Aggressive tree killer or natural thinning agent? Assessing the impacts of a globally important forest insect

Invasive insects and pathogens are prominent tree mortality agents in forests around the world, and the magnitude of their impacts is increasing. Comparative studies across multiple populations can be helpful for the development of new insights and innovative management strategies. We used the Sirex woodwasp, Sirex noctilio Fabricius, as a model system to compare invasion impacts across a range of ecological contexts around the globe: native woodwasps colonizing native trees, invasive woodwasps in non-native plantation trees, and invasive woodwasps attacking native trees. Across 133 stands of eight pine species on four continents, tree mortality associated with S. noctilio attack was positively correlated with stand basal area and stand density, and was mostly confined to smaller, suppressed trees. Larger average tree size and greater distances between trees were linked to lower levels of tree mortality. To more deeply assess the impacts of tree loss due to this pest, we examined mortality in vigorous trees, defined as those with a stem diameter greater than or equal to 90% of the mean diameter for trees in the stand. Sirex noctilio- related mortality in vigorous trees was rare, with one exception where Pinus contorta stands in Argentina lost as many as 300 vigorous trees ha⁻¹. Pine species varied dramatically in their susceptibility to S. noctilio: for example, these losses in P. contorta were in stark contrast to very low mortality in P. ponderosa, the other pine species grown in Argentina. Surprisingly, location did not alter patterns in the influence of stand basal area on tree susceptibility for individual species. Most notably, Pinus radiata had the same relationship between basal area and tree mortality when grown in Spain (where S. noctilio is native and not considered a meaningful forest pest) and South Africa (where S. noctilio is a problematic invasive). Our findings suggest that the availability of optimal pine hosts is a key driver of S. noctilio-related tree mortality across continents and management regimes. Important variables that influence host availability include species-specific susceptibility and environmental and management-related factors that promote or limit the number of stressed trees present both within stands and across the regional forest or plantation landscape.