If Your Honeylocust Leaves Turn Brown in the Late Summer, It Could Be Mimosa Webworm

If one of your ornamental honeylocust trees turns brown in the heat of late summer, and the leaves look like they’ve been wrapped with a fine webbing, then mimosa webworm is a likely culprit. It's a common pest on honeylocust here in Columbia and Central Missouri, and the damage tends to show up quickly once the second generation gets going.

What is Mimosa Webworm?

Mimosa webworm is a worm that feeds on the layers of leaves of honey locust trees, leaving behind brown or lacy leaves coated in webs. When the worms feed on the layers of leaves, they leave behind just a thin layer of leaf veins that turn brown, which is why an infested tree can look scorched by mid to late summer.

You often notice them at the top of the tree working their way down. Normally the first sign is widespread browning of the leaves, so if the crown of your honeylocust starts going brown from the top while the rest of the yard is still green, that's the moment to take a closer look.

How to Tell it Apart from Bagworms

It's easy to confuse one webbing pest for another, but the two behave very differently. Bagworms normally target evergreen trees and are in singular bags made of the plant tissue. Mimosa webworms, on the other hand, will be on your locust trees and cover the leaves in spider-like webs. If the webbing is spread across the honeylocust foliage rather than hanging in individual bags on an evergreen, you're most likely looking at mimosa webworms.

When It Shows Up in Central Missouri

In our area, the small gray moths emerge in late May or early June, and larvae are usually present on infested trees from June into September. A second round of moths shows up in late July or mid-August, and that later group is often the most damaging. Because of that, the worst of the browning usually appears in late summer, right around late August, which is when most homeowners first notice something is wrong.

Will It Kill Your Tree, and Can It Spread?

The good news is that this pest is more of a stressor than a death sentence for an established tree. If your honey locust is otherwise healthy, it should be okay, but this is a stressor on your tree, and repeated years of defoliation take a toll on a tree that is already dealing with heat or drought.

As for spread, the pest is mostly limited to honey locust and won't spread to nonrelated trees, so the rest of your landscape isn't at risk. It does tend to travel down the block, though. More than likely, if you have the pest, your neighbor will too.

Why We Lean on Supportive Care Instead of Spraying

There isn't a practical treatment for the webworms, however supportive care for the tree to help manage the stress is beneficial.

A few things make direct treatment difficult. Pruning is not an option for this type of webworm, and due to the large size of locusts, DIY spray treatments aren’t practical. On top of that, the webworms are enclosed in a water-resistant web that blocks most sprays.

We also don't typically treat for them because by the time they are found the damage is already done. Systemic insecticides could be applied preventatively, but most years would not warrant a treatment because outbreaks are relatively rare. For that reason we don't consider treating the webworm necessary - in most cases. It would need to be done preventively, and severe outbreaks are rare. If a preventive treatment were to be done, it would require one or two applications, with the first before any activity was noticed to significantly reduce damage.

For most honeylocust trees, the more useful path is supporting the tree so it can shrug off the stress and grow new leaves the following season. That's the heart of Plant Health Care: keeping a tree strong enough to handle the pressures it faces rather than reacting after the damage is done.

Contact Ratliff Tree About Your Honeylocust

Mimosa webworm looks alarming when it browns out a tree in late summer, but for an otherwise healthy honeylocust the bigger opportunity is usually supporting the tree through the stress rather than chasing the pest. If your honeylocust is browning out year after year, or you want to know whether supportive care makes sense for your tree, it's worth having it looked at.

Ratliff Tree's Plant Health Care team, led by a certified master arborist, can evaluate your property, confirm what's going on, and recommend a plan that fits your trees. Request a property consultation or call us at (573) 446-2266.