The presence of silken webs in a pecan tree signals an infestation by the fall webworm (Hyphantria cunea), a common caterpillar pest that feeds on the tree’s foliage. While a healthy, mature pecan tree can typically withstand a light infestation without permanent damage, heavy defoliation can reduce the quality of the current year’s nuts and weaken the tree, making it susceptible to other issues. Eliminating these pests requires understanding their behavior and using targeted removal methods, including physical destruction, biological, and chemical applications.
Identifying the Pest and Optimal Timing for Treatment
The fall webworm is easily identified by the large, loose, silken webs it constructs, typically found on the tips of the branches. The webs enclose the leaves and twigs where the larvae feed, providing protection from natural predators and the elements. Inside the web are numerous larvae, which are pale green or yellow, approximately one inch long when fully grown, and covered with long, silky hairs.
Webworms can have two to four generations per year, but the last generation, appearing in late summer and early fall, usually causes the most noticeable damage. Control is most effective when intervening while the webs and larvae are small. Early action ensures the caterpillars have not had time to cause extensive defoliation or expand their protective webbing. Scouting for newly formed webs should begin in mid-summer and continue into the fall.
Physical Removal and Cultural Practices
For accessible nests, physical removal offers an immediate and non-chemical solution. This method is practical for smaller trees or for webs located on the lower branches of larger pecan trees. The most direct approach involves pruning the infested branch, cutting it off just below the webbing.
Pruned branches must be disposed of properly, such as by sealing them in a bag or destroying them, to prevent the larvae from crawling back into the tree. If pruning is not an option, the webs can be mechanically torn open using a long pole, a rake, or a strong stream of water. Breaking the silk web exposes the larvae to natural enemies like birds and parasitic wasps, significantly increasing their mortality.
Beyond direct removal, maintaining tree health through proper watering and fertilization helps the tree tolerate and recover from pest damage. Since the webworms of the last generation often drop to the ground to pupate in leaf litter, keeping the area around the tree clean can reduce the number of overwintering pupae and lessen the severity of the following year’s infestation. Regularly monitoring the trees allows for the early detection and removal of small egg masses before they hatch.
Utilizing Targeted Chemical and Biological Controls
When webworm infestations are too high in the canopy to reach physically, or when the number of webs is excessive, homeowners may need to turn to applied treatments. Bacillus thuringiensis (B.t.), a naturally occurring soil bacterium, is a selective option. B.t. is a biological insecticide that specifically targets and paralyzes the digestive system of caterpillars, making it safe for beneficial insects, birds, and mammals.
For B.t. to be effective, the spray must penetrate the web and be ingested by the larvae, so the application should be directed inside the web after it has been disrupted or broken. Similarly, the insecticide spinosad, derived from a soil microorganism, provides effective, low-toxicity control for webworms. Both B.t. and spinosad are most successful when applied to young, small larvae, and they are appropriate for use on edible crops like pecans.
If the infestation is severe, non-selective insecticides containing active ingredients such as permethrin or carbaryl may be used. However, these products can sometimes trigger outbreaks of other pests, such as mites and aphids, by harming beneficial insects. For very tall pecan trees, which are difficult to spray effectively and safely, it is often best to hire a licensed commercial applicator who possesses the specialized high-pressure spray equipment to ensure the product reaches and penetrates the webs in the upper canopy.