Earwigs, sowbugs and millipedes in the basement in autumn: the mulch and downspout link

Earwigs in the basement, grey sowbugs curled up by the floor drain, millipedes coiled on the laundry room floor: in a GTA house these turn up as the ground outside stays wet and the leaves start to pile against the foundation. None of them is dangerous, and several are useful in the garden. What makes them worth acting on is what they tell you about the ground next to your house. This guide covers how to tell them apart, why they come in, what to change outside this autumn, and when a spray is worth it at all.

Why are these bugs showing up in the basement?

They live in damp, sheltered ground and material right next to the house, and some of them wander through cracks into the lowest, dampest room. Health Canada says sowbugs and pillbugs need moist conditions to survive and live under rocks or debris, feeding on decaying organic matter. It describes earwigs hiding by day in cool, dark, moist places such as stones, garden rubbish, wooden fences and cracks, then crawling over the ground and climbing houses at dusk in search of food and shelter.

The basement is where the house meets that ground. A foundation bed with thick fine mulch, a pile of leaves against the wall, a downspout emptying beside the window well: each keeps the soil at the foundation wet, and each gives these animals a place to live a few centimetres from a crack. Health Canada puts it plainly for sowbugs, pillbugs and millipedes: a persistent infestation indoors may indicate a serious moisture problem within the home, together with a food source like rotting wood.

House centipedes have a specific autumn timing. Health Canada says they come indoors in the fall when the weather turns colder, and invade houses where the sub-floor, drains, basement or bathroom areas are overly damp.

Which one have you got?

Identification matters because the right response is different for each. Sowbugs and millipedes indoors are a moisture signal and will die without help; earwigs are an outdoor-population problem; house centipedes are hunting other insects.

Centipedes, millipedes, sowbugs and pillbugs are arthropods but not insects, and Health Canada calls the group beneficial: sowbugs, pillbugs and millipedes break down organic matter, and centipedes help control other pests. Its advice is to tolerate them as much as possible and to work on the source outside first.

What does mulch have to do with it?

Mulch piled against the foundation holds water next to the cracks and window frames that crawling animals use to get in. For earwigs, one of Health Canada's physical control steps is to create a clean, low-moisture perimeter around the foundation by trimming back vegetation and removing mulch, organic debris and other objects they can shelter under. For sowbugs, pillbugs and millipedes it recommends coarse mulch instead of fine, because coarse mulch lets water drain.

• Pull mulch back from the wall. Leave a strip of bare, dry ground between the bed and the foundation, especially under basement windows and around window wells.
• Switch fine mulch for coarse where the bed stays wet. Finely shredded mulch that mats into a damp layer is the type Health Canada's advice moves you away from.
• Clear leaf litter and dead plant material from the foundation. Health Canada lists leaf litter and decaying vegetation around the foundation as something to remove, along with stones, boards and boxes lying on the ground.
• Prune foundation shrubs. Health Canada says foundation plantings should be pruned and cleaned to improve ventilation around the home, so the soil under them can dry.
• Let the bed dry between waterings. An irrigation zone or soaker hose along the foundation keeps that strip wet all season; Health Canada's advice is to allow the soil to dry between waterings.
• Move stored items off the ground by the wall. Firewood, stacked pots, lumber, a tarp or a bin against the house is shelter. Health Canada also notes house centipedes hide under the bark of firewood stored indoors.

There is a trade-off with autumn garden cleanup. Standing perennial stems in the middle of a bed can shelter overwintering insects you want to keep, as covered in what to cut back in autumn and what to leave standing. The strip right against the foundation is different: that is the one to keep clear and dry, and the rest of the bed can stay as you planned it.

What do the downspouts have to do with it?

A downspout or tap that soaks the ground at the foundation creates exactly the damp strip these animals need. Health Canada's earwig advice is to repair leaky taps and downspouts and make sure water drains away from the foundation, and its sowbug and millipede advice starts with making sure the ground slopes away from the house so moisture does not sit at the foundation.

Halton Region gives the numbers: a minimum slope of 2% away from the house, with 5% ideal, which is a drop of 2 to 5 cm in every metre; downspouts ideally extended 3 m from the foundation onto ground that slopes away; and window wells kept free of debris. The full downspout and sump pump check is in sump pumps, downspouts and the first heavy autumn rain.

