Attack rates ofSirex noctilioand patterns of pine tree defenses and mortality in northern Patagonia

Abstract

AbstractAccidental and intentional global movement of species has increased the frequency of novel plant–insect interactions. In Patagonia, the European woodwasp,Sirex noctilio, has invaded commercial plantations of North American pines. We compared the patterns of resin defenses andS. noctilio-caused mortality at two mixed-species forests near San Carlos de Bariloche, Argentina. We observed lower levels of resin flow and higher levels of mortality inPinus contortacompared withPinus ponderosa. In general,S. noctilioattacked trees with lower resin compared with neighboring trees. Resin production inP. ponderosawas not related to growth rates, but forP. contorta, slower growing trees produced less resin than faster growing conspecifics. For all infested trees, attack density and number of drills (ovipositor probes) per attack did not vary with resin production. Most attacks resulted in one or two drills. Attack rates and drills/attack were basically uniform across the bole of the tree except for a decrease in both drills/attack and attack density in the upper portion of the crown, and an increase in the attack density for the bottom 10% of the tree. Planted pines in Patagonia grow faster than their counterparts in North America, and produce less resin, consistent with the growth-differentiation balance hypothesis. Limited resin defenses may help to explain the high susceptibility ofP. contortato woodwasps in Patagonia.