Several web-making caterpillars can be active in late summer. They cooperate to bind leaves and branches together into unsightly, but protective, nests where the can feed more or less undisturbed. Species active at this time of year include the relatively common fall webworm, the specialist mimosa webworm, and the relatively uncommon ugly nest caterpillar. Usually, they cause scattered aesthetic injury, but occasional outbreaks can be destructive.
Fall webworms are fuzzy caterpillars that have pale green or yellow hairs over their bodies and rows of black spots along their backs. They cooperatively build light gray tents (Figure 1) that enclose the ends of branches of over 100 species of forest trees, shade trees, and shrubs; sourwood, pecan, and persimmon are favorite hosts. Fall webworms feed on leaves inside the webbing and expand the “tent” as they require more food during their 4- to 5-week developmental period. There are two generations of the mimosa webworm each year; the second generation can cause severe defoliation, especially to thornless cultivars. Populations were very high in parts of Kentucky during 2015 and 2016 but seem to be less abundant this year. Fall webworms are primarily an aesthetic issue on healthy, established landscape trees. However, significant infestations on stressed or new transplants can be serious.
Mimosa webworm caterpillars generally produce smaller webbed areas than fall webworms but can make up for it in intensity of infestation (Figure 2). Larvae feed primarily on mimosa and honeylocust. Full-grown gray to dark brown caterpillars have five narrow stripes running from head to tail. Mature caterpillars (about 0.6 inches long) can be nuisances as they rappel down silk threads onto unsuspecting passers-by walking below infested trees. There are two generations of mimosa webworm each year; the second generation can cause severe defoliation, especially to thorn-less cultivars.
Ugly Nest Caterpillars
Ugly nest caterpillars are yellow -green with black heads. They inhabit seemingly slapped-together nests that resemble those of the fall webworm. Full of brown leaves and dark, granular frass, the few nests that occur each season tend to be widely scattered and serve only as minor eye-sores.
Individual webs have little impact on tree health and can be left to natural enemies. An insecticide spray may be needed if the first generation of mimosa webworm is causing noticeable damage, usually during June. This reduces the potential for a larger and more damaging second generation during August and September. Products containing spinosad or pyrethroids can work as contact or stomach poisons. Treatments can be focused on the foliage in and around the tent. Usually, there is no need to spray entire trees.
By Lee Townsend, Extension Entomologist