Plant neighbour identity and invasive pathogen infection affect associational resistance to an invasive gall wasp

Abstract

Theory predicts that mixed forests are more resistant to native pests than pure forests (i.e. associational resistance) because of reduced host accessibility and increased top-down control by natural enemies. Yet, whether the same mechanisms also apply to invasive pests remains to be verified. We tested the hypothesis of associational resistance against the invasive Asian chestnut gall wasp (ACGW, Dryocosmus kuriphilus) by comparing ACGW infestation rates on chestnuts (Castanea sativa) in stands varying in species composition (chestnut alone or associated with oaks, pines or ashes). We investigated the effects of reduced chestnut density and frequency in mixed stands, as well as the effect of biotic interactions between ACGW, its parasitoids and the chestnut blight disease (caused by Cryphonectria parasitica). ACGW infestation rates were significantly lower in chestnut-oak and chestnut-ash mixtures than in pure chestnut stands and chestnut-pine mixtures. Infestation rate decreased with decreasing chestnut relative proportion. The composition of native parasitoid communities emerged from galls significantly differed between pure and mixed chestnut stands, but not the species richness or abundance of parasitoids. The abundance of the introduced parasitoid Torymus sinensis was not correlated with ACGW infestation rates and was independent of stand composition. Blight symptoms modified ACGW infestation rates with taller trees being preferred when they were asymptomatic but avoided when they presented blight disease damage. Our results suggest that conservation biological control based on tree species mixtures could contribute to reducing the damage of invasive forest pests.Excepting for the first and last authors, the order of authors is alphabetical.

Impact of the Asian gall wasp Dryocosmus kuriphilus on the radial growth of the European chestnut Castanea sativa

“…Although we cannot exclude that other stand characteristics such as stem density or species mixture may influence the impact of a pest such as the ACGW (e.g. Fernandez-Conradi et al, 2018), we strongly recommend an immediate release of the antagonist since the first ACGW appearance. This will allow to avoid repeated ACGW attacks that pose a threat for chestnut coppices devoted to high-quality wood production because of the abrupt reductions in radial growth that usually increase the risk of ring-shake (Fonti & Macchioni, 2003;Fonti & Sell, 2003;Spina & Romagnoli, 2010).…”

Spatial propagation and patterns of abundance ofDryocosmus kuriphilusthroughout an invaded region

“…Jactel & Brockerhoff, 2007). Earlier observations of less damage by D. kuriphilus in stands with higher tree diversity (Fernandez‐Conradi et al., 2018; Guyot et al., 2015) also support this hypothesis.…”

Short-term Thermal Changes Affect Parasitism on Asian Chestnut Gall Wasp

“…The cold treatment did not affect the composition of the parasitoid community in any chestnut field, which is congruent with what reported by [75], who did not found differences in diversity, richness, or evenness of ACGW-associated species despite differences in their habitat. Conversely, [76] showed that native parasitoid communities emerging from galls differed between pure and mixed chestnut stands at the same altitude, even if there was no difference in their abundance.…”