The Biology and Behaviour of Bed Bugs

This is not a biology lesson for its own sake. Every habit below has a practical consequence, and we have put the consequence next to the habit so you can see why a treatment is done the way it is.

What a Bed Bug Actually Is

The common bed bug is Cimex lectularius, a small wingless insect that lives on nothing but blood. It has no interest in crumbs, rubbish or dirt, and it does not live on people the way a louse does. It lives near people and visits them to feed.

An adult is flat, oval and brown, roughly the size of an apple pip before a meal and swollen and redder afterwards. It cannot fly and it cannot jump. It walks, and it walks well.

That flat body is the important part. It lets an adult slide into a gap about as thin as a credit card, which is why the places they live are so easy to overlook. For how to recognise them at each stage, see our guide to bed bug identification.

How They Feed

A bed bug finds a host mainly by the carbon dioxide we breathe out and by body heat. A sleeping person is the ideal target: still, warm and breathing steadily for hours.

It pierces the skin with a thin mouthpart and feeds for several minutes. Its saliva numbs the spot and stops the blood clotting, which is why nobody wakes up. Then it walks back to its hiding place to digest, and it may not need to feed again for days.

• Consequence: they spend most of their lives hidden, not on the bed surface you can see.
• Consequence: the person is the bait. Move to another room and the insects follow the carbon dioxide.
• Consequence: bites are a poor guide, because the reaction happens later and varies from person to person. Our bed bug bites guide explains why.

How They Breed and Grow

A fed female lays a few eggs a day, glued into crevices close to where the colony shelters. Each egg is pale, about the size of a pinhead, and very hard to spot.

A young bed bug, called a nymph, hatches looking like a small pale version of the adult. It has to moult five times before it is fully grown, and it needs a blood meal before each moult. In a warm bedroom the whole cycle from egg to breeding adult can take a matter of weeks; in a cold room it slows right down.

How long each stage lasts, and how long they can go without food, is covered in how long bed bugs live.

How They Behave in a Home

Bed bugs cluster. They leave chemical signals that draw others to the same crevice, so you tend to find groups rather than single insects, along with the dark spotting and shed skins they leave behind.

They stay close to the host when they can, which usually means the mattress seams, the bed frame and the headboard. As numbers grow they spread outwards into the bedside table, the skirting, the carpet edge, picture frames and plug sockets, and eventually into neighbouring rooms.

They mostly feed at night, but a hungry bed bug will feed in daylight if someone is lying still. People who work nights are bitten during the day for exactly this reason.

Every rule about bed bugs is really a rule about where they hide and who they follow.

What Their Biology Means for Treatment

• Hiding in cracks: a spray over the mattress misses most of them. Treatment has to go into the harbourage.
• Protected eggs: expect a follow-up, and keep reporting what you see rather than assuming the worst.
• Following the sleeper: stay in your own bed so the insects keep coming to the treated area.
• Walking between rooms: an inspection has to look beyond the bedroom where you noticed them.
• Living without regular meals: leaving a room empty for a while does not starve them out.

This is also why household sprays and foggers disappoint. They land on surfaces, not in the seams, and some of them simply push the insects deeper into the room. Our what to do if you have bed bugs post covers the sensible first steps.

Getting Them Treated

Our bed bug control service starts with an inspection to confirm the insect and find how far it has spread, then treats the places the biology says they will be. The price is £220 for the first room, plus £80 for each additional room, with no call-out fee. The bed bug treatment cost guide explains how rooms are counted.