Have you noticed trees with brown leaves and what looks like spider webbing all over them? The culprit is most likely the mimosa webworm (Homadaula anisocentra)! These caterpillars are occasional pests of the mimosa tree and several varieties of honey locust.
This pest produces two overlapping generations per year. The first generation typically emerges in late May and the second in early August, so caterpillars appear on infested trees from June through September. Like the name suggests, the mimosa webworm will tie leaflets together in tight, protective webs where they can safely feed. This protects the caterpillars from natural enemies (predators or parasitoids) and from insecticides.
Webworm larvae eat the tissue between the veins of the leaves, but do not consume the vein itself. This type of feeding is known as skeletonization. As they feed, patches of the tree’s leaves will brown, taking on a scorched appearance. Feeding is usually heaviest during the second generation in late summer (August through September) and may even completely defoliate a tree. While the silken nests and skeletonized leaves will make the trees look bad, it rarely harms an established tree.
The presence of this pest and the amount of damage it does varies greatly from tree to tree and from year to year. It’s common to see a boom/bust cycle – populations slowly rise year after year (you might not even see them or feeding damage), peak in an “outbreak”, and then collapse.
The best management practice for this pest is prevention! Keep leaf debris and webbed foliage cleaned up from beneath and around your tree to help prevent severe infestations. Chemical control is rarely effective or warranted.
Additionally, avoid large plantings of mimosa or honey locust. We find that the ‘Sunburst’ locust variety is particularly susceptible to this pest. Look to plant ‘Moraine’, ‘Shademaster’, ‘Skyline’, or ‘Imperial’ instead.