John ::. Journal .::

Small Bugs on Windows Screen

Windows Screen Shades / September 26, 2022

In September and October, people who have hackberry trees, or live in neighborhoods where there are hackberry trees, often notice tiny greyish bugs that congregate on their homes, on window screens, front doors and siding. These insects are attracted to lights at night and, at 1/10" long, are tiny enough to pass through ordinary window screen. People describe these bugs as gnats, flies or fleas. The photo above hackberry gall psyllid on the tip of a pencil!

These insects are adult hackberry gall psyllids (pronounced, sill-ids). Another name is "hackberry nipple gall maker". Under magnification, they look like miniature cicadas (what people in Nebraska commonly call "locusts"), which makes perfect sense, because they are in same order (Homoptera) as cicadas, leafhoppers and aphids.

In the fall, these insects are looking for cracks and crevices to squeeze into so they can hibernate without succumbing to lethal temperatures. Normally, they overwinter under the bark of trees, but psyllids don't distinguish between "good" and "bad" overwintering locations so they also squeeze into cracks and crevices around windows, doors and siding. Those that come inside are likely to die.

After adult psyllids come out of hibernation in the spring, they lay eggs on emerging leaves of hackberry trees. After the egg hatches, the young psyllid starts feeding, and the leaf responds by growing abnormally. It develops a small pocket that surrounds the insect, forming a "gall" (photo above). The psyllid spends the rest of the summer sucking on tree sap safely within the small gall. Several species of gall-making psyllids infest hackberry trees. Infested hackberry trees do not seem to be harmed by these galls, but their abundance makes hackberry leaves look pretty ugly.

Hackberry psyllids are not harmful to people or pets and will not attack house plants, stored products or furnishings. They are a temporary nuisance. As temperatures get colder, their activity will decrease and hibernation will set in.


Hackberry psyllids are so annoying that people sometimes ask about spraying hackberry trees to control them. During the summer, psyllids are protected inside the gall (photo right) from insecticides sprayed on the leaves so foliar treatments won't be effective then. Unfortunately, by the time psyllids emerge and start to move out of the trees, it is probably too late to achieve effective chemical control by spraying the trees.

A more effective preventative approach would be to treat trees in the spring to kill newly hatched nymphs before the onset of gall formation. But, because egg laying occurs over a period of several weeks beginning when new leaves unfold from the bud, several foliar insecticide applications would be needed. The labor involved makes this approach impractical, especially with large trees.

Treating hackberry trees with a systemic insecticide to kill psyllids when they feed would be ideal, but this proactive approach means planning ahead. Check out systemic insecticides at your home and garden store. One fairly new systemic product, Bayer Advanced Garden Tree & Shrub Control, contains imidacloprid which provides year-long control. It controls sucking insects, like aphids, psyllids, lacebugs and scale insects. It is even pretty easy to use. After determining the amount needed (based on the diameter of the tree), just mix the liquid insecticide in water and pour around the base of the tree. When using any insecticide product, be sure to read and follow all label directions.


Spraying with insecticides is not recommended. People who are really "bugged" by this problem and just have to do something can try hosing down their siding with water. This is likely to drown many of the psyllids, but, as long as temperatures are warm, more may show up the next day. A fine mesh window screen (18 mesh) may be small enough to prevent entry through open windows. For those insects that get inside, sucking them up with a vacuum cleaner is very effective.

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