Slimy critters’ destructive cycle nears end

A feeding frenzy that has turned tree lines, windbreaks and woodlots across the West into leafless disaster zones will soon be over — at least for this year.

Greg Pohl, a forestry expert with Natural Resources Canada’s Canadian Forestry Service in Edmonton, said tent caterpillar larvae will soon be finished feeding and will enter their pupal stage.

Pupation, which normally begins in mid- to late-June, is where mature caterpillars spin themselves in silk cocoons and emerge as moths, about eight to 12 days later.

Unfortunately, there is no guarantee that a similar feeding frenzy won’t occur next year.

“We usually start to see adult moths in early- to mid-July, but they typically don’t fly all that far,” said Pohl, an insect and disease identification officer.

“They usually stay within a few hundred metres of where they grew up, so they’ll tend to be infesting the same again the next year, depending on environmental conditions.”

Infestations of tent caterpillars can last for five years or longer in any particular area, but most infestation are normally cut short by a cold snap in the spring.

Adult moths typically lay their eggs in July, about a week or two after they emerge from the cocoon.

The eggs will hatch the following spring and produce masses of tiny caterpillars just a few millimetres long.

The egg hatch normally occurs when the caterpillars’ primary food source — trembling aspen trees and poplars — are starting to leaf out in early to mid-May.

Caterpillar larvae are already quite large and nearly finished feeding by the time most people notice them, Pohl said.

However, mortalities can be huge if cold weather occurs when the larva are small or newly emerged.

“All it takes is some poor weather when the caterpillars are just hatching to completely smash an infestation and end it right there,” Pohl said.

“So if the leaves are just starting to flush and the caterpillars pop out and then we get a hard frost or even a lot of cool, wet weather during that whole period, that can kill most or just about all of those little caterpillars.”

Natural predation can also contribute to significant caterpillar mortalities.

The tent caterpillar itself is an important food source for a number of parasitic insects, the most common of which is a parasitic fly that looks like a large, bristly housefly.

“These parasites normally build up in numbers over a couple of years, so by about the third year of a big (tent caterpillar) infestation, there’s an awful lot of these parasites … and they’ll kill 99 percent of the caterpillars in the pupal stage,” Pohl said.

For this reason, the forestry service does not endorse the use of chemicals to control tent caterpillar populations because they would also kill beneficial parasitic insects.

Tree damage caused by tent caterpillars is usually temporary and largely cosmetic, added Pohl.

Bad infestations can have a negative impact on a tree’s annual wood production, but they will rarely kill a tree.

In fact, aspen and poplar trees that have been stripped of their foliage in May and early June will normally produce a second flush of leaves after the caterpillar larva have finished feeding.

“We don’t recommend using chemicals because aspen trees are very resilient to this (pest),” Pohl said.

“They’ve been surviving tent caterpillar infestations for thousands of years and they will reflush a new set of leaves after being stripped.

“It looks horrible of course … but if you use chemicals, you will also wipe out that build-up of natural enemies.”

When caterpillar populations are particularly high, tree foliage can become significantly depleted two to three weeks before larval development is complete.

When this occurs, larval migration follows and populations can suffer high mortalities because of starvation.

Aspen and poplar are the caterpillars’ preferred feed source, but they will also feed on other deciduous tree species, such as maples, birch and elms, as well as coniferous species.

“They’ll eat just about any leafy tree, and they’ll even start chewing on spruce trees and cattails if they run out of leafy trees,” Pohl said.

Land managers who want to minimize damage to trees can choose from a number of pesticides that are registered in Canada for control of tent caterpillars, although the practice is not promoted by Natural Resources Canada.

The bacterial control agent Bacillus thuringiensis (B.t.) is an effective biological insecticide. After eating vegetation treated with B.t., caterpillar larvae will stop feeding and die within five days.

The product is available at specialty garden centres.

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