Hackberry gnats are not actually gnats. They’re tiny jumping insects called hackberry psyllids, members of the genus Pachypsylla, and they develop inside the small bumps (galls) that form on hackberry tree leaves each year. If you’re dealing with clouds of these insects around your home in late summer or early fall, you almost certainly have a hackberry tree nearby. They don’t bite, don’t damage your home, and don’t carry disease, but they can show up in enormous numbers that make them a serious nuisance.
What Hackberry Gnats Actually Are
Several species of Pachypsylla psyllids live on hackberry trees across much of the United States. The most common include the hackberry nipple gall maker, the hackberry blister gall maker, and the hackberry bud gall maker. People call them “gnats” because of their size and swarming behavior, but they’re more closely related to aphids and cicadas than to true gnats or flies.
Adults are small, dark-colored insects with tawny markings and whitish forewings spotted with black. The larger species reach about a quarter inch to the tips of their folded wings and look like miniature cicadas up close. Most of the ones you’ll encounter indoors are smaller than that, often described as tiny black jumping insects. They’re small enough to pass through the mesh of standard window screens, which is why they end up inside homes even when windows appear properly screened.
Their Life Cycle on Hackberry Leaves
Hackberry psyllids produce one generation per year. Adults that survived the winter in bark crevices and other sheltered spots emerge and mate in spring around the time hackberry trees leaf out. Females lay eggs on the undersides of expanding leaves, and nymphs hatch roughly 10 days later.
When a nymph begins feeding on leaf tissue, it triggers the leaf to grow rapidly around the insect, forming a small pouch or gall. These galls appear as raised bumps, typically an eighth to a quarter inch across, on the leaf surface or along the leaf stem. The nymph lives inside this gall all summer, feeding on sap and developing through several stages before emerging as a winged adult in late summer or early fall. If you’ve ever noticed a hackberry leaf covered in tiny pimple-like bumps, each one housed a developing psyllid.
Why They Swarm in Fall
The mass emergence of adults in September and October is what creates the “hackberry gnat” problem. Thousands of psyllids leave their galls within a short window of time and begin searching for sheltered places to spend the winter. In nature, that means bark crevices and leaf litter. Near homes, it means cracks around windows, gaps in siding, wall voids, and any opening they can find.
Large numbers can be present through September and October. On warm, sunny days in mid-winter, psyllids that made it into wall voids or attic spaces may become active again and wander into living areas, confusing homeowners who don’t expect insects indoors in January. This doesn’t mean a new infestation. It’s the same group of adults stirred awake by warmth.
Do They Hurt the Tree?
Heavy galling can make a hackberry tree look rough. A badly infested tree might have dozens of galls per leaf, and the overall appearance suffers. But hackberry psyllids don’t cause significant long-term damage to otherwise healthy trees. The galls divert some energy from the leaf, and heavy infestations reduce the tree’s aesthetic appeal, but hackberry trees are tough, fast-growing species that tolerate the feeding without serious decline. Treating the tree purely for the tree’s sake is rarely necessary.
Keeping Them Out of Your Home
Because hackberry psyllids are small enough to slip through standard window screens, the simplest prevention during peak activity is closing your windows entirely for the few weeks in September and October when adults are swarming. That short-term inconvenience eliminates the main entry route.
Beyond windows, sealing gaps matters. Check for cracks around window frames, door sweeps, siding joints, and utility penetrations. Caulking and weatherstripping these entry points reduces not just psyllid invasions but other fall-migrating nuisance insects as well. Finer-mesh screen material is available if you want to keep windows open during the swarming period, though it restricts airflow somewhat.
If psyllids are already inside, a vacuum cleaner is the most practical tool. They don’t reproduce indoors, so every one you remove is one fewer to deal with. There’s no nest to find and no breeding population inside your walls. The indoor problem is self-limiting once the outdoor swarming season ends.
Managing Psyllids at the Source
Chemical treatment of hackberry trees for psyllids is difficult to justify and hard to time effectively. Because the insects are protected inside their galls for most of the growing season, foliar sprays have a narrow window of usefulness, limited to the brief period in spring when eggs are being laid and nymphs are exposed on leaf surfaces. Once galls have formed, the insects inside are shielded from contact sprays.
If the problem is severe enough year after year, removing the hackberry tree is the most effective long-term solution, as Purdue University Extension notes bluntly. This is obviously a significant decision, and most people find the annual nuisance manageable with exclusion alone. But for homes with a large hackberry directly adjacent to the structure, tree removal does permanently solve the problem.
Replacing a removed hackberry with a non-host species eliminates the psyllid habitat entirely. If you want to keep the tree, accepting some level of fall swarming and focusing your efforts on sealing the home is the realistic approach.