Lady beetles, cluster flies and stink bugs on the house in autumn: keeping them out in the GTA

Lady beetles on the house in fall, a window full of slow flies on the first warm day, a brown shield-shaped bug on the curtain: in the GTA these show up at the same time of year for the same reason. Each one is looking for a sheltered place to spend the winter, and a house wall facing the afternoon sun is where they gather. This guide covers how to tell the three apart, which gaps they use, what to do this week while they are still outside, and what actually works once some are in.

Why are insects gathering on the south side of the house right now?

They are moving to winter shelter, and warm walls are where the search ends. Health Canada describes cluster flies starting to look for winter shelter when the days shorten in late summer and early fall, sunning on the south and west sides of buildings during the day and crawling into small openings in the siding or under the eaves as the sun goes down. Ontario's agriculture ministry says the same shortening day length in August and September sends new brown marmorated stink bug adults toward their overwintering sites, which include homes and woodpiles.

Multicoloured Asian lady beetles follow the same pattern. Health Canada notes they are attracted to homes and buildings with sunny exposures, cluster on outside walls, and eventually work their way indoors through cracks and openings. It also says to focus on the sunny, southwest sides of your home, because shady areas are much less affected. That gives you the order of work: the sunny walls first, the shaded north side last.

Location changes the odds. Health Canada says homes near fields or forested areas are more susceptible to lady beetles, and that cluster flies are more common in houses surrounded by large lawns or backing onto open parks. A house backing onto a ravine, a field or a park in Oakville, Burlington or Milton is in the higher-risk group for at least one of the two.

How to tell the three apart

Identification matters because the right response differs for each one, including whether a product is even registered for it.

The two white bands on the antenna are the feature Ontario's agriculture ministry calls diagnostic for the brown marmorated stink bug. The ministry lists it as established in parts of southern Ontario, and the Invasive Species Centre records its first Canadian detection in Hamilton in 2010. The Invasive Species Centre asks for sightings to be reported, so a clear photo of one on your wall is useful to send.

Where do they actually get in?

Through gaps you can mostly see from a step-ladder or the ground. Health Canada's list for lady beetles is doors and window frames, eaves, utility openings, foundations and wall siding. For cluster flies it adds that they can enter through the tiniest cracks around door and window frames, and it recommends insect screening over the air vents in soffits. Ontario's advice for stink bugs is to seal all cracks, repair or replace damaged screens and remove or cover window-mounted air conditioners before the fall.

• Window and door frames. Look at the joint where the frame meets the siding or brick. Old caulk that has shrunk, cracked or pulled away on one side is an open joint.
• Screens. A torn screen, or one that no longer sits tight in its frame, lets them straight in. Health Canada puts properly fitting screens first on its list for cluster flies.
• Window air conditioners. Ontario's stink bug advice names them specifically: remove the unit or cover it. If it is coming out for the winter anyway, this is the week.
• Soffit vents. A torn or missing vent screen under the eaves gives cluster flies a direct route into the attic.
• Utility entrances. Where the cable, phone, gas, dryer vent and outdoor tap pass through the wall, check for a ring of open space around the pipe or line.
• Siding and foundation. Cracks along the top of the foundation and gaps where siding meets trim on the sunny walls.

Much of this list overlaps with rodent-proofing, which is covered in what autumn yard work changes about rodents. Insects fit through far smaller gaps than a mouse, so a house sealed for rodents can still let insects through the caulk joints around its windows.

What to do this week, while they are still outside

Sealing only works before they are in. Health Canada calls prevention the only way to control lady beetles, and Ontario calls mechanical exclusion the most effective way to keep stink bugs out. The work is small and most of it is a do-it-yourself job for anyone comfortable on a step-ladder.

