For most of the year, subterranean termites are invisible. They live underground and inside wood, and a colony can work in a house for a long time without the homeowner ever seeing one. Spring is the exception. Once a year, when conditions are right, a mature colony releases a swarm of winged termites, and for a brief window they are out in the open.
That swarm is the single clearest warning sign a homeowner gets that termites are present. If you know what to watch for in an Illinois spring, a swarm becomes useful information instead of a missed signal. Here is what to know.
When termites swarm in Illinois
Termite swarm season in Illinois runs March through May, with the peak generally in April as soil temperatures climb. The exact timing depends on the weather. Swarms are typically triggered by warming soil combined with rain and humidity, and they often happen on a warm day after a spring rain. A colony will not swarm until it is mature, usually several years old, so a swarm means an established colony, not a brand-new one.
There is a regional element in Illinois. The warmer Metro East around Belleville, in USDA Zone 6b, sees termite activity earlier and heavier than the cooler north. Downstate river towns and older housing in Rockford, Peoria, and Springfield carry significant pressure. And it is worth knowing that Illinois winters have been warming faster than any other season, which has slowly extended the active season for many pests, termites included.
What a swarm looks like
A termite swarm is a number of winged insects emerging together, often from the ground near a foundation, from a crack in a slab, or, alarmingly, from inside the house, near a baseboard, a window frame, or a basement wall. The swarmers themselves are not strong fliers. They flutter a short distance, the swarm does not last long, and the winged termites quickly shed their wings.
That is why many homeowners do not see the swarm itself at all. They find the aftermath: a small pile of discarded wings on a windowsill, on the floor near a basement window, or caught in a spider web. The wings are all the same size, translucent, and noticeably longer than the body. A pile of shed wings indoors is a serious sign and should not be ignored.
Termite swarmer or flying ant?
This is the single most common point of confusion, and getting it right matters, because the two mean very different things. In spring, ants also produce winged reproductives, and a flying ant swarm is harmless to your house. A termite swarm is not.
Three features tell them apart:
- Waist. A termite has a straight, broad body with no narrowing, the body looks like a single thick segment. A flying ant has a sharply pinched, narrow waist.
- Wings. A termite has four wings, all the same length, and they extend well past the body. A flying ant has four wings, but the front pair is clearly longer than the back pair.
- Antennae. A termite’s antennae are straight and look like a string of tiny beads. An ant’s antennae are bent, with a distinct elbow.
If you find swarmers or shed wings and cannot tell which you have, that uncertainty is itself a reason to have it checked. Treating a termite problem as a harmless ant swarm loses you months, and termites work the whole time.
The other signs to watch for
A swarm is the obvious sign, but subterranean termites leave others, and spring is a good time to look for all of them.
Mud tubes. Subterranean termites build pencil-width tunnels of mud and soil to travel from the ground to wood while staying protected and moist. Look for these on the foundation, on basement walls, on piers, and anywhere wood meets soil. Mud tubes are one of the most reliable signs of an active infestation.
Damaged wood. Termite-damaged wood often looks intact on the surface but sounds hollow when tapped, because they eat it from the inside out. Probe suspect wood gently. Termite galleries are packed with mud and soil, which is one way they are distinguished from the clean galleries of carpenter ants.
Conditions that invite them. Wood-to-soil contact, moisture against the foundation, mulch piled against the siding, a downspout that keeps the soil damp, all of these make a house more attractive to termites. Spring, when you are already doing the work in the spring prevention checklist, is the time to address them.
What to do if you see a swarm
If you find a termite swarm or a pile of shed wings, do not panic, but do not wait either. Termites work slowly, so a house does not collapse overnight, but they also do not stop, and the damage accumulates for as long as the colony is active.
The first step is a termite inspection. An inspector will check the foundation inside and out for mud tubes, examine the sill plate and crawl space, probe suspect wood, and give you a written report on what is active and what damage is visible. In the competitive Chicago market many companies offer a free inspection, though if you need a documented wood-destroying insect report for a home sale, confirm that is what you are getting.
Do not try to treat an active termite infestation yourself. Subterranean termites work underground and inside wood, out of reach of anything a homeowner can apply, and the stakes, your home’s structure, are too high to experiment with. Professional treatment uses either a liquid barrier applied around the foundation or a bait system installed in the soil, both of which address the colony, not just the termites you can see.
Why subterranean termites are the Illinois concern
It is worth understanding the kind of termite Illinois actually deals with, because it shapes everything about detection and treatment.
Illinois termites are subterranean termites, specifically the eastern subterranean termite. They live in colonies in the soil, not in the wood itself, and they travel from the ground to a food source, the wood in your house, through the mud tubes described above. They need contact with soil moisture to survive, which is why moisture conditions around a foundation matter so much, and it is why a liquid soil barrier or an in-ground bait system is the standard treatment: you are treating the route and the colony, not just the wood.
This is different from drywood termites, which live entirely inside the wood and are the reason you hear about tent fumigation. Drywood termites are a southern problem and are not significant in Illinois. If an Illinois company quotes you an expensive whole-house tent fumigation for what should be a subterranean termite problem, that is a reason to get a second opinion. The mismatch between the treatment and the actual pest is a warning sign.
Subterranean termites work slowly. A colony does not destroy a house in a season. But they also work continuously and invisibly, and over years the damage to structural wood adds up to serious expense. The slow pace is exactly why the annual spring swarm is so valuable: it is a once-a-year chance to catch a problem that otherwise stays hidden.
What to check after the swarm season
Even if you do not see a swarm, late spring and early summer are a good time for a deliberate look around the foundation, since you are already doing seasonal yard work.
Walk the full perimeter of the house and check the foundation for mud tubes, paying attention to spots where the foundation is hidden behind shrubs, mulch, or stored items. Look at the sill plate in the basement or crawl space, where the wood framing sits on the foundation, since that is a common first point of attack. Tap suspect wood and listen for a hollow sound. Check porch posts, deck ledgers, and any wood close to or in contact with soil.
While you are at it, fix the conditions that invite termites: pull mulch back from the siding, correct grading or downspouts that keep the foundation damp, and eliminate wood-to-soil contact where you can. None of that replaces treatment if termites are already active, but it makes the house less attractive to the next colony.
What treatment costs
Termite treatment is a real expense, but it is far smaller than the cost of letting the colony run. A standalone inspection runs $75 to $325. A liquid barrier treatment generally runs $400 to $1,200 for an average Illinois home, priced by linear foot of foundation. A bait system runs $800 to $2,500 to install and includes ongoing monitoring. The termite treatment cost guide breaks down the full ranges and what drives them.
The math on termites is straightforward. An inspection is cheap. Treatment is moderate. Structural repair from a colony left alone for years is expensive. A swarm in your yard this spring is the warning that lets you act on the cheap end of that scale instead of the expensive one.
If you have seen a swarm, found shed wings, or spotted mud tubes on the foundation, you can get connected with a licensed Illinois exterminator who covers your area for an inspection. Spring is when termites show themselves. Use the window.