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‘Army worms’ poised to start marching soon


While they’ll be a little later than usual due to the late spring, forest tent caterpillars should be on the march here by the end of the month, Mark Breon, forest health technician with the Ministry of Natural Resources, warned Tuesday.

But whether the infestation will be as bad as last year—when the critters commonly referred to as “army worms” consumed more than 2.3 million hectares of foliage here—isn’t certain quite yet.

“Based on the egg band counts we did in the fall and winter, it appears the population’s going down this year. You may even see some areas where there’s no defoliation,” Breon said.

“We know there were more eggs laid in 2000 than 2001 so that means fewer moths to lay eggs for this year,” he noted. “It usually takes a year or two for the population to drop—hopefully the worst of it is over.”

But he noted those working in his field always are cautious about committing to any predictions on the caterpillar’s population.

“Three to six years is the average cycle but in the Hearst area to the east, they’ve been chewing the forests up for eight years now. You never really know what Mother Nature’s going do,” Breon remarked.

Fort Frances District saw the most damage of anywhere else in Northwestern Ontario last year at 2,351,938 hectares of foliage. In 2000, the worms consumed 1,832,570 ha while in 1999, they destroyed just 93,339 ha.

The region, as a whole, saw almost five times more foliage consumed than northeastern Ontario—and 30 times as much as south-central Ontario.

Larvae normally hatch out of their eggs, which were laid the previous summer, in late April or early May. Then the young larvae start eating.

The young larvae are black and hairy, about two-three mm long, and are visible about the time the buds begin to expand on their host trees. The larvae feed on the expanding leaves, shedding their skin (moulting) as they go through five growth stages.

By mid-June, the mature larvae are 4.5-5.5 cm long. They have white keyhole or footprint-shaped markings down their back, with blue bands and brown hairs on the either side of their bodies.

Larvae of the forest tent caterpillar do not spin silken tents, as their name might imply. Rather, they spin silken threads for pathways to and from their feeding sites on the trees, and they also spin silken mats on which to rest.

The eastern tent caterpillar, which prefers cherry and apple trees, does form a tent and is often confused with the forest tent caterpillar.

Once they are full-sized, the larvae spin cocoons of white silk mixed with yellow powder on trees and other vegetation as well as on fences, buildings, and other structures.

The larvae moult to the pupal stage inside the cocoon. About 10 days later, usually early July, moths emerge. The yellow-brown moths are stout-bodied, with a wingspan of 3.5-4.5 cm and two dark bands on the forewings.

The moths mate and lay eggs before dying.

Each female deposits 150-200 eggs in a single brown band 1.5-2.0 cm long that encircles a small twig. Within three weeks, a young larva forms in each egg, but it remains dormant until the following spring.

Breon noted the monitoring of the forest tent caterpillar now is being taken care of as part of a new MNR initiative called “Science North.”

Part of the service includes a toll-free number, which anyone is welcome to call at 1-866-443-4040 for more information about the forest tent caterpillar and what people can do to defend their foliage.

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