Changes in frequencies of genes that enable Phyllotreta nemorum to utilize its host plant, Barbarea vulgaris, vary in magnitude and direction, as much within as between seasons

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