The Mimosa Webworm in Missouri
Bruce A. Barrett
Department of Entomology
The mimosa webworm, Homadaula anisocentra Meyrick (Lepidoptera: Plutellidae), attacks the leaves of both mimosa and honey locust trees.
Two factors have brought about the increased importance of this pest: the extensive loss of American elm trees due to Dutch elm disease and phloem necrosis, and the increased use of the honey locust and its varieties as a replacement for the American elm in landscape plantings. Increased use of mimosa in landscape work also is a contributing factor.
This pest produces two generations a year in Missouri. However, these generations overlap in that some larvae usually are present on infested trees from June until September. The larvae of the second generation often are so numerous in late August that they cause extensive damage to infested plants.
Mimosa webworms pass the winter as pupae within tough, white silken cocoons. The cocoons may be in crevices in the bark of infested trees or any tree nearby, in cracks in the weatherboarding of a house, under trash on the ground, in old larval webbing on the trees, or in any other protected place near the host plant.
The small steel-gray moths, with small black dots on the forewings, emerge in late May or early June. They have a wingspan of from 3/8 to 7/16 inch and a body length of about 3/16 inch. The moths live for several weeks and lay their eggs on the leaves of the host plant. These eggs are oval, white and very small. Just before hatching, they change in color from white to rose.
The small, dark greenish brown larvae hatch from the eggs and start to feed on the leaflets. They web the leaflets together and feed within this protected area. Some of these larvae reach maturity by mid-July. Others mature by early August.
At maturity, the larvae are slender, about 1 inch in length and grayish brown in color, and have five light-colored stripes, one on the back and two on either side, running the length of the body. They spin additional silk in the webbed leaves or move to a protected area, spin their cocoons and change to pupae.
The second generation moths emerge in late July or mid-August and deposit their eggs. The larvae from this generation often are so numerous that they skeletonize all the leaves on the host trees and leave the ugly webbing over most of the limbs. The larvae mature and change to pupae for overwintering.
Do not make extensive plantings of honey locusts or mimosa trees unless you make plans for chemical control. The sunburst variety of the thornless honey locust is very attractive to this pest and often is completely defoliated.
Some of the insecticides that the homeowner can use to control the webworm are various formulations of acephate (e.g., Orthene), carbaryl (e.g., Sevin), chlorpyrifos (e.g., Dursban), diazinon and Bacillus thuringiensis (e.g., Dipel). Insecticidal sprays are most effective if applied shortly after egg hatch (early June and early August).
Handle insecticides with care. Follow directions on the insecticide label. Keep insecticides off the skin and out of eyes, nose and mouth. Wash immediately with soap and water if concentrates are accidentally spilled on the skin. Store unused insecticides in a dry area, out of the reach of children and pets.