Response of specialist and generalist predators to nonprogressive annual fluctuations in herbivorous insect populations

Abstract

To control sawfly populations that cause a forest management problem due to feeding damage, it is necessary to understand the mechanism of eruptive outbreak in erratic intervals by clarifying the responses of natural enemies to the host population fluctuations. In this study, we investigated annual beech sawfly population fluctuations and its predator, ichneumonid parasitoid wasps (as specialists) and predatory soldier beetles (as generalists). In addition, yearly responses of predators to the host emerging patterns were examined using structural equation modeling. The results revealed that ichneumonid wasps showed a non-delayed synchronous response to erratic fluctuation of the host population, resulting in the maintenance of a constant parasitism ratio. In contrast, soldier beetle population, which tends to moderately fluctuate while maintaining a specific density, showed a low-intensity delayed response without significance in the model. These predator populations without strong delayed density-dependent response are quite unlikely to highly densify causing mass mortality to the host population in the both the current and following years of outbreak. In places where eruptive outbreaks of sawfly species repeatedly occur in erratic intervals, sawfly populations are possibly maintained in high density due to the low mortality ratio caused by predators.