In the fall of the year, we get the phone calls at the Extension office.
"There are these tiny grayish brown gnats all over my window screen and they're so small they're coming inside the house."
"These are not gnats," I say. "They are hackberry psyllids." To an entomologist, this is a very important distinction. The word gnat suggests a small fly, but under a microscope these insects look just like tiny cicadas.
Hackberry psyllids are small insects that cause the galls found on hackberry leaves. These species are specific to hackberry trees and do not develop on any other plants. There are two species. Pachypsylla celtidivesicula is responsible for hackberry blister galls on the upper surface of leaves while P. celtidismamma produce hackberry nipple galls on the underside of leaves. You'lll find a photo of Hackberry galls near the end of this article.
Psyllids are annoying because they show up at the time of the year people want to open windows and enjoy the cooler weather. They may 'bite' people, although not in the same way a mosquito or a flea bites. They may be 'taste testing' or checking for food. Otherwise, psyllids are harmless to people, pets, houseplants, stored products, and furnishings.
These psyllids are tiny - about 1/8 - 3/16 inch long with mottled brownish wings with small black and white spots. The wings are held roof-like over its body and extend past its abdomen.
Adult psyllids emerge from overwintering sites in early spring and fly to hackberry trees to lay eggs on developing leaves. Eggs hatch into tiny nymphs that suck sap out of the leaves. The feeding stimulates abnormal plant tissue that surrounds individual psyllids, resulting in the formation of galls. The immature psyllids live and feed inside these galls for the rest of the summer. Hackberry leaves often have many galls on them, but the leaf injury seems to not effect the health of the tree.
By late summer when development is completed, the adult psyllids leave the galls to spend the winter in protected sites, such as cracks and crevices of tree bark and other sheltered locations. Unfortunately for people who live in the vicinity of a hackberry tree, psyllids are attracted to the sunny sides of buildings and enter through cracks and spaces around windows, doors, or siding. They are small enough that they can pass through most screens and are especially common around windows. Psyllid numbers vary from year to year.
You can reduce the number of hackberry psyllids by cutting down hackberry trees in your yard, but this is not desirable, as these are excellent shade trees. If hackberry trees are common in your neighborhood, removing your trees may have little effect, as the psyllids can fly into your yard.
A second option would be to treat your trees before galls are produced. Spray hackberry foliage during spring with a registered insecticide, such as acephate, when leaves are ½ expanded. With mature hackberry trees, this is difficult logistically and, if the treatment isn't timed correctly, may not be effective.
Another option is to use a systemic insecticide, such as imidacloprid. An example of one imidacloprid product - Bayer Advanced 12-Month Tree & Shrub Protect & Feed Concentrate - is available to the home gardener. Soluble in water, this insecticide should be applied to the base of the tree early enough so it is absorbed by the roots and translocated to the leaves when the psyllid nymphs are tiny. Imidacloprid will work best when applied to soil during late winter to very early spring, or before rainfall or irrigation are expected to facilitate root absorption of the insecticide. The key is to get a lethal concentration into the leaves when the psyllid nymphs are small. Summer application - after galls are noticed - is not likely to be effective.
For psyllids coming inside the house, consider replacing your window screen with a smaller mesh size -18 mesh should work most of the time. Or keep your windows closed. At night, psyllids are attracted to lights at night so either keep your windows closed or you'll want to tightly close your drapes. Keep outside lighting turned off or install lights, such as yellow lights, that are less attractive to insects.
If you find psyllids indoors, use a vacuum to remove them. We don't recommend using insecticides indoors for treating psyllids.
Psyllid problems resolve quickly on their own when weather becomes colder and they settle into cracks and crevices to overwinter.