Our trees are brown in the middle of summer and there are webs everywhere! What is up with that?

This summer as students have returned to campus they may have been alarmed by the clumps of browning trees across campus. They may have also noticed an annoying presence of webs everywhere, often accompanied by a worm. The unsightly appearance of brown trees and annoying webs is the result of an extensive infestation of mimosa webworm on our locust trees.

Mimosa webworm (Homadaula anisocentra Meyrick, 1922), is the main constituent of our current infestation. They are not a native species to North America, having been accidentally introduced from Japan and China in the 1940s. During the larval stage of this insect, they feed on leaves while also spinning a silk web to protect themselves. The feeding and silk spinning is what is responsible for the large clumps of webbed dead leaves on our trees. As we enter fall, these insects will begin to silk down to find places to over winter, leaving silk lines across campus, which you may have discovered while walking.

Non-native species often do not experience the same predation they would typically experience in their native region; thus, it is common for non-native/invasive species to quickly explode in population often wreaking havoc on native environments. However, when it comes to mimosa webworms, they have been here long enough to have been discovered by numerous native predators and parasites, some of which have been found on our campus.

Aside from mimosa webworm, we are also experiencing an infestation of lace bug on our hawthorns. However, unlike webworms, lace bugs are native to the region and therefore are somewhat mitigated by natural predation. But even with natural predation, the damage is still noticeable around campus with a distinct “bronzing” of leaves.

So, what will we be doing about this infestation? Management will begin next spring with non-toxic biological control pesticide BT, Bacillus thuringiensis. This is a bacterium in a water carrier which paralyzes and eventually kills the larvae after they ingest the bacteria. It is entirely non-toxic to people and animals including other beneficial predator insects. We will be treating select trees which have been particularly heavily infested by the pests beginning spring of 2026. Even though the feeding damage is unsightly and even alarming, it is unlikely to significantly impact the health of infested locust and hawthorn trees.

If you are curious about the trees, please scan the QR code below to view more information about all our trees on this campus!