Subterranean termites cause more structural damage to homes in the United States than any other pest. They work from underground, behind walls, and inside wood, and they can go years without revealing themselves. A structure with an undetected infestation for five or ten years may have compromised framing that is not obvious until it fails.
These termites are present across all four states covered here. Pressure is highest in Maryland and the broader Mid-Atlantic, where the climate supports larger, more active colonies. In Illinois, Wisconsin, and Minnesota, colonies are present but tend to be smaller and less active through the cold months. That does not make northern homeowners safe. Termites in a cold climate still cause serious damage if left alone long enough, and annual inspections are the only reliable way to catch them early.
Identification
Reticulitermes flavipes is a social insect with a caste system. What you see depends on which caste you are looking at.
Workers do the actual feeding. They are about 1/4 inch long, soft-bodied, and creamy white to grayish-white, wingless and blind. You almost never see them unless you break open infested wood. Soldiers are the same size but have a large, dark-orange rectangular head with prominent mandibles. Swarmers, also called alates, are the reproductive caste and the form most people see. They are dark brown to black, about 3/8 to 1/2 inch long, with two pairs of equal-length translucent wings that extend well past the abdomen. After a swarm they shed their wings, and finding piles of discarded wings near windowsills or door frames in spring is a strong sign of activity.
The identification mistake homeowners make most often is confusing termite swarmers with carpenter ant swarmers. Three differences settle it.
Waist: Termites have a thick, uniform waist with no constriction. Ants have a narrow, pinched waist. This is the clearest difference when you have one in hand.
Antennae: Termite antennae are straight and beaded. Ant antennae are bent at a clear elbow.
Wings: Termite wings are all the same size and shape, and they detach easily, which is why you find piles of them. Ant wings have a noticeably larger front pair than rear pair.
Behavior and Habitat
Subterranean termites live in underground colonies in soil near a food source. They need contact with the soil to stay moist; they cannot survive in open air for more than a short time, which is why they build mud tubes to travel across non-wood surfaces. Foraging tunnels can extend more than 100 feet from the main nest. The colony eating your sill plate may be nesting under a neighbor’s yard.
Any wood in or near soil is a potential target: sill plates, floor joists, subfloor, stair stringers, wall studs at the base of a frame. Workers follow moisture gradients and signals from decaying wood to find it.
Colonies grow slowly. Reaching maturity takes five to ten years. A mature colony holds 60,000 to several million workers. The primary queen produces thousands of eggs per year. Secondary reproductives supplement her output as the colony ages.
Swarming happens in spring on warm days following rain, usually in the morning. In the Mid-Atlantic, swarms peak from March through May. In the upper Midwest, April through early June is more typical. Finding swarmers does not tell you where the main colony is; it means a mature colony nearby is producing reproductives to start new ones.
Signs of an Infestation
Mud tubes are the most reliable exterior sign: pencil-width tunnels, roughly 1/4 to 1/2 inch across, constructed from soil, wood particles, and saliva. They run along foundation walls, concrete piers, pipes, and up from slabs. Breaking one open and finding white workers inside confirms active infestation. An empty tube does not mean the problem is over; termites abandon and rebuild tubes regularly.
Hollow-sounding wood is the other key sign. Tap structural members, baseboards, door frames, and trim with a screwdriver handle. Gallery damage sounds thin and papery. Soft or blistered paint at the base of a wall may push through into a hollow, mud-lined gallery.
Discarded wings near windows or light fixtures in spring, and wood that looks intact from outside but has a honeycomb interior when cut, are also evidence of active infestation. Unlike carpenter ants, subterranean termites do not leave a visible frass pile. They use waste in building mud tubes, so there is nothing to see at the entry point.
Health and Property Risks
Subterranean termites are not a health risk. They do not bite, transmit disease, or affect air quality.
The risk is structural and financial. Long-term feeding on sill plates, floor joists, subfloor, and wall framing compromises load-bearing capacity. A sill plate eaten to a thin shell, a joist with galleries running its full length, a wall stud hollow at the base: these are failure points. Repair costs for localized damage run several hundred to a few thousand dollars. Extensive framing damage can reach into the tens of thousands.
