Identification

Fall webworm nests: are composed of silky threads wrapped around the leaves and branches encapsulating the caterpillars. The nests are loosely constructed and messy looking, occurring at the branch tips, and enlarging as food runs out. Large colonies can encapsulate an entire tree in their webbing. The caterpillars feed inside the nest (unlike tent caterpillars) and as such the nest is full of frass and dead plant material.

Fall webworm caterpillars: can range in colour from brownish-grey, yellowish, green or all black. The webworms in the northern range tend to have black heads while the ones in the southern range have red heads. Both have brown/black, wort like spots running down the length of their body, with a tuft of long hairs coming out them. They range in length from 25-40 mm (1-1.5″)

Fall webworm moths: are bright white in colour, in their northern range while in their southern range they appear bright white with dark brown/black wing spots. Their bodies are quite hairy and they have a wing span of approx. 30mm (just over and inch) sometimes a little more.

Hosts and Damage

Fall webworms feed on a wide variety of deciduous trees and shrubs such as mulberry, chokecherry, apple, cherry, poplar, trembling aspen, ash, hickory, oak, white birch and many others. For a more extensive list visit the University of Florida site Host Plants and Other Plants Affected. Damage consists of defoliation, sometimes of the entire tree or shrub. In the ornamental gardens their nests detract greatly from the beauty of the garden but by on large both in the ornament garden and the forest setting their damage does not seriously affect the health of the plants. This is because the damage takes place late in the season after the trees and shrubs have already stored up much of their food reserves (Maine Department of Agriculture 2000).

Life Cycle

In it’s northern range fall webworms have one generation per year. They overwinter as pupae in the soil and garden debris, but they sometimes also pupate in cracks and crevices in bark, wood piles, houses and cars. Around mid-June to early July the adult moths emerge and fly as far as a few kilometers away to their preferred food source, where they lay their eggs on the underside of the leaves. The adult moths die and the eggs hatch in about 2 weeks. The newly born caterpillars begin feeding and constructing a silken nest around the leaves they are feeding on, enlarging the nest as more food is required. They actively feed until September, passing through as many as 10 instar stages and several colour changes (Rodstrom R. A. and Brown J.J. 2017), then enter the soil to pupate where they remain until next June.

In their southern range there are between 2 and 4 generations per year, depending on their geographical range. For more information on dates, generations and regions visit https://www.cabi.org/isc/datasheet/28302.

Distribution

Fall webworms are native to North America and appear abundantly across the USA and southern Canada. They were accidently introduced to Europe where they quickly spread. They also occur in eastern China, Northern Korea and Japan (Sourakov A. and Paris T. 2010, rev. May 2021).

Control

Physical controls: If you catch them early enough the easiest method of control is to cut out the nest. You do this by cutting the branches at the outer parameters of the nest. I usually have a large paper yard waste bag ready to drop them into. I then burn it. Alternately you could place the nest in a large tub of warm soapy water or place in a strong plastic bag and seal it shut. If there are to many branches affected you can gather up the nests using a forked stick and dispose of them in the same manor.

Biological controls: Bacillus thuringiensis kurstaki better known as Btk is a biological insecticide that is effective against webworms and many other caterpillars. You will need to tear open the silk nest to get the spray on the caterpillars. Another biological control is Parasitoids. There are many naturally occurring species that lay their eggs in the caterpillars eventually killing them.

Chemical sprays: carbaryl, diazinon, sevin, malathion, acephate and cyonara 9.7 (active ingredient lambda-cyhalothrin) are all registered for use against fall webworms. Note: use of chemical sprays will also kill off natural predators and parasitoids.

Look-a-likes

Fall webworms are sometimes mistaken for tent caterpillars, but tent caterpillars appear in spring, where as fall webworms appear in late summer through fall. The tents of the fall webworms are loosely constructed and messy looking and appear at the ends of the branches, where as the tents of the tent caterpillars are more tightly constructed, neat looking and appear in the crotches of branches.

Fall webworms may also be confused with the ugly nest caterpillar. The ugly nest caterpillar occurs around the same time and also builds it’s tents at the ends of branches but the nests are much smaller and more tightly constructed. The ugly nest caterpillar is smaller and without the black markings running down it’s length; they are also not hairy and more yellowish in colour.

Photo Credits: all photos taken by the author unless otherwise indicated.

References:

CABI. nd. rev. Dec. 2020. Hyphantria cunea (mulberry moth). Retrieved from: https://www.cabi.org/isc/datasheet/28302#toidentity

Maine Department of Agriculture, Conservation and Forestry. April 2000. Fall Webworm Hyphantria cunea (Drury). Retrieved from: https://www.maine.gov/dacf/mfs/forest_health/insects/fall_webworm.htm

Natural Resources Canada, nd. rev. Aug. 2015. Fall Webworm. Retrieved from: https://tidcf.nrcan.gc.ca/en/insects/factsheet/8125

Rodstrom R.A., GreenWood Resources, Portland, Oregon. and Brown J.J., Department of Entomology, Washington State
University 2017. Fall Webworm Insect Pest Management in Hybrid Poplars Series. Retrieved from: http://pubs.cahnrs.wsu.edu/publications/wp-content/uploads/sites/2/publications/fs275e.pdf

Sourakov A. and Paris T. 2010, rev. May 2021. Featured Creatures Fall Webworm, University of Florida. Retrieved from: https://entnemdept.ufl.edu/creatures/trees/moths/fall_webworm.htm on Oct. 16, 2021.

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