Why Are There Suddenly So Many Cockroaches in Your House?

TL;DR

• In most cases they were already there. Roaches are most active at night and can go unnoticed for some time (Health Canada). Seeing them in daylight or in new rooms means the colony has been growing.
• German cockroaches breed fast. Each egg capsule holds up to 48 eggs, so a problem can look stable for months, then seem to explode when several capsules hatch close together.
• Think back a few weeks. Roaches ride in on used furniture, food items and shipping cartons (Health Canada), or travel from a neighbour's unit along pipes and wires.
• Saw more right after spraying, yours or a neighbour's? Sprays and foggers can scatter roaches and push them out of hiding (Penn State Extension).
• Weather rarely explains it. German cockroaches generally live indoors all year, so cold rarely drives them inside. Usually only outdoor species, like Oriental roaches in dry spells, move in with the weather.
• Need help? Get a quote from a recommended local exterminator through Pest Agent's referral form.

A sudden wave of cockroaches in your house is almost never sudden. In most homes the roaches have been there for weeks, breeding behind the stove and inside the cabinets while everyone slept, and what changed is that the colony ran out of room to hide or something new joined it.

The cause matters more than the count. A colony that has only just become visible has been growing out of sight for a while, and the trigger behind it, whether a second-hand couch, the unit next door or a spray that flushed roaches out of the walls, tells you where to start looking.

Why Are There Suddenly So Many Cockroaches in Your House?

Cockroaches in your house seem to appear suddenly, but in most cases they were already there. Cockroaches are most active at night, so, as Health Canada notes, they can go unnoticed for some time. What feels sudden is usually the moment the colony becomes visible, or the moment something new adds to it or disturbs it. These are the six causes to check.

A Hidden Population Got Too Big to Hide

A hidden population that outgrows its hiding spots is the most common reason you suddenly see cockroaches. During the day, German cockroaches stay packed in cracks, crevices and other dark, warm, humid spots. Penn State Extension explains that they tend to be seen in daylight mainly when a heavy population is present or when they are under stress, such as a lack of food or water. According to the University of Kentucky, when populations are large or food is scarce, German cockroaches can also spread from the kitchen and bathroom into bedrooms, closets and other rooms with no food. So roaches in daylight, or in a room where you never saw them before, are both signs the colony has been growing for a while.

German Cockroaches Breed Fast

German cockroaches breed fast enough to turn a few roaches into a visible infestation in a matter of weeks. According to Penn State Extension, each egg capsule holds up to 48 eggs (usually 30 to 48), hatches about 28 days after it starts to form, and the young typically reach adulthood in 6 to 31 weeks. Assuming two generations a year, one female can lead to over 10,000 descendants in a year. That is why a problem can look stable for months and then seem to explode when several egg capsules hatch close together. Our guide on how a cockroach infestation starts explains the early stages.

Roaches Hitchhiked In on Something New

Roaches often hitchhike into a home on something new, and German cockroaches almost always arrive this way. Health Canada says roaches and their egg cases ride in on used furniture, food items, bottles and shipping cartons, and the University of Kentucky adds produce boxes, beverage cartons and grocery bags. UF/IFAS notes that German cockroaches do not come in from outdoors at all: people carry them in. Think back a few weeks: a used appliance, a second-hand couch, a stack of delivery boxes or a recent move are common starting points. Our guide on where roaches come from covers every entry route.

Roaches Are Moving In From Next Door

Roaches moving in from next door is a frequent cause in apartments, condos and semi-detached homes. The University of Kentucky explains that German cockroaches travel between rooms and adjoining apartments along plumbing pipes, electrical lines and openings inside walls. A neighbour's infestation can therefore become yours without anything changing in your unit. The Government of Manitoba also warns that tenants who treat with store-bought chemicals may make the problem worse, and UC IPM notes that sprays may disperse cockroaches to other areas, so a neighbour's spraying may push roaches your way.

A Spray Pushed Them Out of Hiding

A spray or fogger can push cockroaches out of hiding and make them look more numerous. Penn State Extension says residual sprays and aerosol foggers used indoors are of little value and may disperse cockroaches, which makes control harder and longer. UC IPM adds that sprays may repel roaches into other areas without killing them. Penn State also lists a pesticide application among the stresses that may bring roaches out in daylight. If you saw more roaches right after spraying, or after a treatment in a neighbouring unit, this is the likely reason.

Easier Food and Water

Easier food and water can make a resident population grow and roam more. Health Canada says cockroaches are often attracted to foods that contain starches, sugars and protein and tend to favour warm, damp spots like kitchens and bathrooms. A new leak under the sink, dishes left out overnight, a pet bowl that stays full, or a garbage bin without a tight lid can all feed a colony that was small before. Our guide to what attracts roaches lists the main attractants room by room.

Can Weather Cause a Sudden Cockroach Infestation?

Weather rarely explains a sudden cockroach infestation in a Canadian home, because the most common indoor species rarely lives outdoors. Weather can matter for a few outdoor species, though, so it helps to know which cockroach you are seeing. The species tells you a lot about the cause.

German Cockroaches Live Indoors All Year

German cockroaches live indoors all year, so cold weather does not drive them inside. Health Canada describes them as about 1.3 cm to 1.6 cm long and not very tolerant of the cold, yet they have been found in buildings as far north as Nunavut, because heated buildings give them everything they need. UF/IFAS notes that German cockroaches generally do not enter structures from outdoors. If you see small, light brown roaches with two dark stripes behind the head in your kitchen, the cause is inside the building, not the season.

