Forest Health: Forecasting Insect Outbreak Responses to Climatic Change
by Richard A. Fleming

Insects as a whole represent the dominant natural disturbance factor in Canada’s forests. During outbreaks, host trees, especially those in older stands, are often killed over extensive areas. This shifts the forest toward the smaller trees of younger age-classes that contain less biomass and thus store less carbon. One concern is that, together with climate change, insect outbreaks might even alter subsequent forest regeneration so that the original forest ecosystem does not return. Rather, it gets replaced by another with a different species mix, or possibly by an entirely different type of ecosystem (e.g., shrubs, grasses).

In general, as explained in the last issue of Canadian Silviculture, in the article, “Insects are Responding to Climatic Change”, the predicted temperature increases associated with climatic change are expected to lead directly to increases in the potential population growth rates of many forest insects. Increased population growth rates, in turn, potentially lead to expansion of insect ranges northward and upward in altitude, to longer seasons of activity, and to increased peak activity during the year.

The extent to which this potential for increased population growth rates is realized will likely depend on a number of complications. These complications include changes in the abiotic environment, changes in species interactions, and changes in the regimes of natural selection. For instance, the increasing concentrations of atmospheric CO2 constitute a potentially important change in the abiotic environment. The consequent increase in carbon:nitrogen ratios of plants is expected to cause insects to eat more in order to obtain adequate dietary nitrogen. Increases in plant biomass or carbon-based defences may compensate for this effect. For insects, the net result may be slower larval development and increased mortality.

Changes in climate are expected to affect interactions between species, because the direct effects of climate change will almost certainly differ quantitatively among the species in the complex food webs within which most insect species are embedded. The resulting changes in the relative abundances of different species would alter predator/prey, host/parasite, and plant/herbivore ratios, and thus quantitatively affect species interactions throughout food webs. Shifting species interactions and altered atmospheric chemistry make for novel environments, and hence changed regimes of natural selection with which each species must contend.

The most common approach to forecasting how insect outbreaks may respond to climate change involves analysing historical data from a certain region to reveal statistical associations between short-term climatic patterns and the frequency, duration, and extent of outbreaks. For example, colder weather has been associated with shorter outbreaks of the forest tent caterpillar in central Ontario, and less frequent outbreaks of the European pine sawfly in Finland’s boreal forest. Warm, dry summers have been associated with outbreaks of a number of other insect species in Canada’s forests (eastern hemlock looper, mountain pine beetle, western spruce budworm, jackpine budworm, and the spruce budworm). Assuming these same statistical associations hold as climate change progresses, one can infer how the characteristics for that outbreak regime might change in response to the climatic changes projected for the region. In general, this research suggests that the outbreaks of many species can be expected to occur more often, be more extensive, and/or last longer.

This does not necessarily mean that the direct economic impact of these insects will increase - some think increased tree growth will more than offset any increased losses to insects. But there are worrisome possibilities. Climate warming may allow certain insects (e.g., the mountain pine beetle) to extend their ranges into extensive, and previously geographically isolated regions containing vulnerable host species. Overall, the uncertainties associated with climate change influences on insect outbreaks will likely affect depletion forecasts, pest hazard rating procedures, and long-term planning for harvest queues and pest control requirements.

Richard Fleming works for Natural Resources Canada, Canadian Forest Service, at the Great Lakes Forestry Centre. He can be reached at rfleming@nrcan.gc.ca.