The Most Common Pests in Wisconsin Homes, Month by Month

The most common pests in Wisconsin homes aren’t a mystery, and they don’t arrive at random. Each one has a window, driven by temperature and by what it’s looking for, and knowing the window is most of what separates a small problem from a big one.

Here’s the full list, when each one shows up, and what’s worth doing about it.

Ten pests account for most home calls in Southeastern Wisconsin, and each one has a season. Ants and stinging insects build through spring, wasps and mosquitoes peak in late summer, box elder bugs and mice move indoors in fall, and cockroaches, bed bugs, and spiders stay active all winter in a heated house.

What are the most common pests in Wisconsin homes?

Ten species or groups account for the large majority of residential calls in Milwaukee, Waukesha, and Racine counties:

• Ants, mainly odorous house ants and carpenter ants
• Stinging insects, mainly yellowjackets, paper wasps, and bald-faced hornets
• Mosquitoes
• Spiders
• House mice
• Norway rats
• Cockroaches, almost always German cockroaches indoors
• Bed bugs
• Overwintering insects: box elder bugs, Asian lady beetles, and stink bugs
• Pantry pests: beetles and moths in stored food

What follows is the calendar. Two of these ignore it entirely, which is why they’re the hardest ones.

Which pests show up in spring?

Ants first, then stinging insects, then everything that woke up in a wall.

Ants appear in kitchens and bathrooms in March and April, following moisture as the ground thaws. Odorous house ants are the small dark ones that trail along a counter edge and smell faintly like a rotten coconut when crushed. Carpenter ants are much larger, often half an inch, and finding them indoors in spring frequently means a nest in damp wood somewhere in the structure rather than a trail in from outside.

Stinging insects start as single queens in April and May, building nests the size of a golf ball in eaves, grills, and shrubs. This is the cheapest and least risky moment of the entire year to deal with a wasp nest, and almost nobody does, because a nest that size doesn’t look like a problem yet.

Overwintering insects also reappear in spring, moving back out of wall voids and attics toward the light. People often read this as a new infestation when it’s the same population leaving.

Which pests take over in summer?

Mosquitoes and the stinging insects that were golf balls in May.

Mosquitoes track standing water, and the sources that matter are usually within a few dozen feet of the house: clogged gutters, a tarp with a sag in it, a wheelbarrow, a saucer under a planter, a birdbath nobody changes. A container the size of a bottle cap can produce them, which is why yard treatment without water source removal gives short-lived results.

Wasp and hornet nests reach their peak size and aggression in August and September. A yellowjacket colony can hold thousands of workers by then, and late-season nests become defensive in a way they aren’t in June. Nests in the ground, inside wall voids, and above doorways are the ones that put people in urgent care.

Honey bees deserve a separate note, because they get mistaken for wasps constantly. Honey bees are fuzzy, amber and brown, and not interested in your food, while yellowjackets are smooth, bright yellow and black, and very interested in it. Honey bee colonies should go to a beekeeper for relocation rather than being treated, and we’ll help identify what you have before anything happens.

Spiders also become noticeable in late summer, because that’s when the insects they eat are at peak numbers.

Which pests move indoors in fall?

This is the biggest shift of the year, and it happens over about six weeks.

Box elder bugs, Asian lady beetles, and stink bugs gather on sun-warmed south and west walls in late September and October, then work their way into attics, soffits, and wall voids through gaps they find there. They don’t feed or breed indoors and they don’t damage anything. They just show up in your living room on a warm February afternoon, which is unsettling enough on its own.

Mice and rats make the same move for better reasons. A mouse fits through an opening the size of a dime, a Norway rat through one the size of a quarter, and both are looking for warmth, food, and a quiet place to nest. October and November are the heaviest weeks of the year for rodent calls in this region.

Spiders come in with them, following the same gaps and the same insect activity. Pantry pests often arrive in fall too, usually in a bag of birdseed, flour, or pet food that went into a cabinet or a garage shelf.

Which pests stay active all winter?

