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United States Department of Agriculture

Forest Service Northeastern Area

NA-FB/P-26

BRUCE SPANWORM

In the northern part of USA and in Canada, the cater- pillars of Bruce spanworm, Operophtera bruceata (Hulst), attack a variety of hardwood trees, but sugar maple, aspen, and beech are the favored host trees. In areas where these favored host trees are numerous, moderate to severe defoliation will occur. For the past two years, the insect has caused widespread defolia- tion in the New England States. Several hundred thou- sand acres have been defoliated in 1982/1983 with 330,000 acres in Maine alone. Usually, outbreaks last about two years before the population collapses.

The caterpillars can be seen on the unfolding leaves in the spring. They feed from the underside of the leaves, eating everything but the veins.

The full grown caterpillars are about % inch long, bright green, with three narrow yellowish-white stripes on each side of the body. They closely resemble fall cankerworms. Both species have two pairs of well developed prolegs, but the fall cankerworm has a par- tially developed third pair of prolegs. The spring canker- worm also has two pairs of prolegs, but the color is light brown to nearly black.

The caterpillars feed for four to five weeks but no later than the third week in June and then drop to the ground to pupate. The adult moths emerge in late Oc- tober to early November. The males are light brown and have well developed wings.

The females are light brownish grey and are wingless. The females climb the trees to mate and lay eggs. The pale green eggs are laid singly in the crevices of the bark and on larger branches. The color of the eggs changes to orange during the winter.

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Adult male Photo credit: Ronald Kelley, Vermont Dept. Forests and Parks

Caterpillar

Report damage to:

Parker Snowden USDA Forest Service P.O. Box 640 Durham, NH 03824