Spatial propagation and patterns of abundance of Dryocosmus kuriphilus throughout an invaded region

Abstract

AbstractDryocosmus kuriphilus (Hymenoptera: Cynipidae) was first detected in Galicia (NW Spain) in 2014. The wasp then began to spread by stratified dispersal, with the local abundance increasing thereafter. In this study, we analysed location data on wasp‐attacked trees, with the following aims: (i) to determine the relative importance of short distance dispersal (SDD) and long‐distance dispersal (LDD) on the geographical expansion of the wasp; and (ii) to assess whether the spatial clusters of areas with high and low numbers of attacked trees correspond to areas with different infestation levels and thus whether these areas can be considered suitable proxies for pest abundance. A Random Forest algorithm was used to identify the factors that best discriminate hotspot and coldspot areas. Distance matrix analysis was used considering a conservative expansion rate of the population front of 14 km/year as the threshold for distinguishing SDD and LDD events. Gradients of attacked tree abundance were delineated using the Getis‐Ord Gi* statistic. Although LDD events represented a small proportion of the locally dispersing offspring, they determined the rate of spread across the landscape. The infestation level differed significantly between hotspot and coldspot areas, which were therefore used as proxies for D. kuriphilus population abundance. Factors that were favourable or unfavourable in relation to population abundance were mainly associated with maximum temperatures in the summer months. Metrics of composition and configuration of host plant habitat proved less important than the time since invasion and the effect was partly masked by tree diversity at community scale.