Fall webworm

Distribution

Canada

Micro-habitat(s)

Leaf

Damage, symptoms and biology

The fall webworm is easily detected by the presence of numerous silk webs on crown branches in summer. The larvae, which feed on foliage throughout their development, secrete silk which they spin into small webs. As they grow, they enlarge the webs, which can sometimes enclose the entire tree. Even severe infestations have little impact on trees because the damage occurs near the end of the annual growing season. However, the trees may be weakened, making them more susceptible to attacks by other insects or diseases.

The fall webworm has only one generation per year in Canada, the northernmost part of its range. It overwinters in the pupal stage in cocoons and transforms into an adult in the spring. The female deposits her eggs on the underside of leaves and covers them with white hairs from her abdomen.

Life cycle (East of the Rockies)

Other information

The fall webworm is one of the few species native to America to have been accidentally introduced into Europe and Asia. It attacks over 100 species of trees on our continent. Fall webworm outbreaks occur more frequently in eastern and central North America. Between 1939 and 1975, four infestations affecting every eastern Canadian province except Newfoundland have been recorded.

To reduce fall webworm populations on ornamental trees, cut out and burn all branches and leaves on which nests are found when the larvae are still young.

Main host(s)

Apples, balsam poplar, choke cherry, pin cherry, speckled alder, trembling aspen, white ash, white birch, white elm, willow