Boxelder Bugs in St. Louis: Why They're Covering Your House Right Now
Every September, St. Louis homeowners start noticing something unsettling. Hundreds of small black-and-red insects cluster on the sunny side of their house, crawling up siding, pooling around window frames, and working their way into every gap they can find. If you've walked outside to check the mail and done a double-take at your own wall, you're not alone. Boxelder bugs are one of the most common and most startling fall pests in the St. Louis area, and right now is exactly when they show up.
The good news: boxelder bugs aren't dangerous. The frustrating news: when they decide your home is their winter destination, they can show up by the hundreds.
Here's what's happening, why your house is attracting them, and what you can actually do about it.
What Are Boxelder Bugs?
Boxelder bugs (Boisea trivittata) are distinctive enough that most homeowners recognize them once they know what to look for. Adults are about half an inch long, black with bright red-orange lines running along the edges of their body and wings. The nymphs, which are immature boxelder bugs, are smaller and almost entirely red. That's why some homeowners panic and think they have a completely different pest.
They spend most of the year living outdoors, feeding on the seeds, leaves, and flowers of boxelder maple trees and, to a lesser extent, silver maples and ash trees. If your yard or a neighbor's yard has a mature female boxelder tree, the kind that produces seeds, you're essentially providing a summer food source that can sustain a large population. According to the University of Missouri Extension, boxelder bugs mature through the summer and by August and September are fully grown adults ready to seek out a warm place to spend the winter.
That warm place is often your house.
Why Are They Swarming Your Home Right Now?
The timing isn't random. As days get shorter and temperatures start to drop across the St. Louis metro in late August and September, boxelder bugs get a biological signal to find shelter for winter. This behavior is called overwintering. They stop feeding and start searching for cracks, crevices, and voids where they can hunker down until spring.
Your home is an ideal target. Exterior walls, especially south-facing and west-facing surfaces, absorb heat from the sun and stay warm longer into the day. Boxelder bugs are drawn to that warmth. They'll congregate in large numbers on those warm surfaces, crawling toward any opening: gaps in siding, cracks around window and door frames, openings where utility lines enter the house, unscreened vents, and spaces under soffits.
Homes in Chesterfield, Kirkwood, Ballwin, and other St. Louis County communities with established, mature tree canopies tend to see heavier boxelder bug pressure simply because there are more host trees nearby. Boxelder bugs have been documented flying up to two miles to find a suitable overwintering site, meaning even if you don't have a boxelder tree in your own yard, your neighbors' trees can still send a swarm your way.
Are Boxelder Bugs Dangerous?
No. Boxelder bugs do not bite people, they don't sting, they don't carry disease, and they won't cause structural damage to your home. They're classified as a nuisance pest, meaning the problem is volume and annoyance rather than actual harm.
That said, there are a few things worth knowing.
They can stain. When crushed or disturbed, boxelder bugs release a reddish fluid that can leave a faint stain on light-colored siding, curtains, and fabrics. Avoid smashing them on surfaces you care about.
They smell. Like many true bugs, boxelder bugs produce a mild, unpleasant odor when handled or crushed. It's not overwhelming, but it's noticeable.
They come back. If boxelder bugs successfully overwinter in your walls, they'll emerge again in early spring. Sometimes hundreds of them appear inside your home on a warm February or March day as they try to work their way back outside. What looks like a fall exterior problem can turn into a very unwelcome spring indoor surprise.
How to Keep Boxelder Bugs Out of Your St. Louis Home
The most effective time to act against boxelder bugs is before they get inside, which means right now, in early fall, before temperatures drop far enough to drive them into wall voids for the winter.
Seal Entry Points
Walk the exterior of your home and look at it the way a boxelder bug would. Anything smaller than a pencil eraser is a potential entry point. Pay particular attention to:
Gaps and cracks in foundation walls
Spaces around window frames, door frames, and sills
Where utility pipes, cables, and wires enter the house
Torn or missing window and door screens
Gaps in siding, especially at corners
Unscreened soffit and attic vents
Caulk cracks in masonry and around frames. Replace damaged weatherstripping. Install door sweeps if there's a gap at the base of exterior doors. These exclusion steps are useful against boxelder bugs but also reduce entry for mice, stink bugs, Asian lady beetles, and other fall invaders.
Reduce Harborage Around Your Home's Foundation
Boxelder bugs love to hide in leaf litter, wood piles, and debris piled against the foundation. According to Michigan State University Extension, clearing a strip of debris and vegetation from around the base of your foundation, particularly on the south and west sides, reduces the staging area where bugs congregate before moving in.
Keep firewood stored away from the house and elevate it off the ground. Remove piles of leaves promptly. These steps pay dividends against multiple fall pests, not just boxelder bugs.
Consider Professional Perimeter Treatment
DIY sprays have limited effectiveness against boxelder bugs because the sheer volume of insects attempting to enter a home can overwhelm what a consumer-grade product can accomplish. Professional-grade perimeter treatments applied to exterior surfaces, including siding, foundation walls, and around frames, are significantly more effective at reducing the number of bugs that make it inside.
The window for effective exterior treatment is September through early October in the St. Louis area. Once temperatures drop and bugs have already moved into wall voids, exterior spraying has minimal impact on the population that's already inside.
At Pure Pest + Lawn, our approach follows integrated pest management principles, targeting pests strategically and only where needed, without broad broadcast applications that go beyond what's required. If you're seeing heavy boxelder bug pressure on your home in Fenton, Valley Park, Arnold, or anywhere in the St. Louis metro, a fall perimeter treatment is worth considering before they're inside your walls.
What to Do If Boxelder Bugs Are Already Inside
If boxelder bugs have already made it indoors, resist the urge to reach for a can of spray. Spraying insects that are inside wall voids is largely ineffective and can lead to other problems. The most practical approach is:
Vacuum them up. A vacuum with a bag or a detachable canister is the fastest way to remove live boxelder bugs you can see inside your home. Empty the vacuum promptly and dispose of the contents outside.
Don't crush them on fabric or walls. That reddish stain from crushed boxelder bugs can be stubborn.
Call a professional. If you're seeing large numbers indoors, especially if they seem to be emerging from wall voids, baseboards, or window casings, that's a sign of a larger overwintering population that may warrant a professional interior treatment.
The interior population will naturally die off over winter, but they can also emerge en masse on any warm winter or early spring day. Addressing the problem now is preferable to managing it all winter.
The Pure Pest + Lawn Plan
You shouldn't have to choose between living with a wall covered in bugs and guessing at DIY solutions that may not work. Here's how we can help:
Schedule a free inspection. We'll assess the boxelder bug pressure on your home and identify entry points and treatment opportunities.
We'll build a targeted plan. Whether that's a perimeter treatment, exclusion recommendations, or both, we'll design it around your specific situation.
Enjoy your home without the swarm. Our Pure Promise Guarantee means if covered pests return, so do we, at no additional charge.
Pure Pest + Lawn has been protecting St. Louis area homes since 1933. We have over 1,000 five-star reviews and hold the BBB Torch Award for Ethics. When fall pests start moving in, we know exactly what to do.
If boxelder bugs are already showing up on your home, don't wait. The sooner treatment is applied, the more effective it is. Call us at 314-222-PEST (7378) or schedule your free inspection online today.
