Gall size of Dryocosmus kuriphilus limits down-regulation by native parasitoids.
The success of invasive species largely depends on the resistance offered by the native communities. Dryocosmus kuriphilus is a cynipid native from China that is a serious pest of chestnuts worldwide. This species recruits natural enemies in the areas of introduction; nevertheless, their role in regulating pest population is frequently questioned, although the causes are not clear. We sampled parasitoids from 12,525 galls collected in 192 sites in four geographic areas differing in climate and residence time of the wasp of Galicia (NW Iberian Peninsula). We also dissected 2994 galls to assess the effects of gall characteristics on parasitism level. We found a rich community of native enemies that contributed to biotic resistance against the invasive: 19 species of native parasitoids were feeding on the wasp, with mortalities ranging from 14 to 37%. Rarefaction analyses indicated that this species list accounts for most of the native enemies that attack D. kuriphilus in inland Galicia. However, percent parasitism of D. kuriphilus by native parasitoids decreased greatly with time since introduction. This was because galls became larger, more thickly walled, and with more D. kuriphilus per gall as the abundance of D. kuriphilus increased. As the galls became larger and more populated, a declining fraction of the D. kuriphilus within were parasitized by native enemies (decrease from > 25% parasitism to < 2% as gall size increased from 0.2 to 3 cm3). Consequently, native parasitoids may play a role in slowing the invasive at low densities, but this effect becomes increasing inconsequential in highly invaded areas.