El Niño Winter Pests in the Midwest and Great Lakes: What a Milder 2026-27 Could Mean

Cities that could experience increased pest pressure during a mild El Niño winter:

Chicago, Indianapolis, Detroit, Grand Rapids, Milwaukee, Madison, Cleveland, Columbus, Cincinnati, Minneapolis, St. Paul, Des Moines, Kansas City

Key Takeaways

• NOAA favors a warmer-than-normal winter across much of the northern U.S., including portions of the Midwest and Great Lakes.

• A mild winter does not eliminate pests. Some species may stay active longer and remain closer to homes.

• Rodents continue looking for shelter during cold snaps, regardless of seasonal averages.

• Cluster flies, boxelder bugs, brown marmorated stink bugs and Asian lady beetles may become active during winter warmups.

• Freeze-thaw cycles can create gaps and moisture issues that pests take advantage of.

• Winter is one of the best times to seal entry points before spring pest activity ramps up.

How El Niño Weather Can Affect Midwest and Great Lakes Homes

A milder winter may sound like a break from snow shovels and icy mornings. For pests, it can be an opportunity.

Across the Midwest and Great Lakes, El Niño often shifts the primary winter storm track farther south. That can lead to warmer temperatures, fewer prolonged cold periods and reduced snowfall in parts of the region. While that may make winter more manageable for homeowners, it can also allow certain pests to remain active longer than they would during a more typical winter.

As of September 2026, NOAA's Climate Prediction Center reported that El Niño was present and strengthening, with a greater than 90% chance of a very strong event developing through fall and winter.

Short answer: A milder El Niño winter may reduce snowfall across portions of the Midwest and Great Lakes, but it can also keep some pests active longer and increase the likelihood of rodent and overwintering insect activity around homes.

In this guide, you'll learn:

• What NOAA's outlook suggests for the region

• Why mild winters don't necessarily reduce pest activity

• Which pests homeowners should watch this season

• How to prepare your home before spring arrives

El Niño Pest Facts for the Midwest and Great Lakes

• House mice remain active throughout winter and frequently move indoors during cold weather.

• Norway rats often seek shelter inside homes, businesses and outbuildings during sudden temperature drops.

• Cluster flies can emerge during winter warmups and gather on sunny windows.

• Boxelder bugs commonly congregate on south- and west-facing walls.

• Brown marmorated stink bugs may appear indoors during mild winter afternoons.

• Multicolored Asian lady beetles often overwinter inside wall voids and attics.

• Ant species such as carpenter ants, pavement ants and odorous house ants found indoors during winter may indicate nesting activity within the structure.

• Blacklegged ticks can remain active when temperatures rise above freezing.

• Freeze-thaw cycles can create cracks and moisture conditions that support pest entry.

What Does NOAA Forecast for the Midwest and Great Lakes This Winter?

Short answer: NOAA favors a warmer-than-normal winter across much of the northern tier, with slightly increased chances for below-normal precipitation around portions of the Great Lakes.

Here's how the 2026–27 outlook breaks down for the region:

• Temperatures: Above-normal temperatures are favored across much of the northern U.S., including many Midwest and Great Lakes locations.

• Precipitation: The Great Lakes area shows modestly increased chances of below-normal precipitation.

• Snow: El Niño winters have historically trimmed Great Lakes snowfall, including lake-effect snow.

• Storms still happen: Individual snowstorms, lake-effect bands and Arctic blasts can still roll through.

Why Isn't a Mild Winter Necessarily a Pest-Free Winter?

Short answer: Milder temperatures can extend pest activity, while cold snaps still encourage pests to move indoors.

Many pests are well adapted to winter survival.

They spend the colder months in:

• Soil

• Leaf litter

• Woodpiles

• Tree bark

• Attics

• Wall voids

• Crawlspaces

A stretch of relatively mild weather can allow some pests to remain active longer than usual. Rodents continue foraging, overwintering insects become active during warm spells and indoor pest populations continue reproducing regardless of outdoor conditions.

Weather doesn't create pest problems. It influences how pests behave and where they spend their time.

Regional Pest Outlook Across the Midwest and Great Lakes

Great Lakes Cities

Potential concerns include:

• House mice

• Norway rats

• Cluster flies

• Brown marmorated stink bugs

While El Niño may reduce overall snowfall in some areas, homeowners should continue monitoring for indoor pest activity throughout the season.

Upper Midwest

Potential concerns include:

• Boxelder bugs

• Asian lady beetles

• House mice

• Carpenter ants

Overwintering insects typically enter in the fall. So a winter warm up shouldn’t result in increased movement from the exterior but may result in the insects that entered in the fall, thinking it is spring and emerge from their over wintering harborages. This can cause concern on the part of residents and workers in affected structures.

Ohio Valley

Potential concerns include:

• Rodents

• Carpenter ants

• Stink bugs

• Cockroaches

Mild periods may allow some pests to stay active later into winter than expected.

Agricultural Regions

Potential concerns include:

• Rodents

• Cluster flies

• House flies

• Stored-product pests

Barns, sheds, grain storage areas and outbuildings can provide suitable overwintering habitat.

