The Asian chestnut gall wasp, Dryocosmus kuriphilus Yasumatsu, 1951 (Hymenoptera: Cynipidae), is a parthenogenetic species original from China that rapidly invaded Europe since its detection in 2002. The wasps lay eggs in the buds of chestnut trees (Castanea spp.) inducing galls on leaves, stipule, flowers and shoots. The consequences are significant economic damage due to a reduction of nuts and wood production, as well as a progressive decrease in tree vigour, which can make the tree susceptible to other secondary pests. We report here the results of a 4-year study of this species conducted in Galicia (NW of Iberian Peninsula), one of the most important areas for chestnut production in southern Europe. The insect was detected in 2014 but was apparently present at least two years earlier. It has since spread rapidly throughout the territory. It mainly dispersed naturally, although some long distance dispersal events influenced the rate of spread across the landscape. Population abundance varies throughout the territory, and we were able to establish hotspot and coldspot areas, mainly associated with maximum temperatures in the summer months. At a local scale, the trees attacked by D. kuriphilus and the number of galls on each tree were spatially clustered in the first years from the invasion. Tree height significantly predicted both variables, suggesting that gall wasps may use visual cues to locate suitable host trees, at least in the early stage of invasion. The species showed a rapid recruitment of native parasitoids: three years after detection we found 19 species of native parasitoids attacking D. kuriphilus in Galicia. Percent parasitism by native parasitoids decreased greatly with time since introduction. This was because galls became larger, more thickly walled, and with more individuals per gall as the abundance of D. kuriphilus increased. Per capita growth rate of D. kuriphilus was linearly density dependent with an apparent equilibrium of 5.2 galls/shoot. D. kuriphilus in plots with low abundance had higher population growth rates. At high abundance, the number of suitable buds for oviposition decreased because the crowns had thinned, and only current year buds were available. Population abundance was higher in low quality sites. Our studies revealed seven hybrid clones resistant to the pest as well as high variability in the susceptibility of another 24 hybrid chestnut clones. Susceptibility did not depend on insect selection since there was no differences in number of eggs per bud among resistant or susceptible genotypes, and neither on changes of plant secondary metabolites during gall developmental. An early hypersensitive response within attacked buds that received eggs seems to be the most effective defensive trait of chestnut treesagainst the gall wasp.

Helsinki, Finland, July 17-22, 2022