Dynamic Behavior of Douglas-Fir Tussock Moth Populations in the Pacific Northwest

Abstract

Abstract Larval densities of the Douglas-fir tussock moth (Orgyia pseudotsugata [McDunnough] [Lepidoptera:Lymantriidae]) were monitored annually to construct population time series in five different forest ecosystems. The time series were generated in parts of five national forests in three geographical provinces: the northern Blue Mountains of northeastern Oregon, the southern Cascade Range of south-central Oregon, and the northern Cascade Range of north-central Washington. The five series ranged in length from 12 to 23 yr and represented the broad numerical behavior of tussock moth populations in those provinces. Time series with the most variable fluctuations were recorded in the Blue Mountains, a region with a long history of regular tussock moth outbreaks. The least variable series was in the southern Cascades, an area with no previously documented outbreaks. Characteristics of series in the Blue Mountains and the northern Cascades were significantly correlated with one another, but not with those of the series in the southern Cascades. The density-dependent structure of each time Series was examined by autocorrelation and partial autocorrelation functions, which indicated that fluctuations were quasi-periodic, even at low densities, and that the most significant autoregressive process was a second-order feedback. Each time series was fit with a linear second- order autoregressive model and a discrete nonlinear logistic model with a single time delay. The models, based on density-dependent processes associated with preceding insect generations, accounted for an average of 44.6% of the variation in the time series. The remaining 55.4% of variation was due to unknown density-independent factors related to different years in the series. The models showed that the principal dynamics of tussock moth populations are generated by direct and delayed density-dependent processes. In outbreak-prone ecosystems, like those in the Blue Mountains and the northern Cascades, the primary regulation of insect numbers is by delayed feedback processes that result in populations sometimes having extreme fluctuations in density. Conversely, in nonoutbreak ecosystems, as in the southern Cascades, feedback is more balanced between both direct and delayed processes that stabilize populations at lower densities. External factors, like weather, also may influence the numerical behavior of populations to some extent, but their specific contribution to tussock moth dynamics is largely unknown. For. Sci. 42(2):182-191.