Fall Pest Prevention in St. Louis: The Checklist You Should Start in August

Fall pest prevention is one of those jobs that only works if you do it early. By the time you see the first mouse dropping on the counter or the first brown recluse in the bathtub, the animal has already been living in the structure for a while. Around the St. Louis metro the move indoors starts in late August and runs through October, tracking cooling nights rather than the calendar — so mid-August is exactly the right week to start.

None of this is complicated. It is mostly a caulk gun, a roll of hardware cloth, a ladder and an afternoon. Here is what is coming, and what actually keeps it out.

Nothing is “invading” — they are following a gradient

It helps to drop the invasion framing. Pests are not deciding to come after you in September. As overnight lows fall, your house becomes the warmest, driest, most sheltered object for a hundred yards in every direction, and it leaks that warmth and moisture out through every gap in its shell. Insects and rodents follow that gradient to its source, find an opening, and use it. Which means fall pest control is mostly a construction problem, not a chemical one: exclusion first, a properly timed exterior treatment second, and interior work only for what already got in.

The four you will actually deal with here

1. House mice

The house mouse is the number-one fall complaint in Jefferson County, and the biology is why. Per Truman’s Scientific Guide (Ch. 17), a female produces four to seven pups per litter, can have a new litter every three to four weeks, breeds at five to eight weeks old, and needs almost no free water because she gets moisture from food. It is also a nibbler — it feeds a little at many spots rather than one big meal, which is why droppings show up in five different places at once.

A mouse gets through a gap the width of a pencil. Look for droppings along baseboards and inside cabinets under the sink, gnaw marks on the corners of boxes, dark grease smudges where they run the same route against a wall, and scratching in the ceiling or wall at night. Truman is blunt about the fix, and so are we: the permanent solution is exclusion — sealing gaps with quarter-inch hardware cloth — plus sanitation. Trapping alone empties the house and refills it. Details on our rodent control page.

2. Brown recluse spiders

Missouri sits in the heart of brown recluse country, and unlike most pests on this list the recluse is not moving in from outside — established populations live inside St. Louis-area homes year round, in the parts of the house nobody touches. What changes in fall is that you notice them, partly because wandering males are on the move and partly because you are in the basement getting the holiday bins down.

Identification, per Truman: tan body, a violin-shaped mark behind the head with the neck pointing backward, and — the reliable one — six eyes in three pairs, where nearly every other spider has eight. They are genuinely shy and bite when pressed against skin, which is why so many bites happen in a sleeve, a shoe or a bed. The bite is medically significant and can produce a slow-healing wound, so see a doctor if you are bitten.

What helps, in order of how much it helps: get rid of cardboard in storage areas and use sealed plastic totes; pull beds a few inches off the wall and lose the bed skirt; shake out stored clothing, shoes and gloves; keep storage off the floor; and run sticky monitors along the wall-floor junction in the basement, garage and closets, which both catches spiders and tells you honestly how big the population is. Wolf spiders also wander indoors in fall — big, fast, hairy, web-less, and essentially harmless. More on both at our spider control page.

3. Boxelder bugs and brown marmorated stink bugs

These are the ones that show up as a crowd on the sunny side of the house on a warm afternoon in late September, then vanish. They did not leave. Boxelder bugs — black with red-orange lines — mass on south and west walls and then slip into wall voids and attics to overwinter, per Truman Ch. 15. The brown marmorated stink bug, shield-shaped and mottled brown, does the same thing and releases its odor when disturbed.

Two practical points. First, timing is everything: an exterior treatment applied before they cluster on the wall is worth far more than anything done after they are inside, and that window is roughly late August through September. Second, do not spray into a wall void full of overwintering insects. The insects die in the void, and a pile of dead insects in a wall feeds carpet and larder beetles, which then come out through the ceiling in spring. Indoors, use a vacuum. If they are already coming out at a light fixture or a ceiling seam, the real fix is sealing that penetration.