A confirmed infestation also affects home sales. Real estate transactions in these states commonly require a wood-destroying insect inspection, and a positive report with active infestation or prior damage requires disclosure and usually remediation before closing.
Treatment Options
Subterranean termite treatment is not a DIY job. The two professional methods are liquid soil treatment and bait stations. Both are effective. They work differently, and a good technician will explain the tradeoffs.
Liquid soil treatment involves trenching or drilling along the foundation perimeter and injecting a termiticide into the soil. Non-repellent products like imidacloprid or fipronil are standard. Workers pass through the treated zone, carry the chemical back on their bodies, and spread it through the colony. This method works relatively quickly and provides a defined protective zone. It typically comes with a one-year renewable warranty.
Bait stations are installed in the soil at regular perimeter intervals. Workers find the bait, recruit nestmates, and carry the active ingredient back to the colony. The active ingredient is usually a slow-acting chitin synthesis inhibitor or insect growth regulator. This method takes longer, sometimes several months to show full effect, but is a good option when soil conditions make liquid treatment difficult or when a homeowner prefers a lower-chemical approach. Annual monitoring and bait replenishment are required.
Both methods require professional installation. Partial applications fail. Interior treatment of mud tubes found inside is done alongside the perimeter work, not instead of it.
Prevention
The single most effective prevention step is eliminating wood-to-soil contact around the foundation. Sill plates should have a clearance of at least one inch between the wood framing and the soil. Mulch piled against the foundation covers the gap between soil and wood and creates a moisture pathway. Keep mulch pulled back from the foundation wall and use a non-organic ground cover near the base if possible.
Fix moisture problems. Termites need moisture and follow it into structures. Leaking gutters that deposit water against the foundation, poorly graded soil that holds water near the house, and condensation in crawl spaces all create favorable conditions. Ventilate crawl spaces, grade soil to drain away from the foundation, and address any standing water near the structure.
Remove wood debris. Scrap lumber stored against the house, dead tree stumps near the foundation, buried form boards from old construction, and firewood stacked against the siding are all food sources that can support a young colony close to the structure. Remove them.
Annual inspections by a licensed pest control company are the most reliable way to catch early activity. Most termite damage is found during routine inspections, not by the homeowner noticing symptoms.
What It Costs
Termite treatment is the most expensive common pest control service. Cost depends primarily on the linear footage of the foundation, the treatment method, and how much access work is required.
Liquid soil treatment for a typical single-family home runs $800 to $2,500 for the initial job, with an annual renewal inspection and re-treatment if needed costing $200 to $400. Homes with larger footprints, difficult access, full basements with finished walls, or attached structures at the high end of that range.
Bait station systems typically cost $1,200 to $3,000 for initial installation across the perimeter, with annual monitoring and bait replenishment contracts running $300 to $500 per year.
Structural damage repair is separate from treatment costs. Replacing a section of compromised sill plate, subfloor, or floor joists typically runs $500 to $3,000 for localized work. Extensive framing damage can cost significantly more.
Many pest control companies offer a written warranty with termite treatment, covering re-treatment at no additional charge if activity is found during the warranty period. Read the warranty terms before signing: the coverage, exclusions, and annual renewal requirements vary.
When to Call a Professional
Call a professional the moment you see mud tubes, find swarmers indoors in spring, or tap on wood near the foundation and hear hollow spots. Do not wait to see if the problem progresses. Termites work continuously and the damage compounds.
If you are buying or selling a home, a wood-destroying insect inspection is standard practice and required by most lenders. A licensed inspector can determine whether evidence of current or prior infestation is present and whether the damage is limited or extensive.
For any suspected termite activity, this is not a situation where trying a store-bought product first makes sense. No consumer product treats subterranean termites reliably. The treatment requires licensed-grade products and professional application to reach the colony underground. The cost of professional treatment is small compared to the repair bills that follow a few more years of undetected feeding.