Fall Webworms

Earlier in the year I wrote about the tent caterpillars that are prevalent during the spring season.  Now is the season when you will start noticing another web in the trees, the fall webworm.  These insect pests produce a light grey, silky web in trees throughout the Juniata Valley in late summer and early fall.  A difference between the webworm and the tent caterpillar is how they build their nests.  Fall webworm nests enclose both leaves and small branches. The tent caterpillar’s nest are usually only found in the crotch of tree branches.  Although present every year in our area it seems to have major outbreaks every few years.

The fall webworm is a native to North America and has a territory that stretches from Canada to Mexico.  This insect is one of only a few Native American insects that have been introduced into Europe and Asia. 

The larvae of the fall webworm will feed on over 85 species of trees in the United States, including walnut, American elm, hickory, maples persimmon, sweetgum and most fruit trees.  This insect pest eats the leaves of the trees during the late growing season. There is usually little damage done to the infected trees but an infestation of webs can make an area look unsightly and if the weather during a season permits there can be multiple generations of the webworms causing significant defoliation.

During webworm infestation, the caterpillars remain inside of their web nests and when the food supply runs out they encase new foliage.  The caterpillar itself is covered with long white to yellowish tan hairs.  Male caterpillars range in color from pure white to white with a few black spots. When their nests are disturbed the caterpillars makes a very distinct jerking movement in unison. 

Webworms over winter in a pupal stage and usually stay in the ground but can be found in the remains of their old nests, under loose bark and in the leaf litter.  From May through June the adults will emerge and can lay several hundred eggs on the underside of leaves.  Once the eggs are laid the females then place a light scale covering over them.  The eggs will hatch in about a week and the small caterpillars will build a web over the leaf eating it until it looks like a skeleton of a leaf.  The caterpillar larvae mature in about six weeks following hatching and at this stage drop to the ground to pupate and then emerge into moths.

Though their nests make our JuniataValley landscape very unsightly, they do not damage most trees very significantly.  Large outbreaks occur every four to seven years and may last for two to three years and then natural control agents will greatly reduce their population and activity in the trees.

Small nests can be pruned out and easily crushed.  Do not attempt to burn or torch the nests because this may do additional damage to the tree.  There are over 80 species of parasites and predators that have been identified as controls for webworms.  Yellow jackets, paper nest wasps, birds, predatory stink bugs and parasitic flies and wasps are the most common and important control measures.  It is important that you delay destruction of wasp nests until late in August because this is when they change from carnivores to sugar feeders.  Bacillus thuringiensis (Bt) is a bacterial insecticide that is very effective against fall webworms if applied when the larvae are small.  You will want to get a Bt formulation that contains UV protectants and treat the leaves surrounding the nest thoroughly.  The caterpillars will incorporate these leaves into their web and the Bt will be ingested, killing them.  There are also commercial chemicals that can be applied to the nests and surrounding leaves but often they require several applications.  If there is a large infestation on very tall trees making spray application difficult, you can treat the trees with a translocating systemic insecticide.  This chemical treatment is applied to the soil for root uptake by the tree with eventual consumption on the leaves by the caterpillars.  Remember, it is very important when you use any insecticides, make sure to fully read the manufactures label, understand their warnings and follow their specific instructions.