4. The dampness crowd

Camel crickets, house centipedes and silverfish are less about temperature and more about moisture. All three tell you something: a house centipede is a harmless predator that is there because other insects are there, silverfish want damp and starchy material like paper and cardboard, and camel crickets want a damp basement or crawlspace. If you are seeing them, the honest answer is usually a humidity and drainage fix rather than a spray. We cover the whole occasional-invader group on the general pest control page.

The exclusion checklist

Work it top to bottom. Everything here is homeowner-doable with hardware store materials.

• Doors. At night, have someone stand outside with a flashlight while you look from inside. Any light you see is a gap a mouse can use. Replace worn door sweeps and thresholds. The garage door is the worst offender in most houses — check the bottom astragal for compression gaps at the corners, and check the side and top seals too.
• Utility penetrations. Air-conditioning line sets, hose bibs, gas lines, electrical and cable entries are the classic mouse doors. Pack the gap with copper mesh and seal over it. Skip steel wool — it rusts and crumbles within a season.
• Vents. Cover crawlspace and foundation vents with quarter-inch hardware cloth. Screen gable and soffit vents. Confirm the dryer vent flap actually closes; a stuck-open flap is an open door for mice and birds.
• Roofline. Fascia and soffit joints, rotted trim, a lifted shingle at a valley, and an uncapped chimney all let in squirrels, bats and wasps. Seal these after stinging insect season, not during it — see why late-summer yellowjackets make that a bad idea. If you are not comfortable on the ladder, have someone else look at it.
• Windows and screens. Patch torn screens, re-caulk failed glazing and siding gaps, and check where siding meets brick.
• Landscaping. Pull mulch back so it is not piled against the foundation, trim shrubs and limbs so nothing touches the siding or roof, and move firewood at least 20 feet from the house and up off the ground. This one item reduces spider, ant, rodent and termite pressure at the same time.
• Water. Clean the gutters, extend downspouts away from the foundation, fix grade that slopes back toward the house, and repair a leaking hose bib. Moisture is the common denominator under nearly everything on this page.
• Storage. Cardboard out, sealed totes in — in the basement, the garage and the attic. This is the single highest-value thing you can do about brown recluse and silverfish, and it costs an afternoon.

While you are down there: check for mud tubes

The foundation walk you are doing anyway is the best termite check a homeowner ever gets. Pulling mulch back and moving the woodpile puts your eyes on the bottom two feet of the wall, which is exactly where eastern subterranean termites give themselves away.

You are looking for pencil-width tubes of mud running up the foundation, on the inside of a crawlspace pier, or on the joists overhead. Termites live in the soil, need constant moisture, and build those tubes to stay wet and hidden while they travel to the wood (Truman Ch. 8). Mud tubes and springtime swarmers are the two signs you get; the colony itself, which can exceed a million members and feeds around the clock, is out of sight in the soil.

If you find tubes, do not knock them all down before someone can look at them — leave a couple intact so we can tell active from abandoned. Roberts is a Certified Sentricon Specialist® (Corteva branch code 10244082), and the Sentricon® baiting system eliminates the colony rather than just treating the wood, with no trenching or drilling through your slab. Background on the pest itself is on our termite page.

Where a professional actually adds something

Plenty of the list above you can do yourself, and you should. What we add is timing, access and follow-through: an exterior perimeter treatment applied in the window before the clustering and the indoor push begins, treatment of harborage zones you cannot reach, monitoring that tells you whether a recluse population is a handful of spiders or a real one, and exclusion work at the roofline and crawlspace that is unsafe to do off a ladder.

Roberts Pest Control is locally owned, based in Festus, licensed and insured, and has been working Jefferson County and the St. Louis metro for over 20 years. Fall pests do not respect a service date, so if something new shows up between visits, call or text us and we will come back out.

More reading: Rodent control · Spider control · General pest control · Sentricon® termite protection