Cockchafers (Melolontha melolontha and Melolontha hppocastani) are some of the most damaging forest insect pests in Europe. This study evaluated environmental factors that influence the occurrence of cockchafer grubs in forests, including location of the stand relative to open space, the stage of stand development, tree species composition, canopy openness, and cover of forest floor vegetation. To determine grub infestation in soil and to characterise forest stands, 300 sampling pits (1×0.5×0.5 m) were excavated in 12 stands in major centres of Melolontha spp. outbreak in Poland. A hurdle regression model was used to analyse the dependence of the occurrence of cockchafer grubs on environmental factors. Our results show that the boundary between open space and forest is associated with a significantly higher probability of occurrence of cockchafer grubs. Stands adjacent to meadows are particularly susceptible to infestation. The high numbers of grubs predicted by the model were observed in most stages of stand development. Characteristics associated with increased numbers of cockchafer grubs included moderate canopy openness (21–35%) and the presence of dense vegetation cover (>75%). Stands containing beech, birch and oak had slightly more cockchafers than stands containing pine, for which the predicted number of grubs was significantly lower.

Keywords: Hurdle model, Melolontha melolontha, Melolontha hippocastani, Phytoclimate, Stand canopy openness, Stage of stand development.