Droughts drive outbreak dynamics of the invasive forest woodwasp Sirex noctilio in Patagonia

JUAN CORLEY; BRIAN AUKEMA; MARÍA VICTORIA LANTSCHNER

Conferencia; Invasion ecology of forest insects in pine plantations: risks and mechanisms associated with the introduction and establishment; 2019

Invasive forest insects can cause important damage to planted and natural forests, which can result in widespread tree mortality when pest population outbreaks occur. Understanding the spatio-temporal patterns of eruptive insects can provide insights into the mechanisms driving their dynamics and help predict future responses under climate change. The aims of this study were to analyze the spatio-temporal patterns of outbreaks of the woodwasp Sirex noctilio -a major invasive pest of pines- in an invaded region of Patagonia, to assess the relative importance of abiotic and biotic factors on its population dynamics, and to identify the primary factors that influence the magnitude of outbreaks. We used tree ring and insect sampling data of more than 1000 trees to reconstruct S. noctilio outbreaks in 29 pine stands across Patagonia-Argentina over a 16-year period. We found marked spatial synchrony in S. noctilio outbreaks at a regional scale. The occurrence of S. noctilio outbreaks are triggered by an abrupt increase in drought, which likely increases the availability of susceptible host trees. The damage caused by the outbreaks is determined by stand level variables, closely related with tree stress. Rates of tree mortality decreased over time due to negative density-dependence in S. noctilio populations, likely due to the loss of suitable resources over time. Depicting mechanisms of large-scale tree mortality in ecosystems provides insights to the drivers of forest outbreaks and other factors such as responses to a changing climate.