Webworms are common insect pests that create distinctive silken webs on trees and shrubs. These creatures are not true worms but rather the larval stage of a specific type of moth. Though their visible webbing and defoliation can be alarming, understanding their life cycle and habits helps in managing their impact on landscape plants. They are widespread across North America and feed on a variety of deciduous trees, including fruit, nut, and ornamental species.
The Webworm Life Cycle
The life cycle of the webworm, specifically the fall webworm (Hyphantria cunea), involves a complete metamorphosis through four distinct stages: egg, larva, pupa, and adult. Female moths lay their eggs, typically on the underside of host plant leaves. Egg masses can contain hundreds of small, globular eggs, often covered with hairs from the female’s abdomen.
After about a week, the eggs hatch into the larval stage. Larvae immediately begin to construct silken webs and feed collectively within this protective enclosure. As they grow, the caterpillars molt multiple times, typically six to seven, but potentially up to eleven instars, expanding their web to encompass more foliage. The larval stage usually lasts for four to six weeks, during which they cause feeding damage.
Once the larvae are fully grown, they leave the web to enter the pupal stage. They form cocoons in sheltered locations, such as leaf litter, cracks in tree bark, or soil, where they overwinter. In spring or early summer, the adult moths emerge from these cocoons, ready to mate and continue the cycle. Depending on the region, there can be one to four generations of webworms per year, with the later generations often being the most noticeable.
Identifying Webworms and Their Webs
Recognizing webworms and their webs involves observing specific visual cues in affected trees. The most evident sign is large, silken webs that enclose leaves and branches, typically at the tips of branches. Unlike other similar pests, webworms feed and remain inside their webs, which accumulate fecal pellets, dried leaf fragments, and shed skins, giving them a messy appearance.
Caterpillars are about 1 inch long when mature, with hairy bodies that can vary in color from pale green or yellow to darker shades, sometimes with black spots or bumps. They may have either black or reddish heads. When disturbed, larvae within the web may exhibit synchronized jerky movements, which is thought to be a defense mechanism. The damage they cause is primarily defoliation, where they skeletonize leaves by consuming the tender parts and leaving the veins. Though unsightly, healthy, established trees usually recover without lasting harm, as the feeding often occurs late in the growing season.
Managing Webworm Infestations
Managing webworm infestations involves several approaches, ranging from non-chemical methods to the judicious use of insecticides. For smaller trees or accessible webs, physical removal is an effective non-chemical option. This can involve pruning affected branches and disposing of them, or using a stick or strong jet of water to dislodge webs and caterpillars within. Tearing open webs also expose the larvae to natural predators, such as birds, predatory insects, and parasitic wasps, which can help control populations.
Chemical controls can be considered for more extensive infestations or when non-chemical methods are not practical. Insecticides containing Bacillus thuringiensis (Bt) or spinosad specifically target caterpillars and are generally less harmful to beneficial insects. Other insecticides, such as carbaryl and certain pyrethroids, are effective but more toxic to a wider range of insects, so their use is typically reserved for severe cases. For best results, insecticides should be applied when webs and caterpillars are small, ensuring the product penetrates the web to reach larvae. Always consult product labels for proper application instructions and safety precautions.