Fall Webworm 

Robert Bishop 
Frederick County Master Gardener Program

The insect pest commonly known as fall webworm is beginning to make it's annual appearance. Look for their web-like nests at the ends of tree branches at this time of year and through October. People often confuse the fall webworm with another pest, the eastern tent caterpillar, which is active only in the spring season. Fall webworm is only considered a nuisance while the eastern tent caterpillar can; when large populations occur, cause serious problems.

Fall webworm caterpillars (larvae) produce a web of fine silk over the terminal branches of more than 100 deciduous tree species. Some favorite trees are mulberry, oak, willow, poplar, walnut, hickory, sweetgum, elm, linden, ash, and all fruit trees. The caterpillars feed only within their silk nest. Nests become quite unsightly over time as they expand and the branch tips within them become defoliated. 

Because the nests are only found on the ends of tree branches they are easily noticed. Webworm feeding cause no health danger to the tree, it only lowers the overall beauty of the tree because their nests are ugly. Remember that tree leaves are most efficient at producing food during the first few weeks after emerging in spring. As the season progresses into fall the leaves produce less and lesser food but have already stored enough food for next years leaf production and other growth. Leaf loss is less important in late summer and fall.

The caterpillars occur in two forms; black heads with yellowish white bodies; red heads with brown bodies. Both forms have black spots on their backs. Adult moths are about 3/4 inch long and have either all white wings or all white with black spots on the wings. Although there are two generations per year the early hatch in May is very small and usually matures unnoticed.

Since this pest has over 86 different predators and parasites that prey upon it and naturally control them we do not recommend chemical controls. The best approach to controlling fall webworm is to prune out the nest. Pole type pruners work well for tall trees. For people who love to spray, a biological control is also available. 

This product will not harm beneficial predators or parasites and is best used in August while the caterpillars are small. The product is a bacteria known as Bacillus thuringiensis variety kurstaki or B.T. for short. It should be available in most stores. (Read the label to be sure you are purchasing B.T. of the kurstaki variety, it works best.) As always read and follow label directions.

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