The first generation of fall webworm (FWW) is active now. The June brood is usually small and often goes unnoticed, while the August-September bunch is larger and occasionally can be a problem. These caterpillars are covered by pale green or yellow hairs and have rows of black spots along their backs (Figure 1).
FWW can feed on over 90 species of deciduous trees and shrubs. Hickory, walnut, birch, black cherry, crabapple, and mulberry are among its favorites. Usually, the infestation is limited to a branch or two and impact is aesthetic rather than a threat to plant health.
Fall webworm caterpillars work together to produce silk webbing that incorporates the leaves they are eating into a tent that becomes larger as they grow. The hairy caterpillars feed for about 4 weeks and top out at just over an inch in length.
Infestations on small, newly-established landscape trees and shrubs should be watched and treated if necessary to prevent extensive defoliation and unnecessary stress. Large, established trees can tolerate an infestation, which is fortunate because their size can make treatment impractical.
A few scattered tents can be left to natural control by beneficial insects. Accessible unsightly nests can be pruned and discarded, if practical. Tree and shrub insecticides containing Bt, spinosad, or pyrethroids can be used if chemical control is necessary and the sprayer can reach foliage around nests.
The two generations of the mimosa webworm follow a pattern similar to that of FWW, but they feed primarily on mimosa and honeylocust; thornless honeylocust (particularly cv. Starburst) is especially prone to attack. These dark-bodied caterpillars with 5 white stripes along the body thrash about when disturbed.
By Lee Townsend, Extension Entomologist