Are the brown cobwebby masses at the tips of my thornless honeylocust tree branches spider mites?
Spider mites create fine webbing on the foliage of plants, including honeylocust (Gleditsia). If the webbing is thick on the branch tips of your honeylocust, it's likely that the trees have mimosa webworm or fall webworm. Mimosa webworm feeds on branch tips of honeylocust trees beginning in early summer. Adults are moths; larvae are caterpillars that feed on leaves inside webbing they form at branch tips. Additional generations of webworms may develop through late summer. Fall webworm attacks many species of trees. Some of their preferred species are walnut (Juglans), American elm (Ulmus americana), hickory (Carya), apple (Malus), and maple (Acer). Their nests are large and gray. Nests may appear from midsummer to fall.
For either pest, you may be able to physically remove nests on small trees. Prune out affected branch tips. Avoid burning the nests in place; this can cause additional damage to the tree. The bacterial insecticide Bt (Bacillus thuringiensis) is effective if applied when the larvae are small. Unfortunately, many gardeners don't notice the nests until the caterpillars are nearly mature. By then Bt is no longer effective. Spray conventional insecticides when caterpillars are young. Repeat sprays may be needed for succeeding generations.
Community Answers 0