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Termite swarm season in southern Illinois: what March through May means
A termite swarm is a mature colony announcing itself. In the Metro East, eastern subterranean termites usually swarm from March through May, often on a warm day after rain. Swarmers indoors are not an emergency to the hour, but they are a clear signal to book an inspection.
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- Written by Darlene Pemberton
- Updated October 1, 2026
- 6 minute read
Swarmers are winged reproductive termites that leave a mature colony in spring to start new ones. Finding them indoors, or finding piles of shed wings on sills, usually means a colony is in or against the structure. Swarmers themselves do no damage and die quickly indoors. Vacuum them up, save a few in a bag or jar for identification, and schedule an inspection rather than spraying.
Why spring, and why after rain
Subterranean termite colonies take several years to mature enough to produce swarmers. Once they do, they release them on warm days when the soil is moist, which in southern Illinois typically means March through May, often the day after a soaking rain and frequently in the middle of the day. The timing is designed so new kings and queens can land on soft, damp soil and dig in. St. Clair County sits in a zone rated moderate to heavy for termite pressure, so the season is a reliable one here.
Swarmer or flying ant
Spring brings flying ants too, and the two are easy to sort with a close look. Termite swarmers have straight, beaded antennae, a thick waist with no pinch, and two pairs of wings that are the same length and extend well past the body. Winged ants have bent, elbowed antennae, a narrow pinched waist, and front wings noticeably longer than the hind wings. Termite wings break off easily, so piles of identical wings on a sill are a strong termite sign on their own.
| Feature | Termite swarmer | Flying ant |
|---|---|---|
| Antennae | Straight, beaded | Bent, elbowed |
| Waist | Thick, no pinch | Narrow, pinched |
| Wings | Two pairs, equal length | Front pair longer than hind |
| After landing | Wings shed in piles | Wings usually stay on |
Swarmers outside versus inside
A swarm from a stump, woodpile or fence post in the yard confirms termites are active on the property, which is worth knowing but is not proof the house is infested. A swarm from inside, out of a baseboard, window frame, basement wall or slab crack, means the colony has a route into the structure. Indoors swarmers often head for windows and light, so window sills are where most homeowners find them or their wings.
The first 24 hours
Swarmers cannot chew wood, sting or bite, and they die within hours indoors without soil. There is no need to evacuate or spray. Collect a few for identification, vacuum the rest, and note exactly where they came from, since that location helps the inspection. Spraying the emergence point makes it harder to read and does nothing to the colony underground.
- Save a few swarmers in a sealed bag or jar
- Vacuum the rest; no spraying
- Note where they emerged
- Check the foundation for mud tubes
- Book an inspection; colonies move slowly enough for a calm week
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Common follow-up questions
Indoor swarmers mean a colony has access to the structure, and feeding has likely been going on for a while. Damage moves slowly, so a prompt inspection matters more than an emergency response.
A mature colony can swarm every year. Treatment stops the colony; without it, expect a repeat next spring.
Store sprays kill the swarmers you can see and nothing else. The colony is in the soil, which is where professional treatment goes.
Darlene Pemberton is the pest control guides editor at Belleville Pest Control. She writes and maintains these homeowner guides from the field notes our technicians bring back from jobs across Belleville and the Metro East, and every guide is checked against what the crews actually see in local homes.
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