Webworm larvae are actually moth and butterfly caterpillars. These small, fuzzy and inconspicuous creatures create huge webs over foliage, causing damage to trees as they feed. Webworms are very destructive to trees. Chemical insecticides may be used to eliminate webworms, but they may be harmful to plants and wildlife, including beneficial insects such as honeybees and butterflies. Ron Smith, a horticulturalist at North Dakota State University, recommends using a safer, more effective insecticide called dormant oil.
Determine which trees in your area were affected by webworms in years past. Make this assessment in late March or April, when dormant oil will work best at getting rid of webworms. Do not spray the tree during freezing weather or temperatures above 100 degrees Fahrenheit, as this can cause uneven coverage and lower the effectiveness of the insecticide.
Prepare to use the insecticide on a dry day when rain is not likely for a few days. Evaporation of the oil is inhibited if the tree is wet or becomes wet. This can cause further damage to the tree.
Dilute the dormant oil with water according to the manufacturer's directions. The ratio of dormant oil to water will vary by tree type. Once the oil has been properly diluted, pour it into the insecticide sprayer.
Spray the entire tree with dormant oil---following the manufacturer's directions---before the leaf buds emerge. This will destroy any cocoons or pupae that were dormant through the winter months and remain attached to the tree.
Spray the tree again during summer if webworms reappear. Although dormant oil will kill the webworms, the adult moths are likely to be unaffected. Monitor the tree carefully throughout the summer months to ensure webworms do not return.