Why Winter Warmups Can Trigger Sudden Pest Activity

Short answer: Warm spells can temporarily wake pests that were hiding inside walls, attics and other protected areas.

When sunlight warms south-facing walls and attic spaces, overwintering insects can become active and begin moving toward light sources.

Homeowners often notice:

• Cluster flies on windows

• Boxelder bugs indoors

• Asian lady beetles gathering around ceilings

• Brown marmorated stink bugs emerging from wall voids

These sightings may seem sudden, but the insects were likely already inside the structure.

Which Pests Are Most Likely to Show Up Indoors This Winter?

Short answer: House mice, Norway rats, cluster flies, boxelder bugs, Asian lady beetles, stink bugs, carpenter ants and cockroaches are among the most common winter pests in the region.

House Mice and Norway Rats

Rodents remain the most common winter pest concern across much of the Midwest.

Cold snaps often drive them indoors where they can find:

• Warmth

• Shelter

• Water

• Food

A mouse only needs an opening about the size of a dime to gain entry.

Watch for:

• Droppings

• Gnaw marks

• Scratching sounds

• Shredded nesting materials

Cluster Flies

Cluster flies often spend winter inside attics and wall voids.

Warm periods may cause them to appear suddenly around upper-floor windows and sunny rooms.

Boxelder Bugs

These black-and-orange insects commonly gather on south-facing siding during fall and may become active again during winter warmups.

Multicolored Asian Lady Beetles

Often mistaken for native ladybugs, these insects can gather in large numbers inside homes and may leave odors or stains when disturbed.

Brown Marmorated Stink Bugs

These shield-shaped insects frequently overwinter inside structures and commonly appear during sunny winter afternoons.

Carpenter Ants

Seeing large black ants indoors during winter may indicate a carpenter ant colony nesting inside the home, particularly near moisture-damaged wood.

Cockroaches

German cockroaches live indoors year-round and can remain active regardless of outdoor winter conditions.

Blacklegged Ticks

Ticks do not necessarily disappear during winter.

Whenever temperatures rise above freezing, blacklegged ticks may become active. Homeowners should check themselves and their pets after outdoor activities during mild periods.

How Can Freeze-Thaw Cycles Create Pest Problems?

Short answer: Repeated freezing and thawing can widen cracks, loosen weatherproofing materials and create new entry points.

As temperatures fluctuate, materials expand and contract.

This process can create:

• Foundation cracks

• Gaps in caulked structural cracks

• Damaged weatherstripping

• Moisture intrusion

• Damp wood conditions

All of these can increase the likelihood of pest activity around a structure.

How Should Midwest Homeowners Prepare This Winter?

Short answer: Focus on exclusion, moisture management and food storage.

Use this winter preparedness checklist:

1. Inspect areas where pipes, vents, cables penetrate into walls and roofs.

2. Seal gaps and cracks around the exterior.

3. Replace worn door sweeps.

4. Repair damaged soffits and attic vents.

5. Store pet food, birdseed and pantry items in sealed containers.

6. Clean food spills quickly.

7. Stack firewood away from the exterior walls of the structure.

8. Address leaks and moisture issues promptly.

9. Inspect attic spaces during warm spells.

10. Monitor for rodent activity throughout the season.

What Should You Do During Winter Warmups and Cold Snaps?

Short answer: Use warm periods for inspections and cold periods to monitor for rodents.

During Warm Spells

• Inspect attics for overwintering insects.

• Look for boxelder bugs and cluster flies around windows.

• Check exterior caulk and weatherproofing.

• Watch for carpenter ants indoors.

• Remove debris near the foundation.

During Cold Snaps

• Listen for scratching noises in walls and ceilings.

• Check garages and basements for droppings.

• Look for nesting material near stored items.

• Monitor kitchens and utility spaces for rodent evidence.

Common Winter Pest Prevention Mistakes

Short answer: Assuming a mild winter is a low-risk winter is often the biggest mistake.

Common oversights include:

• Delaying exclusion work until spring.

• Ignoring a single mouse sighting.

• Storing birdseed or pet food in unsealed rodent proof bins in garages or sheds.

• Overlooking attic inspections.

• Crushing stink bugs and lady beetles instead of vacuuming them.

• Not taking out the vacuum bag contents to an outside receptacle.

• Waiting until infestations become obvious before taking action.

Use Winter to Get Ahead of Spring Pest Problems

A milder El Niño winter may reduce some of winter's harsher conditions, but it doesn't eliminate pest pressure.

By sealing entry points, reducing moisture issues and addressing pest activity early, homeowners can use the quieter months to get ahead of potential spring infestations.

If you're seeing signs of rodents, overwintering insects or other winter pests, your local Orkin Pro can help identify entry points, locate pest harborage areas and recommend treatment options tailored to your home.

More Resources

• What Could El Niño Mean for Winter Pest Activity?

• El Niño Winter Pests in the Northeast and Mid-Atlantic: What a Wetter, Milder 2026-27 Could Mean

• El Niño in California and the Southwest: How Winter Rain Could Send Pests Indoors