Effects of tree characteristics and climatic conditions on gall midge abundance on European beech (Fagus sylvatica L.)

As a consequence of climate change and damage to coniferous forests, European beech (Fagus sylvatica L.) is the preferred plant species for forest restoration in Central Europe. European beech is generally regarded as pest-resistant. However, its vulnerability to secondary pests, for instance, gall-forming midges, may increase with environmental stress such as long drought periods. We analysed the abundance of two gall-forming insects, Mikiola fagi and Hartigiola annulipes, on European beech at 26 forest sites across the Czech Republic, spanning diverse climatic and environmental conditions, using generalised linear mixed models to evaluate the effects of abiotic factors and host tree characteristics. The results revealed that M. fagi was more abundant on younger trees, in stands with lower canopy closure, and under warmer spring conditions. In contrast, the abundance of H. annulipes declined in drought-affected areas. These patterns demonstrate species-specific responses of gall midges to host tree characteristics and climatic variables, suggesting that climate change may favour higher M. fagi abundance. Accordingly, our findings support the establishment of young beech stands under higher canopy closure, for example, beneath the shading of mature trees.