Long-term dynamics of lodgepole needle miner populations in central Oregon

Abstract

A 30 yr time series of larval densities of lodgepole needle miner (Coleotechnites sp. nr. milleri[Busck][Lepidoptera:Gelechiidae]) populations in central Oregon was analyzed. The series included parts of two needle miner outbreaks and an extended interval between outbreaks when populations were at low densities. Complete data sets and subsets were examined by conventional methods of population and time-series analysis to detect patterns of needle miner abundance and to determine the feedback structure of the series. A 141 yr tree-ring chronology also was constructed for the same area from which the regional history of needle miner outbreaks was determined. Analyses revealed two kinds of concurrent cycles: 8 to 10 yr oscillations of needle miner densities apparently caused by time delays in the regulatory (density-dependent) feedback effect of natural enemies, and recurrent outbreaks at 20 to 25 yr dependent on overall recovery rate of the population between outbreaks and the degree of tree susceptibility to infestation. In general, populations fluctuated around an equilibrium determined by favorability of the local environment for population increase. The equilibrium density was many times higher for a needle miner population on a site with high-risk trees than for populations elsewhere. It is hypothesized that accelerated thinning and harvesting of lodgepole pine and stand-replacing outbreaks of mountain pine beetle (Dendroctonus ponderosae Hopkins) between 1970 and 1990 increased the overall resistance of lodgepole stands to infestation and prevented an isolated outbreak in the 1990s from being more widespread.