Pests of the home & landscape

July 18, 2000
Fall Webworm

The fall webworm is a species native to the United States, and it occurs in most areas of the country. Host plant preferences vary from region to region. Johnson and Lyon (Insects that Feed on Trees and Shrubs. 1976) report that in the Western Appalachia and the Ohio valley, it seems to prefer American elm, maples or hickory. In the western United States, it is common on alder, willow, cottonwood, and fruit trees. While survey data is lacking for Mississippi, the insect is commonly seen on pecan, persimmon, sweetgum and hickory.

Description and Habits

Two races or color phases of this insect are recognized. Larvae of the blackheaded group have black heads and tubercles (small bumps on the body), while the redheaded group have red heads and tubercles. Both groups are covered with long silky hairs that arise from the tubercles. Mature larvae will measure 1 to 1 1/3 inches long. The adult moth is white and medium size with a stout body.

The insects overwinter in the pupal stage in leaf litter and the surface layers of the soil. Emergence time is different for the two color phases. The females of the blackheaded group deposit eggs as a single mass in mid-March, while the females of the redheaded group deposit most of its eggs as double layers in mid-April. The eggs are deposited on the undersides of the leaves. They hatch in about one week and the larvae feed in groups and as they grow, they construct large, loose, irregular, silk webs which cover leaves and small branches at the terminals of larger limbs. The larvae move about within the web feeding on the leaves and as more leaf surface is needed for food, webs are enlarged. When ready to pupate, they leave the webs and find protected places such as bark crevices or, for the last generation of the year, the soil surface. There are two to three generations per year. Data from the Mississippi State Entomological Museum show a difference in color patterns between the early and late emerging adults. The early emerging moths are white with black spots on the front wings and some black banding on the abdomen. Those moths which emerge in mid-April and later are solid white.

Infestations do not occur in trees in thickly wooded areas but rather on those trees that occur on the borders of forested areas or in open lawn situations. Notice the trees along highway right-of-ways. They will be heavily infested, while a tree of the same species located several yards into the woods will not have a single web.

Controls

Defoliation is the physical damage associated with this pest; however, aesthetics also play a role. Webs covering shade trees detracts from the beauty of the plant. Although these problems exist, treatments are difficult. It is hard to spray a tree without the proper equipment that generates the pressures needed for coverage and web penetration. In fact, it may not be a good idea to spray because of this fact, especially if the trees occur in residential areas. Pressure generates small droplets, small droplets mean drift, and one is never sure what the outcome of this drift may be. The pruning out of reachable webs is an alternative; however, many webs may occur in the top portion of the tree.

Given the above facts what is left? In most situations, the only thing that can be done is to maintain good tree nutrition through proper fertilization and water the tree during dry periods, if possible. If a chemical treatment is practical, Orthene Turf, Tree and Ornamental Spray7 is a registered for webworm control on ornamental trees only (Do Not Use On Pecans). Liquid Sevin, (21.3%) may be used on pecan trees. In both cases follow label directions on the product of choice for proper mixing instructions and other safety information.

James H. Jarratt
Extension Entomologist

Richard Brown
Professor

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