The Australian Guide to Cockroaches

Start here: two kinds of cockroach problem

Almost every household cockroach problem in Australia falls into one of two categories. Working out which one you have is the single most useful thing you can do, because the causes, the urgency and the solutions are different.

Indoor breeders. This is mainly the German cockroach: small (10–15 mm), light brown, with two dark stripes on the shield behind the head. German cockroaches live and breed inside, usually in kitchens and bathrooms. They spread on belongings, multiply quickly and rarely go away on their own.

Outdoor visitors. These are American, Australian and smokybrown cockroaches: large (30–45 mm), reddish to dark brown. They live in drains, gardens, sub-floors and roof voids, and come inside mostly on warm, humid nights and after rain.

A useful rule of thumb: many small cockroaches, especially babies, mean an indoor breeding population. An occasional large cockroach means visitors from outside. If you are not sure, see how to identify a German cockroach or use the pest identifier.

Household cockroach species compared

Australia has several hundred native cockroach species. Most live in bushland leaf litter, under bark and in soil, and very rarely come indoors — the large burrowing cockroaches of Queensland are a well-known example. Household problems come from a handful of introduced species that have adapted to living with people.

• German cockroach: 10–15 mm, light tan with two dark stripes behind the head. Lives indoors near food, water and warmth. Breeds fastest of all household species.
• American cockroach: 35–45 mm, shiny reddish-brown with a pale band around the edge of the shield behind the head. Closely associated with sewers, drains and commercial kitchens.
• Australian cockroach: 30–35 mm, reddish-brown with a bright yellow margin behind the head and short yellow streaks at the base of each wing. Common in gardens, mulch and roof voids in warmer regions. Despite the name, it is probably not native.
• Smokybrown cockroach: 30–38 mm, uniformly dark mahogany with no pale markings. Lives in trees, gutters and roof spaces and flies readily to lights on humid nights.
• Oriental cockroach: 20–27 mm, glossy dark brown to black with short wings (males) or wing stubs (females). Cannot fly. Prefers cool, damp places such as drains, and is more likely in southern Australia.

Young cockroaches (nymphs) look like smaller, wingless adults and are often darker. The nymphs of large species lack the two clear stripes of German cockroaches, which is the easiest way to tell them apart.

How cockroaches live and breed

Cockroaches develop by incomplete metamorphosis: egg, then several nymph stages, then adult. There is no larva or pupa. Females produce purse-shaped egg cases (oothecae) containing many eggs.

German cockroaches carry the egg case until the eggs are nearly ready to hatch, which protects the eggs from dryness and many treatments. Each case releases around 30–40 nymphs. In warm conditions a German cockroach can go from egg to breeding adult in roughly two to three months, so a few hitchhikers can become a visible infestation within a season. See how fast cockroaches breed.

Large outdoor species drop or glue their egg cases in hidden, humid places — sub-floors, roof voids, drains and garden debris — and take much longer to mature. Their numbers are driven more by weather and outdoor conditions than by what happens in your kitchen.

Cockroaches are mainly nocturnal. They spend the day packed into tight gaps where their bodies touch surfaces above and below, and come out at night to feed. Regular daytime sightings usually mean hiding places are crowded or a population has been disturbed. See why cockroaches come out at night.

Why cockroaches are in your home

Cockroaches follow three things: shelter, water and food. Kitchens and bathrooms supply all three, and appliance motors add warmth. Water is often more limiting than food, which is why cockroaches concentrate around sinks, dishwashers, fridges, laundries and bathrooms.

Common triggers for a sudden appearance include:

• Hot, humid weather that brings outdoor species out of drains and gardens.
• Heavy rain flooding their outdoor habitat. See cockroaches after rain.
• Dry drain traps in spare bathrooms, laundries and holiday homes.
• New deliveries, cardboard boxes, second-hand appliances or furniture carrying German cockroaches.
• A neighbouring infestation in an apartment building, or a treatment next door pushing cockroaches through shared walls.
• Renovations or moving appliances, which disturb established hiding places.

For specific situations, see why you suddenly have cockroaches and cockroaches in the kitchen.

Where cockroaches hide

A torch inspection after dark is the best way to find where cockroaches are living. Check:

• Behind and under the fridge, dishwasher, oven and microwave, including inside motor housings.
• Cupboard hinges, drawer runners, the underside of benchtops and behind kickboards.
• Under the sink, around pipe penetrations and the dishwasher connection.
• Bathroom vanities, behind toilets and around floor wastes.
• Laundry tubs, the washing machine and the hot water system.
• Outdoors: drains, sewer pits, compost, mulch, palm fronds, woodpiles, sub-floors and roof voids.

Signs include pepper-like droppings, brown smear marks in corners, empty egg cases and shed skins, and a musty, oily smell in heavier infestations. More detail in where cockroaches hide.

