K-State’s Kansas table lists four honey locust tree problems, and they don’t carry the same weight. Leaf spot is cosmetic. Most knots are cosmetic too. Canker is the one that can hold a mature tree in slow decline for years. Wood decay is the one that ends in a removal. Two insects show up on the same table, bagworm and mimosa webworm, and neither one kills a healthy honey locust on its own.
What are the most common honey locust tree problems in Kansas?
Start with the symptom, not with a pest list somebody wrote in another state. On a honey locust the trouble shows up in one of four places: the leaves, the branch nodes, an old pruning wound, or the wood inside the trunk. Here’s how those sort out.
| What you see | What K-State lists | What it means |
|---|---|---|
| Dark-red, sunken areas | Canker | Young trees may decline quickly, while mature trees can weaken slowly |
| Swollen branch nodes | Honeylocust knot | Often unattractive without reducing tree vigor |
| Cosmetic leaf spotting | Leaf spot | Rarely damaging, with no management needed |
| Mushrooms, soft wood, or hidden damage | Wood decay | Structural damage may exist before symptoms become obvious |
| Webbed, scorched foliage near the top | Mimosa webworm | Visible damage means insecticide timing has already passed |
| Bags attached to foliage or twigs | Bagworm | Use the Kansas-specific bagworm identification guide |
We already tell homeowners this is a good tree to plant here. Our Southwest Topeka page says hackberry and honey locust “establish fast on the corridor’s clay soil and tend to develop sound branch structure with early training”, and our Tecumseh page puts it among that area’s newer acreage plantings. What we’d never gotten around to saying is what goes wrong with it. That’s this page.
Which honey locust tree problems does K-State actually list for Kansas?
A much shorter list than you’ll find anywhere else. The November 2023 K-State publication MF3132 gives honey locust four diseases and two insects, and that’s it.
Search this question and you’ll get pages from Colorado, Illinois, Utah, Iowa and Pennsylvania instead. They’ll hand you honeylocust borer, plant bugs, spider mites and pod gall midge. None of those four sits in the Kansas table. That doesn’t mean an insect can never turn up here that K-State didn’t print. It does mean a Colorado checklist shouldn’t be the thing that decides what’s wrong with a tree in a Topeka yard.
Leaf spot is the easiest one to let go of. K-State calls it “Rarely damaging, primarily cosmetic, no management needed.” Spotted leaves on their own aren’t a reason to take a stable tree down. Branch dieback, soft wood and a crown that’s thinning out are the symptoms worth chasing.
Is the webbing at the top of my honey locust mimosa webworm?
Almost certainly, yes. On a honey locust, webbing up in the crown is the signature. K-State’s own write-up puts it this way: “Caterpillars create webs near the tops of trees, which protects them from natural enemies (parasitoids and predators) and insecticide spray applications.” The height isn’t incidental. It’s the reason the insect is hard to hit.
Kansas runs two generations a year. The second one is the late-summer batch, and it’s the one people call about, because that’s when the tree starts to look burnt. K-State says heavily infested trees are “brown or scorched in appearance” “because the caterpillars skeletonize the leaf tissue”. The leaves are still there. The green has been eaten off them.
The caterpillars themselves are about half an inch long full grown, “green to dark brown, with white stripes extending the length of the body.” Poke one and it backs up fast. Later in the season they leave on a silk thread and pupate in bark crevices, or stuck to the side of your house.
If the webbing is on something other than a honey locust or a mimosa, it’s probably a different insect. Our fall webworm guide sorts the two apart by host tree.
Why spraying in late summer does not work
Because you’re already too late. K-State’s Kansas table puts it in one sentence: “Once damage is noticeable, it is too late to apply an insecticide.”
The reason is the webbing itself. As the K-State mimosa webworm guidance explains, “It is too late to apply an insecticide this time of year when trees are heavily infested with webbing because the caterpillars are protected from insecticide spray applications inside the leaf webbing.” Spray hits the outside of a web. The caterpillars are on the inside of it, thirty feet up.
So the honest answer in August isn’t a product. It’s a saw. K-State’s own alternative is that “If feasible, selective pruning quickly removes isolated or localized mimosa webworm infestations.”
That doesn’t mean opening up the whole crown. It means taking the webbed tips out and leaving the sound growth alone. The crews in our Topeka network don’t spray. They can tell you whether the webbing is contained enough that selective tree trimming handles it, or whether it’s a wait-until-spring problem.
What canker does to a honey locust, and where it starts
Canker often starts where an old pruning cut opened live tissue. K-State’s wording is specific: “Sunken, dark-red cankers form in branch crotches or at pruning wounds”. The symptoms it lists are “Branch dieback, reduced foliage and premature defoliation.”
Water stress feeds it from either direction. K-State calls honeylocust canker “Common in trees stressed by drought or over-watering.” Soaking a struggling tree isn’t the safe option people assume it is.
How fast it moves depends on the age of the tree, and the split is sharp. “Young trees may die rapidly while mature trees may remain in a weakened state for many years.” That’s the sentence most homeowners need. A young honey locust with canker can be gone in a season or two. A big one can sit in a holding pattern for a decade, which is why the tree in your yard hasn’t obviously gotten worse and also hasn’t gotten better.
