The mimosa webworm, Homadaula anisocentra, will be showing up in the southern portion of Illinois during this time of year. This pest came into the United States from China in 1940. Heavy infestations of mimosa webworm can destroy the aesthetic value of landscape trees. The larvae (caterpillars) feed on honeylocust, Gleditsia triacanthos, and mimosa, Albizia julibrissin. There are two generations per year in Illinois.
The adult is a silvery-gray moth, with black stippling or dots on the forewings. The females appear in mid-June and lay pearly-gray eggs on tree leaves. The eggs hatch into larvae that are pale lime green in color, with white stripes on their backs. The larvae are about 1/2 inch long when full grown. They rapidly move backward when disturbed. Larvae web leaves together on the ends of branches. The webbing usually starts at the tops of trees and serves as protection from natural enemies. Heavily infested trees appear brown or scorched as the larvae skeletonize the leaf tissue. The larvae eventually drop from trees on a silken strand just before pupating. Mimosa webworm pupates in bark crevices or the pupae can be found glued onto structures. The pupae resemble kernels of puffed rice. It generally takes 3 weeks to go from larvae to pupae. In July, adults emerge and start laying eggs on leaves. The eggs hatch into larvae in early August. This is the start of the second generation.
Second-generation larvae are more numerous and hatch over a more extended time period than the first generation. As a result, their feeding injury can be very noticeable. In addition, second-generation injury may result in plant death, particularly on newly planted trees that have suffered drought stress. In late fall, the larvae pupate in the cracks and crevices of trees or in plant debris. Mimosa webworm overwinters in the pupa stage.
Management of mimosa webworm involves either pruning out early infestations, especially on small trees, and/or using pest control materials. Pest control materials recommended for controlling mimosa webworm include acephate (Orthene), Bacillus thuringiensis var. kurstaki (Dipel, Thuricide, Javelin), carbaryl (Sevin), and chlorpyrifos (Dursban). Sprays of any of these materials should be made in early June when hills-of-snow hydrangea is in bloom or when webs first appear. For second-generation larvae, apply materials in early August, when panicled hydrangea blooms are turning white. Target applications toward the most vulnerable stage, which is the newly hatched and actively feeding young larvae.
Honeylocust cultivars exhibit various degrees of susceptibility to mimosa webworm. The cultivar ‘Sunburst’ is highly susceptible, whereas the cultivars ‘Moraine,’ ‘Shademaster,’ and ‘Imperial’ are less susceptible to attack by mimosa webworm.