How cockroaches get inside

Adult cockroaches can squeeze through gaps only a few millimetres high. Common routes are gaps under doors, weep holes and vents, gaps around pipes and cables, dry floor wastes and shower drains, and connections from roof and wall cavities. German cockroaches are mostly carried in rather than walking in.

Drains deserve special attention. Every floor waste and sink has a water trap that blocks sewer gases and insects. When the water evaporates, the trap becomes an open door for American cockroaches from the sewer. Read can cockroaches come through drains and how cockroaches get inside.

Health implications

Cockroaches are not venomous and very rarely bite, but they are a genuine hygiene and health concern.

• Food contamination. Cockroaches move between drains, rubbish and food areas and can carry bacteria such as Salmonella on their bodies onto food and preparation surfaces.
• Allergens. Their droppings, saliva and shed skins contain allergens that build up in household dust and can trigger allergies and asthma, particularly in children.
• Food businesses. Pest control is part of food-safety requirements for businesses that handle food, and cockroach activity is taken seriously by food-safety regulators.

If anyone in the household has asthma or allergies, dealing with an infestation promptly and cleaning up droppings reduces exposure. See are cockroaches dangerous.

Prevention: a room-by-room checklist

Kitchen

• Wipe benches and sweep floors every night; do not leave dishes, food or pet food out overnight.
• Store dry food in sealed hard containers; rinse recycling and empty bins regularly.
• Clean behind and under the fridge, stove and dishwasher a few times a year, and degrease range hoods, hinges and drawer runners.
• Fix dripping taps and dry the sink at night.
• Reduce cardboard storage, which cockroaches hide and breed in.

Bathrooms and laundry

• Run water into every drain weekly, especially in spare bathrooms and laundries.
• Replace missing or damaged drain grates.
• Seal gaps behind vanities, toilets and tubs; improve ventilation to reduce humidity.

The building

• Seal gaps around pipes and cables where they pass through walls and floors.
• Fit door sweeps and seals; repair fly screens.
• Use weep-hole covers rather than blocking weep holes.

Outside

• Keep mulch, compost and firewood away from the walls.
• Trim palms and trees back from the roof and clean gutters.
• Reduce bright lighting near doors in summer, or use warmer bulbs.

Our wider pest-proofing guide covers these steps for all common pests.

How cockroach treatment works

German cockroaches are usually managed with a program rather than a single spray:

• Inspection to find where they are living.
• Gel baits placed in small amounts into cracks and crevices where cockroaches shelter. Cockroaches eat the bait and share it through droppings and contact.
• Insect growth regulators that stop nymphs developing into breeding adults.
• Sanitation and proofing to remove food, water and hiding places.
• Follow-up visits to catch nymphs hatching from protected egg cases and to rotate products if needed.

Surface sprays and aerosol "bug bombs" generally perform poorly against German cockroaches. They rarely reach hiding places, can scatter cockroaches into new areas, and can make baits less attractive. Some populations have also developed resistance to certain insecticides and aversion to some bait ingredients, which is why German cockroaches are hard to control.

Outdoor cockroaches are typically managed with treatment of drains, sub-floors, roof voids and entry points, combined with exclusion work. The aim is to reduce how many reach living areas rather than to eliminate every cockroach in the neighbourhood.

Whatever you use, read the whole label and follow it exactly — it is a legal requirement in Australia, and it sets out where products may be applied and how to protect children, pets and food areas.

DIY or professional help?

DIY prevention is usually enough for occasional large cockroaches during warm weather. Professional help is worth considering when:

• You are seeing small cockroaches or nymphs regularly (a breeding German cockroach population).
• Cockroaches keep returning after cleaning, sealing and keeping drains wet.
• You live in an apartment and neighbours are affected — coordinated treatment works best.
• Someone in the household has asthma or allergies, or you run a food business.

A good pest manager should explain what they will use and where, how long to keep people and pets away, what results to expect and when they will follow up.

Seasonality and location

Outdoor cockroaches peak in summer and after storms, especially in Queensland and northern NSW, where warm nights extend the season well into autumn. German cockroaches breed all year in heated kitchens, so they are not seasonal. In cooler southern cities, large cockroaches are more strongly seasonal and the Oriental cockroach is more likely.

For local detail, see cockroaches in Brisbane and the Brisbane pest calendar.

Answers related to this guide

• Why Do I Suddenly Have Cockroaches?
• Why Are Cockroaches in My Kitchen?
• Can Cockroaches Come Through Drains?
• How Do I Identify a German Cockroach?
• Why Do I Keep Seeing Baby Cockroaches?
• Why Are German Cockroaches Hard to Control?
• Can Cockroaches Fly?
• Are Cockroaches Dangerous?
• Why Are Cockroaches in My Bathroom?
• Do Cockroaches Bite?

Local information

• Cockroaches in Brisbane

Pests in this guide