1. 1Look during or right after rainWalk the house while water is running. Note any downspout that empties next to the wall, any elbow that has come off, and any spot where water pools or runs back toward the foundation.
2. 2Check the eavestroughs above the wet spotsAn overflowing eavestrough drops water straight onto the bed below. In autumn, look for leaves in the trough or a blocked outlet; see how often eavestroughs need cleaning with mature trees nearby.
3. 3Look at the outdoor tapsA dripping hose bib wets the same patch of soil every day. Fix the drip before the tap is shut off for the winter.
4. 4Open the window wellsLeaves collecting in a window well hold water against the basement window and are shelter for sowbugs and earwigs. Clear them out and check the well drains.
5. 5Match what you found to the basementIf sowbugs keep appearing by one basement wall, start with whatever is wetting the ground on the outside of that wall.

What to do inside the house

Inside, the job is to seal the way in and make the basement too dry for them. Health Canada says to repair cracks in foundation walls or around windows before the fall, and to use a dehumidifier or a small electric fan to dry out damp areas in the basement. Its healthy home guide recommends keeping indoor humidity between 30 and 50%, installing a dehumidifier in damp areas such as a basement, and storing things in plastic bins with lids instead of cardboard.

• Sweep or vacuum up what you find. Sowbugs, pillbugs and millipedes die quickly indoors because the house is too dry for them, so there is nothing to treat once they are picked up.
• Measure the humidity. A hygrometer in the basement tells you whether you are inside the 30 to 50% range or well above it. Silverfish preferring 75 to 95% relative humidity gives you a sense of what a very damp room looks like to these animals.
• Seal the gaps at ground level. Cracks in the foundation wall, gaps around basement window frames, and openings where pipes and lines come through the wall.
• Fix sweating and leaking pipes. Health Canada's healthy home guide says to check basement pipes for condensation or dripping and insulate them to reduce humidity.
• Get cardboard off the floor. Boxes stored on a damp floor are shelter and, in the case of silverfish, food.

Should you spray for earwigs or sowbugs?

Usually not, and not indoors for sowbugs, pillbugs or millipedes. Health Canada says using a pesticide indoors for those three is not recommended, because they soon die from dehydration once inside. For earwigs it says indoor treatment is an option when they are a persistent problem, but that they will eventually die out on their own and the more effective long-term solution is to find and treat the outdoor source.

Timing is the other reason to hold off in September. Health Canada gives the best time to start earwig control as early spring, when the earwigs are young, and the best time to apply insecticides as warm, dry weather in June or early July. It says treatment later in the summer is less effective. In autumn the useful earwig work is the habitat work above, plus trapping if you want to see how many there are.

• Rolled corrugated cardboard. Roll a piece up, hold it with a rubber band and stand it on end near the foundation, where earwigs use it for shelter.
• Upside-down flower pots. Stuff with moistened straw or newspaper and prop one edge up on a small stone.
• Short lengths of old garden hose or bamboo laid on the ground.
• Empty the traps into soapy water. Health Canada says the traps must be checked regularly and the earwigs dropped into a pail of soapy water.

If you do buy a product, Health Canada says to look for a Pest Control Products (PCP) number on the label and to follow the label directions. It warns not to use earwig baits in the home and to keep children and pets away from baits. Diatomaceous earth is the lower-risk option it describes for crawling pests: it works only while it stays dry, and you should avoid breathing the dust when applying it.

Outdoors, Ontario's cosmetic pesticide ban applies. On lawns, vegetable and ornamental gardens, patios and driveways you may only use products whose active ingredients are all on the province's Allowable List, which includes household examples such as soap and borax. The province's exceptions for health or safety cover indoor pests and pests that can cause structural damage to the home. Read the label and the Allowable List before you buy anything for the foundation beds.

What to do this weekend, in order

1. Note where in the basement the bugs appear and which wall that is on the outside.
2. Walk that wall outside: mulch against the foundation, leaf litter, objects on the ground, a downspout or tap soaking the soil, a window well full of leaves.
3. Pull the mulch back, clear the leaves and debris from the strip against the wall, and move stored items away from it.
4. Redirect or extend the downspout so it discharges away from the house, and fix any dripping tap.
5. Seal the cracks in the foundation wall and around the basement windows you can reach.
6. Run a dehumidifier in the damp room and check the humidity with a hygrometer.
7. Sweep up what is already inside, then watch whether the numbers drop before deciding anything else is needed.

Everything on this list is DIY work. The only part worth handing off is the ladder work, if clearing the eavestroughs is more than you want to do; that is what gutter cleaning and repairs covers. For insects gathering on the upper walls rather than crawling in at ground level, see lady beetles, cluster flies and stink bugs on the house in autumn.