Feeding preference of cockchafer populations and response of oak regeneration: a case study in Germany

In view of climate change the risk of biotic stressors in the forests is expected to be enhanced in addition to severe abiotic stress events, like drought. One biotic stress considered to be increased for oak regeneration is the root feeding of young plants from cockchafer. The cockchafers (Melolontha spp.) belong to the scarab beetles (Scarabaeidae) and are widespread pest species throughout Central Europe. In Germany, the most common species is the European cockchafer (Melolontha melolontha Linnaeus), whereby the forest cockchafer (Melolontha hippocastani Fabricius) is also present, mainly on sandy soils. Specifically, in south-western Germany both European and forest cockchafers cause economic losses in agricultural and forest areas. Besides drought, the feeding of roots from cockchafer larvae, act as an additional stress factor for young oak plants. In the present case study, geographically distant oak stands are infested with different intensity.

In the present study, population genetic analysis was used to differentiate the two cockchafer species and was used to estimate population dynamics, as well as possible consequences of climatic changes to the life cycle of forest cockchafer. Additionally, seedlings of two distant oak provenances were exposed to forest cockchafer larvae in a greenhouse experiment and their population genetic and root-based VOC profiles were studied. Larvae preferences for the different plants/populations were estimated and terpene synthase gene expression of the plants was measured.

Chloroplast haplotypes showed patterns of migration from different refugial regions. However, no clear association between genetic constitution of the different provenances and the abundance of cockchafer populations on site was observed. TPS gene expression patterns in response to larval feeding revealed geographic variation rather than genotypic variation. Our results support the assumption that root-released VOC are influencing the perception of roots by herbivores.