The unsightly web you may have noticed lately on the ends of the branches of many landscape trees is formed by the Fall Web Moth.
This native pest is often discovered in late summer, when it builds its nests out of webbed leaves and small twigs. The fall moth is widespread throughout the United States and has been documented feeding on more than 85 species of trees.
Which trees are affected by the webworm?
Preferred host plants are pecan, walnut, American elm, hickory, fruit trees, and some maple species. Ornamental plants such as birch, lilac, wild apple, cherry, and Judas tree are also common host plants.
The life of the autumn spider moth begins as small, yellow or light green eggs that are laid by the female moth in hair-covered masses on the underside of leaves.
Eggs are usually laid from May to July in a single or double layer of several hundred. Eggs hatch within two weeks and larvae begin feeding immediately. Young larvae are pale yellow with two rows of black spots along their bodies. They feed in groups by web-wrapping and skeletonizing individual leaves.
These webs begin near the tips of the branches and gradually spread down the branch toward the trunk.
As they feed, the larvae grow to 1 to 1.5 inches (2.5 to 3.8 cm) long and are covered in whitish hairs that emerge from black and orange warts. They also build additional webs, sometimes enclosing two to three feet of a branch, to feed on the foliage.
When the nest is disturbed, the caterpillars perform noticeable jerky movements in unison.
They feed for four to eight weeks, but there may be several webs on the tree and more than one generation per year. Heavy feeding will defoliate a tree but rarely kill it. Damage is mostly aesthetic, as the webs are unsightly and sometimes persist through the winter. Because feeding occurs late in the season and nests are generally concentrated in limited areas, little real damage is done.
When the larvae are fully grown, they leave the web and spin thin cocoons in which they pupate. This stage takes place just below the soil surface, under loose bark or leaf litter. The larvae remain in the cocoon over the winter and emerge from late May to July as a snow-white moth (about an inch between the wing tips). Some moths may have dark spots on their wings. The females lay their eggs on the leaves and the cycle repeats.
How can you combat bagworms?
There are many methods to control this pest, either with or without chemicals. The following methods have been shown to be effective when used alone or in combination with other methods.
If netting is accessible, cut down the nests or remove them by hand. This works best if done early in the season. After removal, the nests can be crushed or burned. Do not burn the nests while they are still in the trees, as this can cause additional damage to the tree.
Natural enemies such as wasps, paper nest wasps, birds and predatory stink bugs feed on the fall moth and help control this insect. The bacterial insecticide Bacillus thuringiensis (Bt) is also effective against fall moths when applied when the larvae are still small. Spray the product so that it covers the foliage near the nests, as Bt must be ingested to be effective.
Apply insecticides such as Sevin, Bug-B-Gone or Diazinon directly to the netting and nearby foliage. As the larvae walk around on the nest surface or feed on new foliage, they come into contact with the insecticide.
It is not necessary to drive insecticides into the nests. If the nests are difficult to reach, use a translocated systemic insecticide such as Orthene Systemic Insect Killer applied to the soil so that it is absorbed by the roots.
P. Andrew Rideout is the UK Extension Agent for Horticulture and can be reached at [email protected].
