Fall Webworm Outbreaks
Questions and Answers - August 2002

By : Cliff Sadof, Department of Entomology,
Purdue University

What is webbing and defoliating the shade trees?

Fall webworm caterpillars have been covering shade trees with webs throughout much of Indiana. These caterpillars are fuzzy, with black dots on their backs and can extend to about 1 1/2" by the time they complete their development. When unchecked, webs can cover an entire tree, and the caterpillars can consume all leaves in the process. To make matters worse, while these caterpillars defoliate trees, they have a nasty habit of dropping massive amounts of black fecal pellets. Fortunately, most trees can survive a single defoliation. Controls are readily available for home and woodlot owners. Effective use of these controls requires a good understanding of the biology of this pest.

What is the life cycle of the fall webworm?

Fall webworms have two generations each year and feed on a wide variety of shade trees. Wintering in cocoons in sheltered areas, the adult is a white moth that glues large clusters of eggs to the undersides of leaves in June. These eggs are almost always laid on leaves near the tips of branches. Eggs hatch into caterpillars that feed in groups. They protect themselves from predators by keeping themselves covered with webs. In mid-July, these caterpillars make cocoons in the webs. In late July, a new generation of moths comes from these cocoons and lays even more eggs on the trees. By the time the end of August rolls around, trees are just covered with webs. Feeding will continue and webs will continue to grow until mid-September.

Will the fall webworm kill my tree?

Healthy deciduous trees can easily survive losing all their leaves during the month of August. From the tree's perspective, the leaves were busily providing energy from May through July. By August, the leaves already earned their keep. Losing them in August, rather than October, has a minimal impact on tree health. However, repeated defoliation (loss of over half the tree's leaves) by fall webworms over the course of several years can weaken trees. Trees that suffer other stresses caused by drought, recent planting, or construction are less likely to survive.

Now that my tree has been defoliated, what can I do to help it survive?

Right now, in August, there is nothing you can do. Next spring, be sure the tree gets enough water to produce healthy leaves and new growth. Most trees need at least one inch of irrigation, or rain, each week. Fertilizer is not really required for established trees.

What is the best way to prevent extensive webbing?

The trick to managing this pest is to target the first generation of caterpillars. Look for early signs of webbing, and kill the caterpillars before they engulf the tree in the second generation. Small webs can simply be pruned off and destroyed. When webbing covers too much of the tree, pruning away the affected area may simply not be practical-especially if you ruin plant structure or remove most of the tree. In this case you should apply an insecticide (see below).

Can the webs be burned to control the caterpillar?

Absolutely NOT!! The fire is likely to kill the tree. It could also get out of control during drought conditions and burn down nearby homes. Many parts of the state require permits for open burning.

What controls should be used in late summer when the webs get large?

Many insecticides can be used to kill caterpillars and prevent further defoliation. The insecticides (see below) will not remove the webs from the trees. High-pressure sprays are needed to knock the webs from the trees.

Will the webworms be back next year?

Probably yes. However, there is a definite cycle to the webworm problem. A wide variety of insects and diseases feed on webworms. Eventually, these beneficial organisms will bring the webworms back to a more tolerable level. Homeowners can help them along by refraining from unnecessary insecticide use, and by using biorational pesticides when they need immediate control.

Recommended Pesticides

Homeowners can get very effective control with a new biorational material called spinosad. It is marketed under a number of labels including Fertilome Spray (used for borers, bagworms, leaf miners, and tent caterpillars) and Bulls-Eye Bioinsecticide. This material provides the added benefit of not killing many of the beneficial insects. A second biorational, the microbial insecticide Bacillus thuringiensis (e.g., Dipel or Thuricide) is effective on younger caterpillars. Traditional broad-spectrum insecticides that can provide good control include cyfluthrin (Bayer Advanced Product), permethrin (Eight), carbaryl (Sevin), and acephate (Orthene).

Story Resources

 

 

 

Fall webworm defoliation in
LaPorte County, Indiana.

 

 

 

Adult fall webworm
laying eggs on crabapple leaf.

 

 

 

Small fall webworm nest on
contorted filbert.

 

 

 

Adult fall webworm.