Larvae leave leafless aspen in their wake

They are called aspen defoliators – and if you have been out in some of our forests this year, you may have seen the aftermath of their work.

This year, surveys done by forest personnel with Alberta Agriculture and Forestry have found two types of caterpillars, the large aspen tortrix and the forest tent caterpillar, are busy munching their way through some of the province’s forests.

The large aspen tortrix infestation extends over much of the southern and western parts of the province and the forest tent caterpillar outbreak appears to cover much of the central, eastern and northeastern areas of the province. Surveys will continue throughout the year to determine the exact extent of the infestation.

The Ugly Truth

These guys may look small but they can cover a large area in a very short period of time. While not very pretty, this is normal and occurs periodically in most forests and does not impact long-term forest health, in fact these types of insects are both important to cycle nutrients and actually play an important role in maintaining our province’s forest productivity.

Future Outlook


Large-scale outbreaks generally last for about three to five years. Typically, the caterpillars move on to other areas after one or two years.  Consecutive years of defoliation may leave affected trees more susceptible to other insects and diseases as well as lower their growth rates. In most cases, however, the trees grow their leaves back and actually benefit from the nutrients generated by the insects.

The good news is that, if history has anything to say about it, we will likely wait about another decade before another cycle of this proportion. The Alberta Agriculture and Forestry website offers information on forest health, aspen defoliators and other forest pest conditions in Alberta.

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