Mimosa webworms are a common pest of ornamental trees such as mimosa and honeylocust. These caterpillars feed within silken webs that they construct over leaves, protecting themselves from predators while consuming foliage.
Although rarely fatal, repeated infestations can weaken trees and reduce their aesthetic value, making them a concern in urban landscapes.
The Silken Architect: Mimosa Webworms
The Mimosa Webworm (Homadaula anisocentra) is a destructive caterpillar known for “skeletonizing” and binding the foliage of its two primary hosts in the United States: the Mimosa tree (Silk Tree) and the Honeylocust. While these trees are prized for their feathery, delicate leaflets, a heavy webworm infestation can quickly turn a lush canopy into a scorched-looking mass of brown silk and dead tissue.
Nesting and Feeding Behavior
Mimosa Webworms are masters of structural silk. Unlike Fall Webworms, which build large, conspicuous nests at the ends of branches, Mimosa Webworms work on a smaller, more intimate scale. Their feeding cycle follows a predictable pattern:
- Leaf Binding: The larvae use silk to tie several small leaflets together, creating a private “pocket” or envelope.
- Surface Rasping: They feed inside this pocket, rasping away the green surface of the leaflets. Because they leave the veins and the opposite skin intact, the leaves turn a characteristic “rusty” brown.
- Expanding the Colony: As the larvae grow, they expand their webbing to encompass larger sections of the branch. In severe outbreaks, the entire tree can appear as if it has been draped in a fine, dirty veil.
Identification and Lifecycle
The larvae are small (about 15mm) and highly active, often wiggling violently or dropping on a silk thread when the web is disturbed. They range in color from pale gray to dark brown, usually with five white longitudinal stripes. In most parts of the U.S., they produce two generations per year, with the second generation in August usually being much more destructive than the first.
Management and Resilience
While the “scorched” look of an infested Honeylocust is visually alarming, the damage is rarely fatal to established trees. However, repeated defoliation can weaken a tree’s resistance to Thyronectria canker or boring beetles. Management strategies include:
- Variety Selection: Some Honeylocust varieties, such as ‘Sunburst’, are highly susceptible, while others like ‘Skyline’ show better resistance.
- Physical Removal: For smaller trees, the webbed clusters can be pruned out and destroyed before the second generation emerges.
- Biological Control: Because the larvae are exposed on the leaves, Bacillus thuringiensis (Bt) or Spinosad are highly effective if applied early in the feeding cycle (June and August).
Identification
Larvae are small caterpillars with striped bodies. Webbing on leaves is often the first visible sign of infestation.
Damage
Damage includes browning leaves, webbing, and defoliation. Heavy infestations can stress trees.
Control
Control includes pruning infested branches, encouraging natural predators, and applying insecticides when necessary.