Why are bugs covering the sunny side of my house every fall, and how do I get their stains off the siding?

Why do bugs cover the sunny side of a house every fall?

Bugs cover the sunny side of a house every fall because they are looking for a sheltered place to spend the winter, and a warm wall full of gaps is the best option they can find. In nature these insects pass the winter beneath bark. As days shorten they enter diapause, a resting state in which they stop reproducing and eat little or nothing, and they gather on warm surfaces while they search for a crevice to crawl into.

In Michigan the gathering runs from late summer into fall. Michigan State University (MSU) Extension puts the start of stink bug gathering at about the second or third week of September, once daylight drops below 12.5 hours, and boxelder bugs and lady beetles arrive on the same walls in the same late-summer and fall window. The siding, soffits and window frames of a house offer the same nooks bark does, and many of those gaps lead indoors.

What are boxelder bugs and Asian lady beetles, and how do you tell them apart?

Boxelder bugs and multicolored Asian lady beetles are two common fall invaders of Michigan homes, according to MSU Extension, and they belong to different insect orders. The boxelder bug is a native true bug that feeds on boxelder and related maples, and MSU Extension notes it can be quite abundant in some parts of Michigan in certain years. The Asian lady beetle is a beetle that spends the summer eating aphids in fields and gardens. Brown marmorated stink bugs and western conifer seed bugs can turn up on the same walls in late summer or fall, and MSU Extension notes that none of these true bugs transmit disease or sting, and that they do not reproduce over the winter.

Why do they gather on south and west walls?

Fall invaders gather on south and west walls because those walls collect the most afternoon sun and stay warmest as nights cool. MSU Extension identifies south- and west-facing walls as the most susceptible to large populations, and University of Minnesota (UMN) Extension says boxelder bugs are drawn to buildings with a lot of southern or western exposure. UMN adds that lady beetles favor the sunny sides of tall or prominent buildings, especially ones with strong contrast between dark and light.

That describes many houses across Macomb and Oakland County: a light vinyl colonial with dark shutters, or a brick ranch with white trim, facing south or west across an open lawn. UMN also notes that boxelder bugs can fly several blocks and as far as two miles, so a house does not need a boxelder tree in the yard to collect them.

What stains do they leave on siding, trim and windows?

Boxelder bugs and lady beetles leave two kinds of stain on siding, trim and windows: excrement spots and a yellowish fluid. UMN Extension notes that boxelder bug excrement can stain walls, curtains and other surfaces, and that lady beetles release a strong-smelling yellowish liquid from their leg joints that can stain light-colored surfaces. MSU Extension adds that lady beetle secretions can stain fabric. Neither insect chews or damages siding itself.

The marks concentrate where the clusters sit longest: under soffits, around window frames and corner trim, along the top of the south wall and on the sunny side of the garage. White and almond vinyl, painted trim and window screens show them first. Window tracks and sills also collect dead insects over the winter, because both insects move toward sunny windows once indoors and many die there. Spring window cleaning clears tracks, screens and frames along with the glass.

Should siding be washed while the bugs are still clustered on it?

Siding is usually better washed after the fall clusters have moved on than while the bugs are still on it. MSU Extension notes that a strong force of water can knock insects off exterior walls, but that more are likely to return on the next warm day, and UMN puts boxelder bug flights at up to two miles. A wall washed mid-cluster looks clean for an afternoon and can be speckled again within the week.

Weather sets the timing. UMN Extension reports that in Minnesota lady beetles fly to buildings from late September to late October on days above 65°F, so the walls are quietest after a cool spell has sent the clusters into hiding for the winter. Some homeowners book the wash for late fall, and others leave the stained wall for the spring wash, when the insects have left. If the house has to be washed sooner, knocking clusters into a bucket of soapy water or vacuuming them up before the wash keeps the count down. The rest of the calendar is in our Michigan house-washing timing guide.

How do you clean the stains without pushing bugs into gaps?

Insect stains come off siding without pushing bugs into gaps when the wall gets a low-pressure soft wash with the spray aimed down the wall, never up into the laps. An upward stream drives water, and any insects in its path, behind the laps, as our guide to whether pressure washing can damage vinyl siding explains.

1. Clear the clusters first. Brush or vacuum loose insects off sills and trim instead of crushing them against light surfaces.
2. Pre-wet the plants below the wall. Wet foliage sheds any cleaner that drifts.
3. Apply a house-wash solution at low pressure. Let it dwell for several minutes so the spots and yellow residue loosen.
4. Brush the trim and frames by hand. A soft brush lifts stains from window frames, corner boards and soffit edges.
5. Rinse top to bottom at low pressure. Finish with screens, sills and window tracks.

Those steps describe a soft washing job, which Macomb Power Washing Solutions prices from $349 for a full house. Washing removes the stains; it does not treat the insects, so sealing and cleaning work as a pair. Spider webs and fish fly marks on the same walls are covered in our guide to removing fish flies and spider webs from siding.

How do you seal entry gaps without blocking weep holes?

Entry gaps get sealed with caulk, screen and weatherstripping, while weep holes in brick or stone veneer are left open and fitted with ventilation plugs. UMN Extension advises sealing openings 1/8 inch or larger, because lady beetles fit through gaps that small. It recommends silicone, elastomeric latex or silicone-acrylic caulk for cracks, and caulk, expanding foam or copper mesh where utility lines, pipes, outdoor faucets and dryer vents enter, plus door sweeps, a rubber seal along the garage door and repaired window screens.

Weep holes are the small openings near the bottom of brick or stone veneer, and they let water that gets behind the masonry drain back out. UMN Extension says they must not be sealed and suggests ventilation plugs that let water out but keep insects from crawling in. For older houses with vinyl siding, UMN notes there may be too many gaps to keep boxelder bugs out completely, while newer houses with house wrap under the siding keep most insects out. UMN times the work before the end of August for boxelder bugs and by the end of September for lady beetles.

What does fall bug cleanup look like at a glance?

Fall bug cleanup is a short sequence: seal in late summer, leave the clusters alone while they are active, and clean once they have gone. The table lays out that year for a Macomb or Oakland County home.

When you book house washing, mention which walls the bugs favor and where the stains sit. Quotes are itemized, written and delivered inside 24 hours, with travel never billed.