Cockroach infestation: what you need to know

Finding a cockroach is unsettling, and finding one usually means there are more out of sight. Cockroaches are nocturnal, hide in the smallest gaps and breed quickly, so the few you see are rarely the whole story. Here is how to identify them, what the signs are, and why they need treating thoroughly.

01Is it a cockroach?

Before anything else, make sure it is a cockroach. Large brown beetles, especially the cockchafer or May bug that blunders into lit windows in late spring, are often mistaken for them. Our guide on telling a cockchafer from a cockroach shows the differences. Cockroaches have flat, oval bodies, long whip-like antennae and fast, darting movement, and they run for cover when a light goes on.

02The two common species

• German cockroach (Blattella germanica). Small, about 10 to 15mm, light yellowish-brown with two dark stripes behind the head. It likes warmth and is usually found in kitchens, behind and under appliances, inside motors and around hot water pipes. It can climb smooth surfaces and breeds very quickly.
• Oriental cockroach (Blatta orientalis). Larger, about 20 to 25mm, dark brown to black and glossy. It prefers cooler, damp places such as cellars, drains, ducts, under floors and ground-floor rooms, and struggles to climb smooth vertical surfaces.

The species matters, because it tells us where to look and how to treat.

03Signs of an infestation

• Live cockroaches, usually at night. One seen in daylight often means they are short of hiding space.
• Droppings. Small dark specks like ground coffee or pepper, or dark smears on surfaces, near where they shelter and feed.
• Shed skins. Young cockroaches moult several times as they grow, and the empty skins collect near their hiding places.
• Egg cases. Brown, purse-shaped capsules a few millimetres long, tucked into cracks and behind fittings.
• A smell. A musty, oily, unpleasant smell that builds up where there are a lot of them.

Check under and behind cookers, fridges and dishwashers, under sinks, around boilers and pipework, and inside cupboards where food is kept.

04Why cockroaches are a health risk

Cockroaches feed on almost anything, including food waste, rubbish and drains, then walk over worktops, dishes and food. They can carry bacteria such as Salmonella and E. coli on their bodies and in their droppings, and so can contaminate food and surfaces. Proteins in their droppings, saliva and shed skins are a known trigger for allergies and can make asthma and eczema worse. If you are unwell with stomach symptoms or breathing problems, speak to your GP or pharmacist.

For a business that handles food, a cockroach sighting by a customer or an inspector is also a serious reputational and legal problem. Our commercial pest control page explains how we work with businesses.

05Why they are so hard to clear

• They hide well. Cockroaches flatten themselves into very narrow gaps, so the size of an infestation is hard to judge.
• Egg cases protect the next generation. The case shields the eggs from many insecticides, so young cockroaches keep appearing after a treatment. The German cockroach carries its egg case until it is nearly ready to hatch.
• They breed quickly, especially German cockroaches in a warm kitchen.
• They can resist insecticides, and some populations have learned to avoid sweet baits that once worked well.
• They are hardy. They can go weeks without food, though they need water regularly, which is why they gather near sinks, drains and leaks.

Over-the-counter sprays tend to kill the cockroaches you can see and scatter the rest deeper into the building.

06What treatment involves

We start by identifying the species and finding where they are sheltering, feeding and coming in. Treatment uses professional baits and insecticides placed where cockroaches live, not just where they are seen, and we follow up to deal with nymphs that hatch from protected egg cases. We will also point out the conditions keeping them there, such as gaps around pipes, leaks and food left out, because fixing those is what stops them coming back.