When it comes to insects that bite humans, there’s simply not a very long list of “common suspects”—especially during the cooler months. Things such as bed bugs, fleas, and lice are all fairly straightforward to confirm. However, I do occasionally bump into other creatures that can bite, such as bird mites, pirate bugs, and others. I also bump into cases where clients are experiencing biting or crawling sensations, but no insects of concern are found. One of my tasks at the UW Insect Diagnostic Lab is to evaluate a situation to see if any of the less-common possibilities might be at play. If not, then I start to suspect delusory parasitosis (aka Ekbom’s Syndrome), which entomologists encounter on a fairly regular basis. In one recent case, I was scratching my head for a while until I was able to confirm the presence of hackberry psyllids (Pachypsylla spp.)—tiny, jumping, biting insects that pop up under the right conditions.
Hackberry psyllids (pronounced “sill-ids”) resemble miniature cicadas and are about 1/10th inch long. They have mottled grayish bodies and are sometimes called “jumping plant lice” or “hackberry nipple gall makers”. True to their name, these insects are associated with hackberry trees (Celtis occidentalis), which are commonly planted in the landscape as both yard and street trees.
In spring, overwintered psyllids lay eggs on emerging hackberry leaves. After the young psyllids emerge, their feeding causes unusual distortion of the leaf tissue, resulting in small “nipple-like” lumps (galls) on the leaves. Such galls are actually very common and most hackberry trees possess the characteristic galls to some extent. They may be alarming in appearance, but the galls are harmless to the trees and are essentially a minor “cosmetic” issue. The young psyllids feed and develop within the protection of their leaf galls. Eventually, they complete their development and the next generation of adult psyllids emerges from the galls.
At this point, you might be wondering how these tiny plant-feeding insects end up bugging humans. Similar to boxelder bugs and Asian lady beetles, hackberry psyllids seek out sheltered overwintering spots in the fall and can easily invade homes and other structures. With their tiny size, hackberry psyllids can be a bit harder to keep outdoors. They are often overlooked and can easily squeeze through most window screens. Indoors, these insects face death by desiccation due to the dry conditions, but can be a nuisance as they jump or fly around. Occasionally, they’ll invade in fall and their activity resumes during warm spells over the course of a winter.
In addition to being a nuisance, hackberry psyllids can “bite”. These insects feed on plants (hackberry trees), but they do have a habit of “testing” various surfaces they land on to assess if another food source has been found. If they happen to land on exposed skin, they’ll use their slender, beak-like mouthparts to probe, which can feel like a small pinch. When they do this, hackberry psyllids don’t feed on blood or inject any kind of venom, but it certainly can be unpleasant.
The good news is that unless you have a hackberry tree in your yard or very close by, you probably won’t bump into appreciable numbers of these tiny insects. If you do encounter them at your home, leaving windows closed on warm fall days (especially on south and west-facing sides of your house) or replacing window screens with a finer sized mesh can go a long way towards keeping them outside.