The spruce budworm (Choristoneura fumiferana) is a destructive forest pest. The caterpillars of these moths in the family Tortricidae feed on balsam fir, white spruce, red spruce, and black spruce. During an outbreak in the mid-1970s, which lasted 15-20 years, they defoliated nearly 52 million hectares of boreal forest in eastern Canada, and a more recent outbreak in Quebec that began in 2006 occupies 6.3 million hectares and is still growing.
For more than a century, entomologists have been trying to figure out what triggers these population increases, and what slows them down. Factors under consideration have included forestry practices, weather conditions, resource availability, and the number of natural enemies like birds and parasitoid wasps.
Although hundreds of papers have been published on this topic, there is still no general consensus on what drives the population oscillations. However, the authors of an article published in Environmental Entomology have attempted to make some sense of it by reviewing the literature and the various hypotheses.
Since these insects are difficult to study when their populations are low, they found that there is not much field data about how populations go from low densities to high ones. However, progress has been made concerning the peaks and the declining phases of spruce budworm outbreaks, and they appear to be driven by the number of predators that feed on them.
“Control by natural enemies and, to a lesser extent, declines in resource availability appear to be responsible for the collapse of outbreaks,” according to the authors. “Control by natural enemies is the primary agent keeping populations at low levels between outbreaks.”
That leaves a lot left to learn.
“There are still many data gaps, particularly in the low-density to rising phase of outbreaks, and these need to be filled before we can be certain of the key processes driving and shaping the dynamics of budworm outbreaks,” the authors concluded.
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