The fall webworm is native to North America and Mexico. It is now considered a world-wide pest, as it has spread throughout much of Europe and Asia. (For example, it was introduced accidentally into Hungary from North America in the 1940’s.) Hosts include nearly all shade, fruit, and ornamental trees except conifers. In the USA, at least 88 species of trees are hosts for this insect, while in Europe at least 230 species are impacted. (All are not included here.) In the history of this pest, it was once thought that the fall webworm was a two-species complex. It is now thought that H. cunea has two color morphs – one black headed and one red headed. Black headed caterpillars are yellowish-white in color and red headed caterpillars are brown in color. Both are covered in long, silky gray hairs. These two color forms differ not only in the coloration of the caterpillars and the adults, but also in their behaviors. Females of the black headed morph lay eggs in a single layer mass during mid-March. Redheaded individuals lay eggs in double-layered masses in mid-April. Caterpillars may go through at least 11 molts or instars, each stage occurring within a silken web they produce over the host. When alarmed, all caterpillars in the group will move in unison in jerking motions that may be a mechanism for self-defense. Depending upon the location and climate, 1-4 generations of fall webworm can occur per year. Fall webworm adult moths lay eggs on the underside of the leaves of host plants in the spring. Adults are approximately 3/4 inch in length with either all white wings or white wings with black spots. Eggs hatch in late June or early July depending on climate. Young larvae feed together in groups on the undersides of leaves, first skeletonizing the leaf and then enveloping other leaves and eventually entire branches within their webs. Webs are typically found on the terminal ends of branches. All caterpillar activity occurs within this webbing, which becomes filled with leaf fragments, cast skins, and frass. Fully grown larvae are approximately an inch long and then wander from the webs and pupate in protected areas such as the leaf litter where they will remain for the winter. Adult fall webworm moths emerge the following spring/early summer to start the cycle over again. There may be 1-2 generations of fall webworm in Massachusetts per year.
Foliage of many deciduous trees and shrubs may be fed upon and coated in webbing. Fall webworms typically do not cause extensive damage to their hosts. Nests may be an aesthetic issue for some, as they can be large, typically found at the branch ends encircling twigs, shoots, and containing shredded leaves. Typically, this insect is mostly a visual concern in the US and does not require chemical management.
Monitor for silken webs on the branch tips which can be pruned out before they become extensively large. Webbing may first be seen on the south side of tree crowns. Webs become most obvious near the end of larval feeding and can persist for months afterwards. Larvae are feeding 3 to 4 weeks prior to webs becoming obvious. Small webs begin to appear in mid-July in Massachusetts, but are often not noticed until the fall.
If in reach, small fall webworm webs may be pruned out of trees and shrubs and destroyed. Do not set fire to H. cunea webs when they are still attached to the host plant. This may cause more damage than these caterpillars ever could.
50+ species of parasites and 36+ species of predators are known to attack fall webworm in North America. The egg parasite, Telenomus bifidus, is cited as one of the most effective at managing fall webworm populations. Two notable parasites of fall webworm caterpillars include Apanteles hyphantriae and Meteorus hyphantriae.
Abamectin (NL)
Acephate (NL)
Bifenthrin (NL)
Bacillus thuringiensis subsp. kurstaki (NL)
Carbaryl (L)
Chlorpyrifos (N)
Chromobacterium subtsugae (NL)
Cyantraniliprole (NL)
Cyfluthrin (NL)
Deltamethrin (L)
Emamectin benzoate (L)
Flonicamid + cyclaniliprole (N)
Gamma-cyhalothrin (L)
Horticultural oil (L) (eggs)
Indoxacarb (L)
Lambda-cyhalothrin (L)
Malathion (L)
Permethrin (L)
Pyrethrins+piperonyl butoxide (L)
Spinetoram + sulfoxaflor (N)
Spinosad (NL)
Tebufenozide (NL)
Active ingredients that may be applied systemically include: abamectin (injection), acephate (injection), cyantraniliprole (soil drench, soil injection), and emamectin benzoate (injection).
When used in nurseries, chlorpyrifos is for quarantine use only.
Make insecticide applications after bloom to protect pollinators. Applications at times of the day and temperatures when pollinators are less likely to be active can also reduce the risk of impacting their populations.
Note: Beginning July 1, 2022, neonicotinoid insecticides are classified as state restricted use for use on tree and shrub insect pests in Massachusetts. For more information, visit the MA Department of Agricultural Resources Pesticide Program.
