The Richmond Register

September 3, 2013

Fall webworm showing up on trees

By Amanda Sears
Extension Agent

RICHMOND — There have been several calls into the office about the webbing that is showing up on trees.

The culprit behind this is a caterpillar called fall webworm.

Fall webworm caterpillars build light gray silk tents that contain the ends of branches of trees and shrubs. These large, conspicuous webs contain caterpillars, dead or partially eaten leaves, and lots of small black droppings.

The fall webworm is about an inch long, is very hairy and is pale green or yellow. They have either a red or black head.

The blackheaded larvae have black spots along the back while the redheaded have orange to reddish spots. The blackheaded larva will create a flimsy web while the redheaded larva makes a larger, more dense web.

The caterpillars overwinter as pupae in cocoons that are hidden either in the bark of trees or in the soil. The moths emerge from mid-March to mid-late April. They are satin white with brown wing spots.

After mating, the female moth lays her eggs on the undersides of leaves in masses of 200-500. This first generation of caterpillars start to feed sometime in mid spring to early summer. A second generation of webworms appears in August or September. This second generation causes more defoliation than the first.

This insect can feed on over 90 species of deciduous trees, including hickory, walnut, birch, black cherry, crab apple and mulberry.

Fall webworm larvae incorporate the leaves they are eating into their tent which is enlarged to include more foliage as the caterpillars grow. They can be numerous enough to completely defoliate small trees and shrubs, but this is not common. Usually, little real damage is done to trees but the ugly webs and brown are unsightly.

Accessible nests can be pruned out and discarded, if practical.

Insecticides containing Bacillus thuringiensis (Bt), spinosad, Sevin, or pyrethroids can be used if chemical control is necessary.

If an insecticide application is made, it should be made when the larvae are small and easiest to control, not when they are full grown and have already done their damage.

The insecticide should be applied in the evening or early morning when the insects are in the nest. A high pressure spray may be needed in order to get the insecticide into the tents.

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