Four, and they’re the ones that cost the most.

Mice and rats breed year round inside a heated building. A population that got in during October isn’t waiting for spring, and droppings in a second room four weeks after the first sighting is the normal progression rather than bad luck.

German cockroaches have no season at all. They live entirely indoors, near warmth and moisture, and reproduce continuously. Kitchens and bathrooms in January look the same to them as in July.

Bed bugs are the same story with a travel component. They move on luggage, clothing, and used furniture, so holiday travel and secondhand furniture are two of the bigger winter introduction routes in Wisconsin.

Overwintering insects technically stay too, dormant in wall voids, which is why they reappear on warm days and then again in spring.

Which of these can you handle yourself?

Some, honestly. Here’s a straight assessment, because the usual version of this article says call a professional about all ten and nobody believes it.

Reasonable to handle on your own:

• Pantry pests. Throw out the infested product, empty and vacuum the cabinet, wipe it down, and move dry goods into sealed containers. This works and it’s most of what a treatment would do anyway.
• Overwintering insects already inside. Vacuum them and empty the canister outdoors. Don’t crush Asian lady beetles, since they stain.
• Mosquito source reduction. Walking the yard weekly and dumping standing water does more than most products sold for the purpose.
• A single small wasp nest in spring, in an accessible spot, if you’re comfortable with it and nobody in the house is allergic.

Worth getting help with:

• Any ground nest, wall void nest, or late-season wasp or hornet nest. These are the stings that send people to the hospital.
• Carpenter ants indoors, because the question isn’t the ants, it’s where the damp wood is.
• Mice and rats beyond one mouse in a trap. Store-bought traps catch individuals, and they don’t find the entry point that let the population in.
• German cockroaches. Over-the-counter sprays scatter them into wall voids and adjacent rooms, which makes the eventual treatment harder and longer.
• Bed bugs. There’s no version of this that works without a defined multi-visit protocol, and attempts to handle it at home routinely spread it through a house.

What should you do next?

Match your action to the calendar rather than to the sighting. In November, that means rodent exclusion and winter rodent service, checking stored food, and looking at what’s coming in on boxes and decorations. In April it means wasp nests while they’re small and ants before they trail. The work done ahead of a season is always smaller than the work done in the middle of one.

Ehlers Pest Management has handled residential pest work across Milwaukee, Waukesha, and Racine counties since 2010, and a certified technician can tell you in one visit which of these you actually have. You can talk to our experienced team at (414) 303-6926 or schedule service online.

Frequently asked questions

What is the single most common household pest in Wisconsin?

Ants, by call volume across a full year, with odorous house ants making up the bulk of it. Mice take the top spot for a few weeks in October and November, and in a bad mosquito summer, mosquitoes generate more complaints than anything else. Ants are the steadiest year over year.

Does cold weather kill off pests?

It pushes them indoors rather than killing them. Outdoor insect activity drops hard after the first sustained freeze, which is why yards get quiet, and that same freeze is what drives mice, box elder bugs, and spiders through gaps in your exterior. Anything already inside a heated house is unaffected.

Why do I see bugs in my house in the middle of winter?

Usually overwintering insects waking up. Box elder bugs, Asian lady beetles, and stink bugs spend winter dormant in wall voids and attics, and a stretch of sun on a south-facing wall warms them enough to start moving. They head toward light, which puts them at your windows.

Do I need year-round service, or just when I see something?

It depends on the history of the house. A home with no recurring problems often does fine with treatment when something turns up. A house with a rodent history, a carpenter ant history, or a lot of entry points usually comes out ahead on a scheduled plan, because the recurring issues are the expensive ones to keep re-solving.

How do I tell a carpenter ant from a regular ant?

Size first. Carpenter ants are large, often a quarter to half an inch, and black or black and red, while odorous house ants are about an eighth of an inch. Carpenter ants also produce small piles of what looks like sawdust near their nest. Finding them indoors in winter is a strong sign the nest is inside the structure.