Case history of population change in a Bacillus thuringiensis-treated vs. an untreated outbreak of the western spruce budworm.

Abstract

Larval densities of the western spruce budworm (Choristoneura occidentalis Freeman) were monitored for 12 years (1984-95) on permanent sample plots in northeastern Oregon. The time series spanned a period of general budworm infestations when populations increased rapidly from low densities, plateaued for a time at high-outbreak densities, and then declined suddenly. Midway through the period (1988), an area with half of the sample plots was sprayed with the microbial insecticide Bacillus thuringiensis (B.t.) in an operational suppression project. The other sample plots were part of an untreated area. In the treated area, B.t. spray reduced numbers of larvae by more than 90 percent; however, populations returned to an outbreak density within 3 years. In the untreated area, populations remained at outbreak densities and continued to fluctuate due to natural feedback processes. Natural decline of the population (1992-95) in the monitored area was largely unexplained and coincided with an overall collapse of the budworm outbreak in the Blue Mountains.

Citation

Mason, R.R.; Paul, H.G. 1996. Case history of population change in a Bacillus thuringiensis-treated vs. an untreated outbreak of the western spruce budworm. Res. Note. PNW-RN-521. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 12 p