Outdoor Cockroaches and Dry Weather

Outdoor cockroaches, mainly Oriental and American cockroaches, can come inside when conditions outdoors change.

Penn State Extension notes that Oriental cockroaches, dark and about one inch long, are generally found outdoors in warm weather, and that in periods of drought there may be considerable movement into buildings. They come in under doors, through air ducts and along plumbing. American cockroaches can forage from sewers into the ground floor of buildings (UC IPM), and both species may enter through sewer and drain lines (University of Kentucky). If large, dark roaches show up near floor drains, in the basement or in a tub, read how to stop cockroaches from coming up the drain.

Wood Cockroaches in Spring and Summer

Wood cockroaches can show up in numbers in spring and summer, especially in homes near woods. Orkin Canada says the Pennsylvania wood cockroach is one of the most common wood roaches across Canada and that males may fly toward lights in May and June while looking for mates. Penn State Extension explains that wood cockroaches come in on firewood, through gaps around doors and windows, or drawn to porch lights, and that indoors they usually die within a few days from dehydration and do not feed or reproduce. A sudden wave of them is a nuisance, not an infestation.

Is a Sudden Increase a Sign of a Big Cockroach Infestation?

A sudden increase in cockroaches in your house usually means the infestation is bigger than what you see. The good news is that a sudden infestation says nothing bad about you, and acting early limits how far it spreads.

Seeing Roaches in Daylight

Seeing roaches in daylight is one of the clearest size signals. UF/IFAS says that when cockroaches are out in the open during the day, there is a high probability that the population is large, and the Government of Manitoba also lists daytime sightings in bright areas as a sign of infestation. For a full checklist of droppings, egg cases and odours, see our guide to the signs of a cockroach infestation, and for the numbers question, read how many cockroaches is an infestation.

Does It Mean Your House Is Dirty?

A sudden cockroach problem does not mean your house is dirty. Health Canada says the presence of cockroaches does not necessarily mean unclean conditions exist, and the University of Kentucky notes that even the cleanest home can become infested. Montreal public health estimated that in 2018 about 3% of households in the city had had cockroaches, and stresses that anyone can have them, regardless of social class or origin. Cleanliness still matters, because crumbs and spills help a colony grow faster.

Health Risks

The health risks of cockroaches come from contamination and allergens, not bites. Health Canada explains that cockroaches contaminate food and food preparation areas by walking over them after passing through garbage or sewers, and that people with asthma may react to their droppings and body parts. The Government of Manitoba notes that they can carry bacteria such as salmonella and E. coli; NC State Extension says they can transmit Salmonella. If anyone in your home has asthma or allergies, it is one more reason to act quickly.

What Should You Do About a Sudden Cockroach Infestation?

A sudden cockroach infestation is easiest to stop in the first weeks, before the next egg capsules hatch. Start with these steps, then follow our full guide on what to do if you have cockroaches.

Set Sticky Traps

Sticky traps show where the roaches are coming from and how many there are. UC IPM calls them the best way to detect and monitor cockroaches. Place them flat against the wall where floors meet walls, under sinks and beside the refrigerator, and check them every day. The traps with the most roaches point you toward the hiding spot, and a room with sudden catches near a shared wall or pipe often means roaches are coming from a neighbour.

Cut Off Food and Water

Cutting off food and water slows the colony and makes baits more attractive. Montreal public health recommends these habits:

• Do not leave dirty dishes on the counter, especially at night.
• Keep food in sealed glass or plastic containers, because cardboard and paper are not cockroach-proof.
• Do not leave your pet's food or water bowl out.
• Put garbage in containers with tight-fitting lids and take it out regularly.
• Fix water leaks, including drips under sinks.

Seal Utility Openings

Sealing utility openings cuts off the paths roaches use between rooms and units. The University of Kentucky recommends caulking or plugging every opening where plumbing pipes or wires pass through walls or floors, which is especially useful in apartments. Montreal public health also suggests rubber seals under outer doors and fine-mesh screens on conduits.

Use Baits Instead of Sprays

Baits work better than sprays on a sudden cockroach infestation, because sprays scatter roaches into new spots. UC IPM says baits give better control than sprays but do not work instantly, often taking 7 days or longer to show results. Health Canada notes that bait stations kill roaches about 48 to 96 hours after they eat the active ingredient. Choose a product with a PCP number on the label, which shows it is registered by Health Canada, follow the label, and keep baits out of reach of children and pets. Do not spray near the baits, as UF/IFAS warns that repellent products make baits ineffective. Our picks for the best cockroach killer compare the options.

Check What You Bring Home

Checking what you bring home stops new roaches from restarting the problem. Inspect used furniture, appliances and food deliveries before they come inside, and unpack boxes quickly instead of storing them. When moving, Montreal public health advises checking electrical appliances, furniture and food for insects before settling in.

Tell Your Landlord

Telling your landlord right away is the most important step if you rent. The Government of Manitoba says that once landlords are told, they are generally responsible for making sure units are pest-free, and that getting rid of cockroaches takes teamwork in multi-family buildings. In Montreal, if the owner does not act within a reasonable time, tenants can contact their borough by calling 311. Check your provincial tenancy rules for the process where you live.

When to Call a Pest Control Professional

Calling a pest control professional makes sense when you see roaches in daylight, in several rooms, or still after a few weeks of careful baiting, and in any multi-unit building where the problem may span several units. Montreal public health advises tenants not to treat their home themselves and to let a professional handle it, an option the University of Kentucky also mentions. A licensed pest control professional will inspect, identify the species and combine baits, dusts and monitoring. Read what a cockroach exterminator does, or get a quote from a recommended local exterminator through Pest Agent's referral form.