Honey locust is one of the toughest street and yard trees, shrugging off road salt and many diseases that kill other trees. Its delicate new leaves, though, draw a steady crowd of small insects and mites, and they cause most of the browning, thinning and early leaf drop owners notice. The rarer bark and root diseases are the ones that end a tree’s life, and they mostly follow stress an owner can often see or prevent, such as dry soil, wounds and trampled roots.
Bumpy Pods and Spotted Leaves on New Growth
Tiny fly larvae of the honey locust pod gall midge feed on new leaflets and make them swell into pod-shaped growths called galls.1College of Agricultural Sciences. Honeylocust Pod Gall Midge The galls can be green, brown or reddish.2UC Statewide Integrated Pest Management Program. Honey Locust—Cecidomyia gleditsiae (honey locust pod gall midge) The larvae finish growing inside the pod, and once the adult midges leave, the leaf tissue dies and drops early.3College of Agricultural Sciences. Honeylocust Pod Gall Midge Heavy infestations can destroy all new growth, leaving bare stretches of branch and a thin-looking tree.4Agricultural Biology. Honeylocust pod gall midge
Spotting and curling have a different cause. In early spring, young honey locust plant bugs crawl into unfolding leaves and feed just after the buds open.5Penn State Extension. Honeylocust Plant Bug They inject a toxic saliva that kills cells around each bite.6Rainbow Ecoscience. Common Problems With Honey Locust Trees Each bite becomes a yellow spot that later turns brown.7Penn State Extension. Honeylocust Plant Bug Heavy feeding causes yellowing, rolling, twisting and stunted leaves.8HYG Pest newsletter. Honeylocust Plant Bug
You can tell the two apart with a leaf in your hand. Midge damage is a closed, puffy pod, while plant bug damage is flat spots on a leaf that may be curled. Split a fresh pod and you may find tiny larvae inside, unless they have already left. Plant bug damage shows up within weeks of leaf-out, while the weather is still cool. Both can appear on the same tree in the same season.
The tree often looks better by midsummer. Both pests attack tender new growth, so older, hardened leaves suffer less, and honey locust often pushes out new leaves after an early drop. For an otherwise healthy tree the damage is mostly cosmetic.
Rake and bag fallen galls and leaves to reduce the next round. Water deeply in dry spells so the tree can replace what it lost. For a young or prized tree, an arborist can time a spray, but the timing has to match leaf-out. On large trees spraying is impractical, and accepting some damage is the sensible choice.
Bronzed Canopy and Webbed Leaves
Honey locusts, especially on dry sites, are almost always carrying honey locust spider mites.9Colorado State University Extension. Spider Mites Mite numbers climb on city trees prone to drought stress, so severe outbreaks can bronze or brown the whole canopy, stunt growth and cause early leaf drop.10Agricultural Biology. Mite pests of honeylocust
Up close, the damage starts as fine pale stippling on individual leaflets, which you can see by turning a leaf over in good light. As feeding continues, the stippled leaflets dull and turn bronze, so the canopy looks faded rather than spotted or chewed. Bronzing often begins on the sunniest, most exposed side of the tree or on the inner leaves, then spreads outward.
To confirm mites, hold a sheet of white paper under a branch and tap the branch sharply. Mites show as tiny moving specks that streak when smeared. Check during hot, dry weeks, when they peak.
Mimosa webworm caterpillars are a different problem. They web leaves together and eat the leaflet surface, leaving a see-through skeleton.11Purdue University Extension. Mimosa Webworm Webbed clumps of brown leaves point to webworm rather than mites.
Deep watering in drought is the main defense against mites. A strong water spray knocks them off small trees. Prune out small webbed clumps by hand when you see them. Broad insecticides can kill the predators that feed on mites and make an outbreak worse.
Sticky Branches and Black Mold From Scale Insects
Soft scale insects suck sap and excrete large amounts of sticky liquid called honeydew onto branches, leaves and the ground.12Purdue Landscape Report. Keep Honeylocust Trees from Becoming a Pest Nightmare Calico scale, a white and dark brown bump about 1/4 inch across, makes heavy honeydew in spring.13University of Kentucky Department of Entomology. Common Problems With Honey Locust Trees Twigs and leaves can end up coated with black sooty mold growing on the honeydew.14University of Kentucky Department of Entomology. Common Problems With Honey Locust Trees Calico scale can also severely weaken trees and make them more vulnerable to drought.15University of Kentucky Department of Entomology. Common Problems With Honey Locust Trees
Cottony maple scale can infest honey locust too. It forms white cottony egg sacs on twigs in late spring to early summer.16Colorado State University Extension. Insect and Mite Pests of Honeylocust Its heavy infestations only occasionally yellow leaves, but they draw a sticky mess and nuisance insects.17Colorado State University Extension. Insect and Mite Pests of Honeylocust
To find scales, look for still bumps on the undersides of twigs rather than on leaves. Wasps and ants crowding a branch often signal honeydew above.
Black mold wipes off. It grows only on the surface of the honeydew and does not invade the leaf, so the blackening is mostly cosmetic.
Scrape or prune heavily infested small branches. On small trees, dormant horticultural oil in late winter targets scales that overwinter. Any summer treatment should be timed for when young crawlers hatch, with an arborist’s advice. Keep the tree watered, since scale plus drought is the dangerous combination.
