One of the few leaf-chewing insects endemic to North America is the fall webworm (Hyphantria cunea). As the name implies, this small caterpillar causes most injury in late summer in northern reaches, and throughout the year in the South. The larvae are known to feed on a wide range of species, nearly 100 in all, including virtually all popular shade trees and many commercial fruit and nut species. It is one of a few species that has invaded Europe and Asia, and although it does not appear to have spread to Australia and New Zealand, both countries have developed strict biosecurity guidelines to ensure that the fall webworm remains offshore. The fall webworm arrived on the Eurasian continent sometime before 1950, and it has since established itself as a significant pest.
Like virtually all larva-forming insects, the webworm has four distinct life stages: egg, larva, pupa and adult. Females are known to lay up to 500 eggs at one time, and depending on climate, there can be multiple generations—up to four—in a single year. Although there have been attempts to characterize fall webworm as the activities of more than one species, current thinking suggests there is only one species, but at least two varieties.
Adults are relatively small in comparison to other foliage-chewing insects, with a wingspan of less than 1.5 inches. They are usually white in color, but some also exhibit forewings with small dark spots. Females are almost always white, but there are regional and varietal differences. Eggs are laid in light green masses, often lightly coated with hairs from the female’s backside. Although most egg masses are placed on the underside of leaves, some will also lay eggs on the top of leaves. Depending on weather conditions, eggs will hatch within a week to 10 days.
Larvae begin forming webs almost immediately after they begin feeding, probably as a protective mechanism. As a larva increases its feeding area, the web is expanded to include more leaves. Since larvae are gregarious, impacts tend to be focused in specific areas of the crown. However, when infestations are severe, an entire tree crown will take on the appearance of one giant web.
Larval colors vary from black to reddish headed with tan to off-white body colors, the differences most often associated with varieties. Both varieties show larva body colors that darken during the larval phase, and both have lightly hair-covered bodies. When fully developed, larvae are usually no more than 1 to 1.5 inches in length. Feeding is completed in six to eight weeks after which larvae drop to the ground and pupate. Depending on the season, the length of pupation varies from a few weeks to many months, especially in northern areas where the webworm overwinters as a pupa.
In northern regions, the fall webworm is more nuisance than pest, since defoliation is usually concentrated to portions of the crown, and populations are highly susceptible to climatic conditions that can cause collapse. In the South, where the insect has a high fecundity with up to four generations per year, the webworm is a pest to be reckoned with. Damage is caused by defoliation that can weaken a tree, leaving it susceptible to other stress factors. In landscapes, the extensive webbing and tattered foliage is unsightly and often disturbing to homeowners. Although the fall webworm is only rarely the direct cause of mortality, it is often a contributing factor when trees are stressed by other factors simultaneously; factors such as drought, vascular diseases, root injuries and/or severe climatic conditions.
In most landscape settings, the fall webworm is rarely a worrisome pest since population explosions are always followed by a rapid collapse. Natural control methods include parasites of egg masses, larva and pupae; larva predators (such as birds, reptiles and small mammals), and temperature extremes, both high and low, have been known to kill larvae, especially earlier instars.
For homeowners who do not want to live with fall webworm in their landscape, there are other control measures. Carbaryl is a well-known and generally accepted pesticide sold under the trade name Sevin. It is fairly inexpensive, easy to use and highly effective, especially in large populations and during the early instars (before larvae become more than half their adult size). There are, however, risks with nontarget insects, especially bees, so the conditions at the time of application, and the location of active hives, are factors that must be considered. Also, carbaryl is only effective against larvae and, to a lesser extent, adults; the egg and pupa stages of virtually all larvae-forming insects are nearly impossible to control with chemicals.
Bacillus thuringiensis (Bt) is another well-known and battle-tested biological control agent that is effective when used on exploding webworm populations. It is not, however, a contact pesticide; the larva must ingest it to become infected. For this reason, application timing is crucial. Bt must be on the foliage before the insect begins feeding, and rainfall following an application can remove it altogether.
The good news is fall webworm infestations rarely cause mortality and only infrequently contribute to other factors that, combined, can cause trees to die. And, the insect is very easy to control, leaving homeowners with a sense of having saved their trees when the webs disappear.
The author is a professor and extension forester with the Rubenstein School of Environment and Natural Resources at the University of Vermont.