The fall webworm (Hyphantria cunea) is the larval stage of a common North American moth, best known for the large, silken, tent-like nests that appear on tree branches during late summer and early fall. These gregarious caterpillars can cause significant aesthetic damage to foliage, leading to frequent public concern. However, to directly address the primary question, the fall webworm is neither poisonous nor venomous to humans or pets.
Toxicity and Safety Concerns
The fall webworm is neither poisonous (lacking internal toxins) nor venomous (it does not sting or bite). Its defense mechanism poses only a minimal risk to humans and domestic animals. The primary safety concern is a mild, temporary skin irritation caused by contact with the caterpillar’s body hairs.
These urticating hairs can cause contact dermatitis or an allergic reaction in sensitive individuals. The reaction is typically mild, resulting in a localized, itchy rash that resolves quickly. For pets, the risk is similarly low. If a dog or cat chews or swallows the caterpillar, the hairs may cause temporary irritation to the mouth or digestive tract, but ingestion is not life-threatening.
Identifying the Fall Webworm and Its Habitat
Confirming the identity of this caterpillar helps differentiate it from other species. The fall webworm larva is quite hairy, appearing pale yellow or greenish with a black head, though the body color can vary. These larvae are distinct because they live and feed entirely within their conspicuous, loosely constructed silk webs.
The web itself is a defining characteristic, often growing quite large, sometimes reaching three feet across by late summer. This messy, silken nest is found exclusively at the very ends of tree branches. This placement distinguishes it from the Eastern Tent Caterpillar, which builds a tighter, denser web earlier in the spring, typically situated in the crotch of a tree branch.
The fall webworm is a general feeder, attacking over 100 species of deciduous trees, including pecan, walnut, elm, and fruit trees. Because the infestation occurs late in the growing season, the defoliation they cause is mostly an aesthetic nuisance. The damage rarely causes long-term harm or tree death, as the tree has generally completed its active growth for the year.
Safe Removal and Control
Physical removal is the most straightforward and safest control method for homeowners concerned about tree appearance. If the web is within easy reach, the most effective technique is to prune the infested branch tip and dispose of the web and caterpillars. Webs can also be removed by tearing them open with a long stick or using a strong jet of water to dislodge the larvae.
When handling the webs or caterpillars directly, wear protective clothing, such as gloves and long sleeves. This precaution mitigates the risk of contact with the irritating hairs, preventing any potential mild rash.
Chemical control is usually unnecessary and is only recommended when the webs are small and the larvae are young. Since the damage is largely cosmetic and the caterpillars are a native species with numerous natural predators, the least interventionist approach is often best. Healthy, mature trees recover from the late-season defoliation without issue.