What is a mimosa webworm?
If you have a mimosa or honey locust tree you may notice brown leaves or patches of dense webbing like material in the tree. These are symptoms of a mimosa webworm problem. Mimosa webworms are web building caterpillars much like tent caterpillars or fall webworm. As adults, mimosa webworms are dull colored moths. They have a half inch wingspan and are grey in color with small black dots on their wings. Their eggs are small, oval, and white but will turn rose-colored as they near hatching. The caterpillars are about an inch long when mature. In terms of color they are gray to dark brown with five white stripes and a brown head. If disturbed they will drop down from trees on a silken line.
Adults emerge in late May or early June to begin mating and laying eggs. These first eggs and caterpillars will fully mature into moths by the end of July or beginning of August. These early summer moths that emerge will mate and begin a second generation of caterpillars that feed until September. Once the second generation finishes feeding, they will leave the tree and construct a pupae on nearby buildings or trees in order to overwinter. These cocoons will produce moths the next spring to begin the cycle anew.
Integrated pest management tactics
The mimosa webworm prefers thorn-less varieties of honey locust. Avoiding cultivars like “Sunburst” will help to prevent dealing with the webworm in the future. Preferable cultivars would be “Shademaster” or “Imperial”. If tree-owners want to be proactive, they should scout their trees and use a pole pruner to remove visible nests in June or July. There are insecticidal options including, bifenthrin, carbaryl, chlorantraniliprole, cyfluthrin, and malathion. Organic control options include spinosad and Bt products applied in June to control caterpillars while they are small. Thorough coverage of the leaves (top and bottom) will be needed no matter what insecticides is applied. It would be best if an arborist made applications to large, mature trees. The spray schedule would be June for the first generation larvae and August for second generation larvae.