or susceptible to the defense of one of its host plants, Barbarea vulgaris R.Br. G‐type (Brassicaceae). Previous findings suggested that the frequency of resistant beetles on host plants other than the G‐type of B. vulgaris had decreased over time within the period of 1999–2003. In 2008 and 2009 new sampling was performed to investigate whether or not this decrease in frequency of resistance of the flea beetles formed a continuing trend and whether or not the frequency of resistant beetles also varies within the year. The frequency of resistant beetles on different host plants was determined during the reproductive season of the flea beetles in both years. Overall, the frequencies of resistant beetles on B. vulgaris (G‐type) remained close to 100%, as found before, but those on other host plants did not consistently decrease across the years, in contrast to what had been suggested. Furthermore, the repeated sampling revealed that the frequency of resistant beetles differed significantly within a season. The present data show that relative frequencies of different resistance phenotypes of P. nemorum on other host plants than B. vulgaris (G‐type) are highly dynamic, both within and across years. Therefore, monitoring the changes in these resistance frequencies should involve season‐wide sampling efforts. Although the monitoring in this study does not provide an explanation for the observed dynamics, we propose a testable scenario.

Abstract. Flea beetles are characterized by their tendency to jump. They can also fly. First, the effects of starvation on flight activity in the flea beetle, Phyllotreta nemorum L. (Coleoptera: Chrysomelidae) were determined. After starving P. nemorum for five days a greater number of individuals of both sexes flew than when fed continuously for the same period. In addition, the effect of the mating status of females of P. nemorum on their movement was determined. Mated females were more active than virgin females. It is concluded that starvation stimulates the flight activity of both sexes and mating activates the movement of females of P. nemorum.