Forest Tent Caterpillar

Invasion of the Worms!

By Doug Collicutt

(Click links for more images.)

Article Updates

2017 Update: Forest Tent Caterpillars (FTC's) are again wreaking havoc in parts of southern Manitoba. The City of Winnipeg sprayed various areas with the biological agent Bt. In south Whiteshell, where I had predicted a major outbreak this spring, infestation has proved to be less than expected. There are lots of individual stripped aspen trees, but it looks like, as of June 8, that there will not be a major defoliation event this year. There's already lots of Flesh Flies around suggesting that FTC numbers have likely peaked and will decline in the coming years. But we'll see what happens in 2018!?

2016 Update: The City of Winnipeg is experiencing a significant outbreak of Forest Tent Caterpillars this spring. It's nothing like the defoliation that Whiteshell Park and other regions in the southeast had to deal with in 2000, but it's still enough "worms" crawling about to make woosy Winnipeggers whine.

However, on a recent trip to the Whiteshell I noticed pretty significant numbers of FTCs, similar to the numbers that were around in 1999, the year before the HUGE outbreak in 2000. Could cottagers and campers be in for another complete defoliation in 2017 like we saw that year? My guess is, yes, we are likely on track for another amazing entomological event. We'll see what happens next spring!

2013 Update: Back in 2000 Forest Tent Caterpillar numbers spiked, prodigiously, in Manitoba's Whiteshell Provincial Park. Forests were denuded of leaves and the park was nearly emptied of people, too. The local cottagers association fought with Manitoba Conservation demanding something be done. Conservation argued, rightly, that there was nothing that needed to be done; that this was a natural process and caterpillar numbers would fall of their own accord next year.

In 2001 the cottager's association organized a localized aerial spraying with Bt insecticide, but another infestation failed to materialize, as had been foretold by provincial entomologists. In the process the association angered a lot of cottagers with heavy-handed tactics at fund raising for the spraying program and a refusal to accept the advice of entomologists. Now, some 12 years later, we may soon find ourselves facing a plague of "worms" again in the near future, so it's worthwhile information to review. (Doug C.)

As a Whiteshell Park cottager, I was pleased to hear that Manitoba Conservation WON'T be doing anything about the recent population explosion of forest tent caterpillars. At least the notion of aerial spraying with Bt has been squashed for 2001, so far. And so it should be. The Whiteshell and other provincial parks are, after all, natural areas and we should let nature take its course wherever and whenever its reasonable to do so.

Caterpillars swarming the cottage.

I experienced the full brunt of the tent caterpillar invasion in 2000. In our part of the south Whiteshell, the defoliation was complete. By mid-June there was barely a shred of leaf left on any of the deciduous trees and shrubs. Even the blueberry plants were stripped bare. It was rather eerie to walk about in a forest with no leaves, but with bright green grass and ferns covering the ground. Oh yeah, there were actually two kinds of plants that the caterpillars wouldn't eat, our native maples: the Manitoba maple (Acer negundo), a tree, and mountain or white maple (A. spicatum), a tall shrub. They nibbled on the white maple when all else was gone, but they wouldn't touch the Manitoba maple.

Caterpillars on hazel leaves.

At its peak in June the mass movement of caterpillars was astounding; it was as though the grass around our cottage was alive. You couldn't walk anywhere without squooshing caterpillars. We kept a broom by both doors so we could sweep them out of the way as we left or entered. As a biologist I was quite enthralled by the whole process. It was one of the most amazing biological spectacles I've ever witnessed. OK, so it wasn't quite "herds of wildebeest streaming across the Serengeti", but it was still pretty cool. And at worst it was just a minor nuisance. It actually improved the early summer for me, because it kept so many people away!

The other fascinating aspect of the summer of 2000 was how quickly the forest trees rebounded. By mid July you couldn't tell that anything had happened, other than the cocoons everywhere. The trees and other shrubs had re-leafed fully. The infestation will certainly have effects on the other local wildlife, particularly other leaf eating insects, but these will be transient and it's nothing that all these species haven't had to deal with before. In another year or two the caterpillars will be gone again and people will have to find something else to complain about. Hey, remember "personal water craft" and 200 horsepower outboards!

Adult moths attracted to a light on the cottage.

Anyway, here's what Manitoba Conservation has to say about forest tent caterpillars. At Nature North we're happy to help "spread the biological word".

Carry on for Caterpillars, Caterpillars, Caterpillars!


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