Spatial variability in generalist predation on the Douglas-fir tussock moth

Abstract

The Douglas-fir tussock moth (DFTM) is a forest defoliating insect that is subject to periodic population outbreaks. These outbreaks are sometimes spatially synchronized across hundreds of kilometers. The DFTM’s complex population dynamics are thought to result primarily from two regimes of population control: at outbreak-level population densities, DFTM populations are subject to control by viral infection, whereas endemic-level insect densities are maintained by generalist predators. Generalist predation delays the onset of insect outbreaks. In this paper, we show that variation in generalist predation can be modeled as a result of variability between locations and that predation is reduced when the accessibility of larvae to flying predators is restricted. However, protecting larvae from flying predators did not increase larval survival by the same amount from site to site. Thus, local effects of predation show considerable spatial variation, even within a 10-km area. The effects of this variability on the spatial synchrony of population outbreaks remain unclear.

Citation

Hovland, Charlotte; Polivka, Carlos M.; Dwyer, Greg; Skalisky, Kirsten C. 2021. Spatial variability in generalist predation on the Douglas-fir tussock moth. Res. Note. PNW-RN-582. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station.