Now the part that’s easy to misread. MF3132 puts the cankers “at pruning wounds” and then, in the very next column, tells you to “Maintain tree health with proper pruning, fertilization, and watering.” Both sentences are the publication’s own. They aren’t in conflict. Pruning isn’t the problem. Careless pruning is, because every cut is an opening, and this species keeps a record of the bad ones.
So the guidance isn’t to leave the tree alone. It’s to cut deliberately and in the right place: “Prune out all cankered branches to 1 foot below visible damage.” A foot below what you can see, not flush with it. There’s a second instruction on the same line: “Avoid mechanical wounds to the trunk.” In a yard, that’s the mower and the string trimmer.
That’s four different problems that all start with a tree looking off, and telling them apart from the ground is genuinely hard. A tree health assessment is how you separate canker from webworm damage, cosmetic spotting and decay you can’t see yet.
Are the knots on my branches a problem?
Honeylocust knots are usually more alarming than damaging. K-State describes “Small, swollen galls at branch nodes. Shoot blighting may also occur.”
Their cause remains unsettled. MF3132 says they are “Suspected to be caused by a bacterium and/or herbicide damage, but causation not fully understood.” Appearance alone can’t settle which suspected factor contributed.
K-State adds, “In many cases this condition does not affect tree vigor, just causes unattractive knots.” That makes monitoring reasonable when foliage, branch growth, and structure remain stable.
K-State does say “Some cultivars are more susceptible than others.” It doesn’t name which ones, so neither will we. Nobody can read a cultivar off a photo of a knot anyway.
If affected branches do need cutting, clean tools matter. K-State’s instruction is to “Sanitize tools between cuts using 70 percent alcohol or 10 percent bleach”. It’s a small step that keeps one cut from seeding the next.
When decay means the tree comes out
Wood decay becomes a removal issue when structural risk develops. K-State warns, “Symptoms of decay may not be visible until extensive structural damage to the tree has resulted.”
Look for crumbly, spongy or discolored wood, and for mushrooms or conks at the base. A full green crown doesn’t rule any of that out. The tree can look fine from the driveway and still be hollow where it counts.
There’s no fixing it once it’s started. K-State is blunt about that: “There is no control once the fungus has colonized the wood.” Its instruction from there is short. “Trees with indications of decay should be evaluated by a tree-care professional.” And then: “Trees that are a structural risk should be removed by a certified arborist.”
That reads differently in Kansas than it does in a mild climate. K-State’s own decay section says the loss of strength “poses a risk to structures, cars, or people, especially during windy weather or ice storms”, and Topeka gets plenty of both.
A tree risk assessment examines the trunk, roots, branch unions, targets, and likely failure points. The site’s guide to recognizing a dead or declining tree helps homeowners organize visible symptoms before that visit.
When decay has already compromised the structure, pruning can’t rebuild wood. A second opinion through an arborist consultation can clarify whether monitoring remains reasonable. Confirmed structural risk may require professional tree removal.
Frequently asked questions
Which honey locust tree problem is most serious?
Wood decay, without much competition. It’s the only one on K-State’s list with no treatment, and it can stay hidden until the structural damage is already extensive. Canker is second, mostly because it can take a young tree down fast. Leaf spot, knots and webworm are all things a tree lives through.
Will honeylocust knots kill the tree?
Usually not. K-State says that in many cases the condition doesn’t affect tree vigor and just leaves unattractive knots behind. Shoot blighting can happen alongside them, so a branch that’s actively changing is still worth a look.
Should I spray visible mimosa webworm damage?
No. Once you can see the damage, the window has closed. The caterpillars are inside the webbing and the spray stays outside it. Selective pruning is what still works this late, and only where the webbing is contained. Next year’s answer is to treat when the caterpillars first show up, before they’ve built anything.
Can a cankered honey locust be pruned?
Yes, and it should be, but the cut has to go past what you can see. K-State’s spec is a foot below the visible damage. Sanitizing between cuts matters too, since the same publication puts cankers at pruning wounds in the first place.
Does K-State list honeylocust borer as a Kansas problem?
No. MF3132’s honeylocust table lists two insects, bagworm and mimosa webworm, and honeylocust borer isn’t one of them. It shows up all over the national results because Colorado and Utah extension pages rank for this question. Don’t let an out-of-state list decide what’s wrong with a tree here.
When should I think about removing and replacing it?
When someone qualified has looked at it and found structural risk from decay. That’s the threshold, not a bad-looking season. If it gets there, our tree removal cost guide covers what the work runs and our replacement planting guide covers what comes next. Neither one is a substitute for someone standing under the tree.
Get it identified before the next cut
Four problems, four different answers. Leaf spots need nothing. Contained webworm gets pruned. Canker gets cut a foot back from the damage. Decay gets an inspection and, often enough, a removal. Guessing between them is how a fixable tree becomes a hazard tree.
Topeka Tree Pro connects homeowners across Shawnee, Jefferson, Jackson, Osage and Wabaunsee counties, including Topeka itself, with insured local crews. They’ll tell you which of the four you’ve got and what it actually needs. Call (785) 333-4052 and we’ll get a crew matched to your yard.