Understanding patterns in forest tent caterpillar outbreaks
Forest tent caterpillar (Malacosoma disstria), an insect pest native to North America, has historically caused extensive defoliation of trembling aspen, oak, ash, maple and white birch. The area defoliated by forest tent caterpillar ranged from 14.3 million hectares in 2001 to 150,000 hectares in 2009. Widespread outbreaks have occurred in much of the boreal forest at intervals of 10 to 12 years and typically last 3 years or less at the stand level and up to 6 years at the landscape level, depending on natural control factors such as weather, host-parasitoid interactions and forest structure.
Trees are weakened by repeated defoliation, which makes them more susceptible to stresses such as drought or other pests. Two or more years of heavy defoliation can also result in a severe reduction in the radial growth of trees and may cause considerable branch and twig mortality. Forest tent caterpillar is one of the causes of aspen decline reported in Alberta and Ontario, and tree mortality has been shown to increase with the duration of sustained defoliation.
Population outbreaks of forest tent caterpillar have not been as widely studied as those of other cyclic insects, such as the spruce budworm and gypsy moth. However, forest tent caterpillar outbreaks represent a model system of forest insect disturbance ecology. In addition, from a timber supply perspective, the decline caused by forest tent caterpillar defoliation could have important implications for management planning. Furthermore, if climate change alters the pattern of future outbreaks, the overall health of the boreal forest in Canada could be affected, with potentially serious environmental and economic impacts.
For these reasons, researchers at Natural Resources Canada’s Canadian Forest Service have been working with collaborators to improve our understanding of the disturbance ecology of the forest tent caterpillar by examining historical records of outbreaks. This will help resource managers develop effective pest management strategies. The information will be further used in predicting the influence of climate change on outbreaks and the effects of insect outbreaks on carbon budget estimates.
Patterns of outbreaks
Records of forest insect defoliation in Ontario and Quebec have been maintained since the 1930s. The researchers used Natural Resources Canada defoliation maps from 1938 to 2002 to study the frequency, severity and return interval of forest tent caterpillar outbreaks at a broad scale, in order to better understand the processes driving these outbreaks.
The maps showed that six major outbreaks had occurred, with the infestations lasting 2 to 5 years and recurring every 7 to 11 years. The largest average intensity of defoliation occurred during the period 1951–54.
Degree of synchronization between outbreaks
The researchers also wanted to determine to what degree outbreaks were synchronized among the various regions, as well as the patterns and processes governing synchronization among populations.
The outbreaks recurred periodically and somewhat synchronously among regions of Ontario and Quebec. Three regions—northwestern Ontario, eastern Ontario/western Quebec and southeastern Quebec—showed the strongest large-scale, synchronized fluctuations. However, defoliation in the vast surrounding hinterlands tended to be infrequent and sporadic. In addition, there was one area in northeastern Ontario that stood out as having experienced persistent defoliation between 1992 and 1999.
Factors that influence outbreaks
Previous studies that analyzed data from Ontario found that outbreak cycles of forest tent caterpillar were sensitive to local climate, which can influence temporal processes governing population growth and host-parasitoid interactions.
The Canadian Forest Service researchers and their collaborators sought to determine whether topography or climatic factors had a greater influence on the synchronization of outbreaks. They hypothesized that the ability of insects to disperse in the landscape was more important, with the relatively flat topography of Ontario and Quebec allowing for greater dispersal than the mountainous regions of the west, where there has been less synchronization. Understanding this effect is important in management efforts, because it could help determine the survey range required around new infestations to accurately detect their extent.
Effects on overall health of aspen
Repeated defoliation by forest tent caterpillar may not allow trees to recover to a normal state of health, which can lead to decline. This appeared to be the case in northeastern Ontario, where an area that experienced eight consecutive years of defoliation starting in 1992 was subsequently mapped as in decline in the early 2000s.
The researchers wanted to know if this pattern of outbreak was consistent with the other populations in the insect’s range and if there was an increasing trend in outbreak severity over the entire northeastern region. They concluded that this particular population occupied a region of marginal habitat for forest tent caterpillar and that the population fluctuations followed those of the other populations only if weather and tree health at the time of the outbreak were conducive.