10 Neat Things About Forest Tent Caterpillars
by Dorothy Dobbie
by Dorothy Dobbie

The Local Gardener magazines, Ontario Gardener, Manitoba Gardener and Alberta Gardener, are published by Pegasus Publications Inc.

Drawing on her 30 years' experience as a senior executive in the magazine publishing industry, Dorothy launched Manitoba Gardener in 1998, initially running the business out of her home. Two years later, Dorothy's daughter Shauna, living in Ontario, jumped into the fray with Ontario Gardener. And two years after that, they started Alberta Gardener. Visit us at and register for our "Ten Neat things" newsletter. Watch Shaw TV for garden tips and Listen to CJOB for the Gardener Sundays at 9:08

April 10, 2017

1. Cuddle up a little closer.

While we don't know for sure what stimulates an outbreak of forest tent caterpillars, we do know that they are social animals, clustering together for body heat, which speeds up metabolism and growth. Getting though their instars quicker appears to be a protection strategy against predators such as birds.

2. The caterpillars that stay together.

When a nest of 100 to 300 eggs hatch, the resulting larvae stay together and feed together. Feeding together also seems to stimulate growth. If one starves, they all starve, very few going off on their own to forage for new sources of food. Larvae (caterpillars) from more than one egg mass often join together, prompting some to call them army worms for the way they march together.

3. Night time gluttons.

The forest tent caterpillars feed at night, resting on their silken sheets during the day unless they are on the march for new food on the next tree. They don't build tents but instead lay down a silken mat on large tree branches or trunks where they retire to rest from feeding. As they move from resting mat to feeding site, they lay down silken pheromone-laden strands of silk which help them stay on the tree and also to find their way.

4. Two months of living it up, 10 months of sleeping.

Their active lifespan is short. The eggs hatch in mid-May to early June depending on the weather. The larva feed for five to six weeks, gradually getting larger and larger until they reach about two inches (50 cm) long. They then pupate in a pale yellow cocoon which the caterpillar spins over a 24-hour period and they stay there for the next three weeks. Then the adult moth emerges and is in a hurry to mate and lay eggs for the next generation, as this stage lasts only about five days. In about three weeks, egg masses hatch into "pharate" larvae (meaning they partly change to the next stage as larvae) but stay in that stage for the next 10 months, until ready to emerge in mid-May.

5. Looking for love.

Once the caterpillar has reached its final size, it hastens to build a cocoon, where it can pupate for the next three weeks, metamorphosing into an adult moth. The moths are brown and furry with pretty feathery antennae and they also are nocturnal, setting out about 5:30 in the evening on their search for a suitable female. She, in turn, sends out pheromone signals, and when a male finds her and connects they make the most of it - the act of copulation lasts 202 minutes, quite a long time.

6. The conjugal band.

Once mating is complete, the female winds her abdomen around the nearest twig, usually in the higher branches of the tree, and deposits her eggs in an organized band around the twig. This, she covers with a sticky, frothy substance called spumaline. It hardens to a shiny black band.

7. Too cold, too hot.

The forest tent caterpillar eggs are vulnerable to extremes of temperature. If temperatures in winter drop to -41 C or F, the half-formed animals within the egg masses die off. If the temperatures at mid-summer exceed 100 F (38 C), the viability of the eggs is severely compromised.

8. Black and blue, white and orange.

The caterpillar, taken singularly, is not unattractive, with a black body stamped with short orange stripes. Its sides are bordered by blue dashes separated by white dots, and the whole body is covered in black hairs.

9. Oh, the trembling aspen.

Actually, almost any hardwood would do, depending on where the infestation takes place. They do like poplar (trembling aspen), but they also like fruit trees, oaks, maples.

10. The rest.

Infestations occur every decade or so and last anywhere from two to six years. You can spray your trees with Bti, but trees generally refoliate with 30 days and can recover quite well if the infestation doesn't last too long. Be patient. The lives of this insect on the move are short lived and soon pass.

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