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Webworms, tent caterpillars and other web-making insects

Several species of caterpillars and sawflies create conspicuous silken webs on the branches of shrubs and small trees that they feed on. Inside the webs, insect colonies feed and rest and are protected from birds and other predators. As the season progresses, the webs are enlarged by the growing insects and begin to fill with their frass and shed skins.

Larger webs and their caterpillars can be removed by hand and destroyed. Caterpillars in smaller and numerous webs may require application of a systemic chemical such as acephate or dimethoate. Fortunately, many parasites, predators (such as chickadees and nuthatches) and disease organisms eventually cause sharp declines in these insect pests, thereby making chemical control unnecessary.

Eastern tent caterpillars

Time: April, May and June
Trees: apples, cherries and plums

Silken tents are constructed on branches of apples, cherries or plums beginning in late April or early May. They are enlarged as these caterpillars feed on leaves elsewhere on the tree and return to their tents when not feeding. Caterpillars spin white or yellowish parchment- like cocoons on tree trunks, fences, and other available objects in late May and June. Adult moths emerge from these cocoons in about three weeks. Eggs masses overwinter on twigs.

Caterpillars are black-headed and black-bodied with a white stripe along their backs and many brownish diagonal marks on their sides. A row of pale blue spots are also present on each side of their bodies. Fine, long, light brown hairs cover their bodies. Full grown caterpillars are 2 and 1/4 inches long.

Forest tent caterpillars

Time: May and June
Trees: aspen, basswood, birch, ash, oak

Inaccurately named, forest tent caterpillars do not form a Inaccurately named, forest tent caterpillars do not form a tent or web to house the colony of caterpillars. Instead, this species constructs silken mats on the stems of host trees where the colony rests. FTC commonly feed during May and June on trembling aspen, basswood, birch, ash, and oak but shun red maple. This insect becomes very common during outbreaks which occur at ten year intervals and last three to four years. Cottony, white cocoons are spun in late June and moths emerge two to three weeks later. Egg masses overwinter on twigs.

Caterpillars are light blue and black with white spots down their backs that look like footprints or keyholes. Long brown hairs sparsely cover their bodies. Full grown larvae are 2 and ½ inches long.

Fall Webworms

Time: July and August
Trees: All, except conifers

Webs are constructed in July and August by this species and they gradually are enlarged by the colony. All shade, fruit and ornamental trees, except conifers, serve as hosts for fall webworms. In early fall, caterpillars spin thin cocoons in the duff and overwinter as pupae. Adult moths emerge in spring and lay eggs. All larval stages ( as many as eleven) feed within the web.

Caterpillars have pale yellow bodies with a row of dark red or black spots along their backs. Long white or yellow hairs cover their bodies. Full grown larvae are 1 and ½ inches long.

Oak webworms

Time: June and July
Trees: Oaks

The webs of these caterpillars are commonly three or four inches wide and nearly two feet long by July. Red and black oaks are favored hosts in our area. Webs fill with feeding caterpillars, excrement and bits of leaves, and are the locations where pupation and moth emergence occurs in August and September. Overwintering eggs are attached to stems and trunks. Caterpillars are grayish green. Their heads and prominent neck shields are black. Full grown larvae are 3/4 of an inch long.

Ugly-nest caterpillars

Time: May through August
Trees: Cherries, hawthorn and more

These caterpillars spin a dense web around the feeding site that becomes filled with pieces of leaves and their frass. Favored hosts are roses, cherries and hawthorns. Eggs hatch in May and June and caterpillars immediately begin building the web where the colony enlarges the web during July and August until they emerge as adults in late summer. Egg masses overwinter on stems. Caterpillars are yellowish-green with shiny black heads. When fully grown, they are 3/4 of an inch long.

Pine webworm

Time: June and July
Trees: Pines

These caterpillars form conspicuous webs on terminal twigs which are heavily laden with brown excrement particles and needles. Webs are found in late June and July on jack, red, white and Scots pine. At maturity, pine webworms drop to the ground and overwinter there as pupae. Adult moths emerge in the spring and lay eggs.

Caterpillars have yellowish heads and thoracic shields with brown markings, and their bodies are yellowish brown with dark brown longitudinal stripes. Full grown larvae are 5/8 of an inch long.

Pine false webworm

Time: May, June and July
Trees: Pines

This introduced species of sawfly is very common. Favored host trees are white pines and red pines; other ornamental pines are also hosts. Sawfly larvae are active in May, June and into July. Defoliated twigs, stems and branches are coated with unsightly webbing to which much frass is attached. Older needles are stripped and the current year's growth extends prominently beyond the colony's web. Larvae have vacated webs found in mid- summer and overwinter as pupae.

Sawfly larvae have clay-yellow heads with dense, small dark brown spots. Their bodies are green with purple- reddish stripes. Full-grown larvae are ½ to 3/4 of an inch long.