Be watchful for mimosa webworm on honeylocust. This insect is adversely affected by cold winter temperatures; it is likely that large numbers of this insect will be present this year due to the mild winter. Mimosa webworm is likely to appear throughout the state but may be most noticeable in northern Illinois, where (the ordinarily cooler) winter temperatures make this webworm an uncommon pest.
Mimosa webworm has two generations per year. The first generation is usually small in number and easily overlooked. The greenish, brownish, or grayish slender larvae feed on the leaflets of honeylocust and mimosa (silk tree). When disturbed, mimosa webworm larvae move quickly and violently, which helps them escape. The first-generation larvae web together two or three leaflets and feed on the leaflet undersides. This feeding causes the leaves to appear silvery at first, particularly from a distance. As damaged areas dry, they turn brown. Large numbers of first-generation infestations scattered throughout a tree call for an insecticide application to reduce the more seriously damaging second generation.
First-generation caterpillars pupate in the webbed leaves and emerge as small grayish moths. These moths mate and tend to lay eggs back into first-generation webbing. The second generation is usually much larger in number than the first. These caterpillars typically web together two to six compound leaves, causing damage that is much more obvious. Although treatment of the second generation is usually successful, considerable damage may have occurred before treatment is initiated.
Fully grown second-generation caterpillars migrate out of the webbing to pupate in protected areas such as under loose bark on tree and shrub trunks and under building siding. Many webworms pupate under siding and around windows of heated buildings where temperatures are a few degrees warmer--which explains why higher populations of mimosa webworms are found in trees near heated buildings and in years following mild temperatures.
Mimosa webworm is controlled with a variety of insecticides, with Bacillus thuringiensis kurstaki (Dipel, Thuricide) the most environmentally friendly. If an insecticide is used, high-pressure application will move more of it through the webbing to the caterpillars.