Sunken Bark and Dying Branches From Cankers
Cankers are sunken, dead patches of bark caused by fungi.18Colorado State University Extension. Honeylocust Diseases Thyronectria canker and black spot Nectria canker are common on honey locust, with coral spot Nectria seen occasionally.19Colorado State University Extension. Honeylocust Diseases Symptoms include branch dieback, thin or yellow foliage, early fall color and early leaf drop.20Colorado State University Extension. Honeylocust Diseases
Variety matters a great deal. Sunburst honey locust is the most canker-prone, while Imperial, Skyline, Trueshade and Thornless are the most resistant.21Colorado State University Extension. Honeylocust Diseases
To spot a canker, look for bark that is sunken, cracked or discolored compared with the bark around it. Coral spot shows tiny pink-orange bumps, and black spot shows dark ones. Leaves beyond the canker wilt or yellow while the rest of the tree stays green.
The fungi usually enter through bark that is already injured or weakened, so a healthy tree with intact bark resists them better. Mowers, string trimmers and bad pruning cuts are common starting points. Drought, sunscald, frost cracks and storm breaks also create openings, which is why cankers often show up after a hard season. If you find one, look back for what injured that spot.
Prune infected branches well below the canker in dry weather, and clean your tools between cuts. No spray cures cankers, so preventing wounds and stress is the real treatment. A canker on the main trunk calls for an arborist visit.
Root Rot and Shelf Fungus at the Base
Ganoderma is a fungus that rots roots and the base of the trunk, entering through wounds there.22CES Pest newsletter. Stressed Honeylocust Environmental stress and wounding make trees easier to infect.23CES Pest newsletter. Stressed Honeylocust A large shelf-shaped fungus on the trunk, usually at a wound, is the visible sign.24CES Pest newsletter. Stressed Honeylocust On one university lawn, many honey locusts died of Ganoderma, and soil packed by foot traffic was thought to be a stress factor.25University of Illinois Extension. Stressed Honeylocust
The fruiting body is a hard bracket that grows out of the trunk near the ground, often at an old wound. Check the base of the trunk and the root flare, because a bracket there points to decay below the surface. Other fungi can also form brackets, so a bracket on a honey locust calls for an expert’s identification rather than a final diagnosis.
It is a safety question as well as a health one. By the time the shelf shows, decay inside is usually advanced, and rotted roots can let a tree fall without warning. Have a certified arborist assess any such tree near a house, car or play area. There is no cure, so removal is often the outcome.
Prevention comes down to protecting the roots. Keep mowers and trimmers off the trunk base with a mulch ring. Avoid parking or heavy foot traffic over the root zone. Don’t pile soil or cut big roots during construction.
Weed Killer, Salt and Other Non-Pest Damage
Honey locust is sensitive to growth-regulator herbicides, the type in many broadleaf lawn weed killers.26Purdue Extension. Diagnosing Herbicide Injury on Garden and Landscape Plants One turf herbicide, aminocyclopyrachlor, caused brown, twisted shoots and leaves, dieback and sometimes death of trees next to treated lawns.27Weed Technology. Potential Damage to Sensitive Landscape Plants from Wood Chips of Aminocyclopyrachlor Damaged Trees
Herbicide injury usually affects new growth across the tree or one whole side, rather than showing scattered spots or bumps. Leaves look cupped, twisted or strapped, and you find no insects, galls or webbing. The damage may appear days to weeks after spraying, so owners often miss the connection.
To check for drift, ask whether you, a neighbor or a lawn service sprayed in the past few weeks. Damage often shows on the side facing the treated lawn. Read lawn product labels for warnings about tree roots. Don’t use mulch or chips from herbicide-damaged trees around other plants.
Some suspects can be ruled out. Honey locust tolerates road de-icing salt.28USDA Forest Service. Honeylocust29Highway Research Record. Salt Tolerance of Trees and Shrubs to De-Icing Salts It also resists Verticillium wilt, so that disease is an unlikely cause of limp leaves.30Extension Ask Extension. Limp Locust Leaves #183159
Winter is a different story. Like other thin-barked trees, honey locust is among the trees most often hit by sunscald and frost cracks on the trunk.31Kansas State University Agricultural Experiment Station and Cooperative Extension Service. Common Problems With Honey Locust Trees Splits usually appear on the south or southwest side. A light tree wrap in winter helps young trees, but remove it in spring to avoid trapping moisture and insects.
Breakage and Roots That Lift Pavement
Weak branch forks are considered the main reason honey locusts break in ice storms.32Arboriculture & Urban Forestry. Ice Storm Damage to Urban Trees Where two upright stems of similar size grow tightly together, bark can get trapped between them and the joint stays weak. Ice adds heavy weight, so a weak fork splits where a well-attached branch would hold. That is why pruning for structure early matters more than any treatment after the damage.
On young trees, pick one main trunk early and remove competing upright stems. Favor wide-angled branches over tight V-shaped forks, and prune in late winter while the tree is dormant. Large trees with split forks may need cabling by an arborist.
Roots crack sidewalks as they thicken, mostly within about 10 feet of the trunk, because the biggest surface roots sit right beside the tree.33Extension Knowledgebase. Reducing root size on large locust tree during sidewalk replacement Cutting big roots close to the trunk can destabilize the tree and invite rot. Paths can curve or be raised instead, and an arborist should weigh in before any root cut beside the trunk.
Choosing a Healthier Honey Locust to Plant
Choosing Imperial, Skyline, Trueshade or Thornless over Sunburst lowers canker risk. A site with enough soil water and no foot-traffic compaction reduces mite outbreaks and root rot.
Match the site to the tree. Honey locust prefers full sun, so a shaded spot will leave it thin and stretched. Leave enough open soil around the trunk for roots to spread, since roots squeezed into a narrow strip lift pavement sooner. Check overhead wires and the distance from buildings, because it grows into a large tree.
At planting, set the tree at the right depth with the root flare visible. Water deeply once a week for the first two summers, and mulch a wide ring kept off the trunk. Avoid lawn weed killer in the root zone. Planting several tree types limits losses to any one pest, and your local extension office can say which species suit your area.