1. 1Walk the south and west walls on a sunny afternoonWarm afternoons are when the insects are on the wall, so they show you where they collect. Note every window and door on those sides, and any spot where they are crawling into the siding or trim.
2. 2Check every screen on those wallsReplace or repair torn mesh and reseat any screen that has come loose in its frame. A window that still gets opened on warm autumn days needs a good screen until you stop opening it for the season.
3. 3Re-caulk the gaps you foundHealth Canada recommends caulking around windows and door frames and sealing cracks and crevices in the siding and foundation. Cut out caulk that has lifted before applying new, or the new bead sits on a joint that is already open.
4. 4Add or renew weatherstrippingBoth Health Canada pages list weatherstripping on windows and doors. If daylight shows under a closed exterior door, fit or replace the door sweep.
5. 5Deal with the window air conditionerTake it out, or cover it and seal around it if it stays in over the winter, as Ontario's stink bug advice recommends.
6. 6Look up at the soffit vents and utility linesNote any torn vent screen and any unsealed gap around a pipe or cable. Vent screens under a second-storey eave are a ladder job at height; decide honestly whether that one is yours to do.

Once they are inside: vacuum them up

Vacuum them up and throw the contents out. Health Canada says to sweep or vacuum lady beetles, then seal and discard the bag so they cannot escape back into the house. Ontario says stink bugs can be removed with a vacuum or shop vac, and to dispose of the bag or contents promptly because the odour will persist. For cluster flies, Health Canada notes that squashing them releases an unpleasant odour and can stain surfaces, so a vacuum is the cleaner option there too.

Sprays are where the three insects differ most, and the table below is the reason identification comes first.

Even for cluster flies, Health Canada puts screens, weatherstripping and caulking first and treats products as the option for a bad infestation, with a professional pest control operator as a further choice. It also warns against trying to control cluster flies by killing earthworms in the lawn, even though cluster flies are parasites of earthworms, because earthworms keep the soil fertile and aerate the lawn.

Why they reappear in the windows in March

The insects in the windows on the first mild days of spring mostly got in during the autumn. Health Canada describes lady beetles hibernating in attics and wall voids, coming out in mild weather, and a larger movement for several days in late winter or early spring as they try to get back outside. It describes cluster flies the same way: hibernating in the walls or attic and gathering in windows when warmer days make them active.

Two things follow. First, a spring window full of flies is a sign of which gaps were open last autumn, so note which rooms they appear in and check the outside walls of those rooms next September. Second, dead insects left in wall voids are not always harmless: Health Canada notes that cluster flies dying in wall voids may attract larder beetles, which feed on them and then move into other parts of the house.

The cleanup: sills, tracks and the outside of the glass

Autumn invaders leave a mess where they gather. Dead insects collect in window tracks and on sills, cluster flies leave stains on walls and curtains, and anything crushed against the glass leaves a smear on the outside. The order that avoids doing the job twice is to finish sealing first, let the insects that are already inside die off or be vacuumed up, and then clean the sills, tracks and glass.

• Vacuum the tracks with a crevice tool before wiping them. Wiping first pushes insect remains into the corners and the drainage slots at the bottom of the frame.
• Empty and dispose of the vacuum contents after a stink bug cleanup, not at the end of the week.
• Clean the outside of the south and west glass after the sealing work, since caulking and screen work leaves fingerprints and dust on the glass.
• If the exterior clean is the last one of the year, time it with the leaves and gutters as described in the last outside window clean before winter.

What to do yourself, and when to call someone

Most of this is a homeowner job. Checking screens, caulking frames within reach, fitting a door sweep and vacuuming the insects that got in need no one else. Call someone when the gaps are at height, such as soffit vents and trim on a second storey; when the insects are coming from inside a wall or the attic in numbers that vacuuming does not reduce, which is when Health Canada mentions a pest control operator; or when the cleanup includes second-storey glass, screens and tracks you cannot reach safely. Our window cleaning service covers exterior and interior glass, screens and tracks, and the eavestroughs above those soffits are covered by gutter cleaning.