GardenLine | Common Saskatchewan Pests | Forest Tent Caterpillar

Bonnie Willie

Description:

The adult moths are a light yellow to buff brown in colour with 2 oblique dark bands on their forewings and a wingspan of 35-45 m. They are quite short lived with an expected adult life span of approximately 2 to 10 days: just long enough to mate and lay eggs. The eggs are grey and laid in honey-comb like bands of 150-200 eggs around small twigs of host trees in the mid-summer. After the eggs are laid, the female secretes spumaline, a shiny dark-grey foam-like material over the egg mass.

The larvae are misnamed as they do not form a tent, but they do spin a trail of silk wherever they go. The mature larvae are about 45mm in length and are covered in fine, light coloured, silky hairs. The most common species of the forest tent caterpillar found on the prairies has blue sides; a black back with a narrow white strip down the centre and irregular orange markings.

Life Cycle:

The forest tent caterpillar, has one generation per year. The larvae hatch in early spring (mid-May) approximately the same time as aspens leaf out. The larvae (caterpillar stage) goes through five instars. In mid to late June the caterpillars will migrate and swarm and search of food or cocooning sites. They reach maturity by mid July and pupate between leaves. The adult moth emerges 10 days later. The female will mate, lay eggs and die. The embryo develops within the egg into the first instar larva, overwintering in the egg bands until hatching the following spring.

Hosts: Trembling aspen (Populus tremuloides) is the tent caterpillars preferred host, although the caterpillars will attack a wide variety of deciduous trees and shrubs. Under high population conditions, the larvae will devour almost any green foliage in their path; even spruce and tamarack will be attacked in wooded areas where preferred hosts are not present.

Control:

Biological controls

There are at least 40 different species of parasites known to attack the forest tent caterpillar, the most important being the flesh fly (Sarcophaga aldrichi, Park.), which resembles large house fly. The flesh fly deposits a living larvae on to the cocoons of the pupating forest tent caterpillars. In an urban situation, where only a few trees are of concern, the egg-bands can be slit, removed by hand and then burnt.

In June, when the caterpillars are migrating, a collar may be placed around the base of trees requiring protection.

The bacteria, Bacillus thuringiensis, Berliner var. Kurstaki, is used as an organic insecticide. It is sprayed onto the foliage of host plants, which are later ingested by the caterpillar. The toxin produced by the bacterium destroys the digestive tract of larvae and usually causes death within a few days after ingestion. The bacteria may be sold under the trade names Thuricide or Dipel.

Chemical control

Products registered for the chemical control of the forest tent caterpillar include methoxychlor, malathion, diazinon, carbaryl, resmethrin and permethrin. Spraying the insecticide at night will allow for a better control as the caterpillars return to their nesting site at night.

Whatever product you choose for control, insure that the product is safe to use on the infested plant. Always follow label directions carefully and use proper safety precautions that are specified by the label.

Sources:

Insect Control Notes: Forest Tent Caterpillar; Sask. Agriculture Plant Industry Branch; Crop Protection Section (Ornamentals 275/627); Clarence C. Peters; February 1979

The Forest Tent Caterpillar; Canadian Forestry Service: Fact Sheet; September 1975; Ottawa.

Forest Tent Caterpillar; Northern Forest Research Centre; Edmonton, Alta.; V. Hildahl; 1977.

Tree and Shrub Insects of the Prairie Provinces; Information Report NOR-X-292; Northern Forestry Centre; W.G.H. Ives and H.R. Wong.

Effective Tent Caterpillar Control; Agriculture Canada, Plant Industry Directorate Pest Note 7; March 1993.

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Sustainable horticultural information, offered free of charge to the public with the support of the University of Saskatchewan Extension Division, the Department of Plant Sciences and the Provincial Government.