Above photo by Purdue University in the article, "Into to Trees of Indiana: Honey Locust"
Today we’ll discuss a tree that is very different when viewed in natural woodlands versus home landscapes because of plant breeding: honeylocust. You may also see it referenced in name as honey locust, honey-locust, honey-shucks locust, sweet locust, thorn tree, and a few others.
Honeylocust is widely planted as a hardy and fast-growing ornamental, but if you find it in the woods, you’ll see some long, nasty thorns. Thornless and fruitless varieties have been developed by the horticultural industry for use in landscapes. One characteristic of this tree is that while it offers shade, it’s not as dense a shade as you might get with a sugar maple tree. So, it allows more light to the grass in your lawn, giving it a better chance to flourish. As a bonus, leaves exhibit a beautiful yellow color in the fall.
While this happy compromise is achieved for tree and turf, honeylocust is not without a few issues. Purdue Extension entomologist, Cliff Sadof, authored an article outlining some of the pests of honeylocust, namely: honeylocust plant bugs, mimosa webworm caterpillars, scale insects, and spider mites.
“Active in May and June, honeylocust plant bugs can distort and disfigure young, expanding leaves when they suck on leaf tissue,” he said. “Although these insects can be found every year, they rarely cause enough damage to significantly thin tree canopies.”
As one who has had a thornless honeylocust in my landscape, I can attest to the annual damage inflicted by this second pest.
Mimosa webworm caterpillars will fold leaves and cover the canopy with brown, unsightly webs. It has two generations per year. “Defoliation can weaken the tree but is unlikely to kill it,” said Sadof. “Focus management efforts on the first generation to reduce the number present in the second generation.” He adds that if few webs are present and within reach, it is possible to prune out injury. When webs are too numerous or out of reach insecticide may be needed to avoid unsightly injury.
Sadof said that soft scale insects can threaten the health of honeylocust trees when they suck out plant sap and excrete copious amounts of liquid excrement (aka honeydew) on branches, leaves, and creating a sticky mess on the ground below. “When the honeydew gets covered with black sooty mold, the tree and the cars parked beneath them appear to be covered in soot,” he said. “Water stress and heat generated by paved surfaces make trees in parking lots and streets especially susceptible to these pests.” Sadof referenced calico and European fruit lecanium scales as being common problems on honeylocust.
While spider mites are not insects, in the absence of predators they can cause enough damage to defoliate trees in mid-summer, said Sadof. “Mites live and feed on leaves by piercing leaf tissue and lapping up the liquid that bubbles up from the wounds,” he said. “With multiple generations, they can grow from an egg to an egg-laying female in about a week during the heat of the summer.”
Find Sadof’s original article, which includes suggested control measures for pests mentioned at https://www.purduelandscapereport.org/article/keep-honeylocust-trees-from-becoming-a-pest-nightmare/. Additionally, find information about Indiana's native honeylocust in woodlands at https://ag.purdue.edu/news/department/forestry-and-natural-resources/2022/09/intro-to-trees-of-indiana-honey-locust.html.