Webworms are the larval stage of a small moth, constructing silken nests in tree branches during the late summer and early fall. These pests, most commonly the fall webworm, create large, messy webs that enclose the leaves they feed upon. While an infestation can be alarming, control methods range from simple mechanical removal to targeted biological treatments. Understanding the webworm’s life cycle and the risk they pose is the first step in choosing the most effective intervention.
Identifying Webworms and Assessing Risk
Webworms are often confused with Eastern tent caterpillars, but their timing and nest location are distinct identifiers. Fall webworms build loose, dirty-looking webs at the tips of branches in late summer and autumn. The caterpillars remain inside this web to feed, filling the nest with cast skins and fecal droppings, known as frass.
In contrast, Eastern tent caterpillars appear in the spring and construct tighter webs in the crotches of branches, leaving the nest to feed. Although the damage is unsightly, it rarely kills a healthy, mature tree because feeding occurs late in the season before leaves drop. The primary risk is defoliation, which can weaken young or stressed trees, making intervention necessary for aesthetics or to protect new plantings.
Mechanical Removal Strategies
Mechanical removal is the primary way to control webworms without using chemical agents. If the webs are within easy reach, prune the infested branches several inches below the webbing to ensure the entire nest and all larvae are removed. The cut branch and web should be immediately destroyed, either by sealing it in a plastic bag or dropping it into soapy water.
For webs that are too high to prune safely, use a long pole or stick to physically tear open the silk webbing. Tearing the web exposes the caterpillars inside to natural predators, such as birds and parasitic wasps, and to the elements, which causes the colony to perish.
Selecting and Applying Targeted Treatments
Targeted insecticide treatments may be necessary when mechanical removal is impractical due to the tree’s size or the extent of the infestation. Since the thick, silken webs are protective, any spray must penetrate the web or be applied after the web has been physically broken open. Systemic insecticides are ineffective for webworms because they feed only on external foliage, not boring into the wood.
A biological control option is Bacillus thuringiensis subspecies kurstaki (Btk), a naturally occurring soil bacterium. Btk is a species-specific gut poison that affects only caterpillars and is safe for beneficial insects, birds, and mammals. For Btk to be effective, webworms must ingest the product while actively feeding, and it works best on younger larvae.
For severe infestations, contact insecticides, such as those containing carbaryl or synthetic pyrethroids, provide a rapid knockdown. These chemical sprays should be considered a last resort and must be applied strictly according to the product label directions. Regardless of the treatment chosen, the webbing must be disrupted first, or the spray must be applied with enough force to breach the silk barrier and thoroughly coat the leaves the larvae are eating.
Long-Term Prevention and Tree Health
Promoting overall tree vigor and health is essential for preventing severe webworm damage. Well-maintained trees are less susceptible to lasting harm from defoliation than those already under stress. Maintenance includes ensuring the tree receives proper moisture, especially during dry periods, and applying mulch around the base to retain soil moisture and regulate temperature.
A proactive measure targeting the webworm life cycle is the application of dormant oil in the late winter or early spring. Dormant oils work by suffocating the overwintering eggs of the webworm moth before they hatch. This treatment should be applied when the trees are dormant, typically before the buds break, and when temperatures are within the range specified on the product label.