Defoliation by the western spruce budworm, Choristoneura occidentalis, causes a range of damage to its Douglas-fir host, from tree mortality to reduction in growth rates, depending upon duration, severity and frequency of defoliation. The structure, composition and density of stands influence both the dynamics of the insect as well as the resultant stand damage. Stands within the Interior Douglas-fir biogeoclimatic zone (IDF) are most susceptible and suffer the greatest losses due to chronic budworm activity. Within the IDF the two understory layers of multi-story and selectively harvested stands are most severely affected by the budworm, often with associated tree mortality. The budworm occurs in other biogeoclimatic zones such as the ICH but outbreaks here usually cause little damage and are of shorter duration. Information on the outbreak dynamics of western spruce budworm is critical in the long-term management of interior Douglas-fir forests. There have been eight major outbreaks since aerial mapping and detection began 100 years ago, with each outbreak lasting between 5-8 years. Each outbreak did not have the same range, and the periodicity between outbreaks varied. Within the IDF, outbreaks in the IDFxh (very dry, hot) were the most sustained followed closely by stands in the IDFdk (dry, cool). Over 2,000 Douglas-fir and non-host cores were collected and analysed and the results show periods of defoliation throughout the ring chronologies. Some periods are sustained in duration, between 6 to 9 years, and synchronous across most geographic areas. Other outbreaks are very eruptive in nature, building rapidly and lasting only a few years; yet they are severe in intensity. The number of years between outbreak periods ranged from 10 to 48 years. In the North Thompson region, the outbreaks are very distinct and are of longer duration with fairly long time intervals between outbreaks. In contrast, the region around Princeton has less distinct outbreak periods, which are of shorter duration. Outbreaks in the Okanagan are somewhat intermediate and have outbreak periods that vary both spatially and temporally. Overall, the frequency of defoliation events seems to be increasing in all sites with the period between defoliation periods decreasing in the past 50 years.
Lorraine Maclauchlan, Julie Brooks.
Maclauchlan, Lorraine E., Brooks, Julie E.. 2004. Spatial and temporal patterns of western spruce budworm outbreaks in southern British Columbia. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report