Influence of host log age and refuge from natural enemies on colonization and survival of phoracantha semipunctata

The eucalyptus longhorned borer Phoracantha semipuncata (F.) is native to Australia and has become established in most of the regions of the world where its Eucalyptus L'Heritier host trees have been introduced. To study the population dynamics of the insect in its native range, we investigated the influence of host log age (fresh or aged), protection from ant predation, and protection from parasitism by specialist larval parasitoids on numbers of initial larval feeding galleries, larval mortality, larval survivorship, and parasitization. There were greater numbers of initial larval galleries when ants were excluded, and greater numbers on fresh logs than on aged logs. However, mortality of larvae was greater in fresh logs, as well as when larvae were exposed to parasitism. Survivorship was greater when ants and parasitoids were excluded. Larvae achieved a larger size in fresh logs and in logs protected from parasitism. Fresh host material may provide a fitness advantage because it is a higher quality resource, but greater infestation of that material also increases the probability of mortality from competition among larvae. The results suggest that there are potential ecological interactions among food quality, host resistance, natural enemies, and competition that must be considered in evaluating the population dynamics of this species in native and novel environments.