Mimosa Webworm

Mimosa WebwormThe Mimosa Webworm attacks the leaves of both Mimosa (Albizia julibrissin) and honeylocust (Gleditsia triacanthos).

Initially, the eggs are tiny, oval and pearly white, but turn pink when close to hatching. The larvae vary from gray to brown with five white stripes. The pupae are yellowish-brown and are located in silken cocoons. The adult, rarely seen, is a silver-gray moth with wings that are covered with black dots.

Adult mimosa webworms are small, silvery-gray moths which have wings stippled with black dots. Moths of the first generation appear in early June, while those of the second generation are seen in late July and early August. Adults lay eggs on foliage and in old webbing. Eggs are tiny, oval and at first pearly-white, but turn pink as hatching time nears. Full-grown caterpillars are about 1 inch long and grayish-brown in color with five lime green stripes running lengthwise down the body. They have a dark head, and the body is sometimes tinged with pink.

Life Cycle: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.

Damage :Mimosa webworms attack both mimosa and honeylocust trees in Indiana. Widespread use of thornless honeylocusts as ornamentals has heightened interest in the control of this insect, because all varieties of the tree are susceptible to attack.Damage is done by webworm caterpillars, which web foliage together and skeletonize leaflets. The insect has two generations each year. However, injury is most noticeable by August when second generation larvae are at the peak of activity. Continued feeding may cause infested trees to turn brown as if scorched by fire.

There are many insects and birds that feed on the larvae.Prevention is the most effective way to control mimosa webworm. Species of honeylocust (Gleditsia triacanthos) reported to be less susceptible are ‘Moraine’, ‘Shademaster’ and ‘Imperial’. Gleditsia triacanthos ‘Sunburst’ is very vulnerable. Clearing away leaf debris around the host tree will decrease chances of infestation.Young larvae should be targeted with insecticide just after eggs hatch in mid-late June and in August.

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