Bed Bug Monitors and Interceptors: What They Show and Miss

A good monitor is like a smoke alarm for bed bugs. It will not fix anything, but it can tell you early whether bugs are present and whether a treatment is working. It also has limits, and it helps to know them before you trust a clear result.

This guide explains how monitors and interceptors work, what the research says, and how to use them in a London flat.

Why monitoring matters

Bed bugs are small, nocturnal and secretive, so they are hard to find. A review of detection methods says that visual inspections are unreliable when only a few bugs are present. It also warns that detection by bites alone is inadequate, because reactions to bites are non-specific and often misdiagnosed.

For that reason, many people add a trap to their routine. A trap works all night while you sleep, unlike a torch check that takes a few minutes. Therefore it can catch a single bug that a quick search would miss.

Passive monitors and interceptors

The same review describes two broad types. Passive monitors mimic the places where bed bugs naturally hide, and they usually use an adhesive to trap the bugs. Interceptors are a common passive device, and they sit under furniture legs.

An interceptor is a small pitfall dish with a smooth, steep wall. A bug climbs in from the outside, but it cannot climb out the inner wall. The 2025 study in Insects says pitfall interceptors and sticky glue boards are the most commonly used devices for monitoring bed bugs.

An interceptor does a second job as well. It cuts the route between floor and bed, because bugs that climb the leg fall into the dish. Researchers have used interceptors under furniture legs and in room corners as part of a managed programme in low-income housing.

Sticky traps work differently. A study of sticky trap design found that smoother surfaces around the adhesive made capture more likely. In practice, dust and fluff reduce how well any trap works, so keep them clean.

Active monitors

Active monitors do more than wait. They give off carbon dioxide, heat, a pheromone or a mix of them to attract bed bugs. The review says that these devices exist alongside passive ones, and that DNA tests and electronic noses are innovative but impractical and expensive for widespread use.

A 2013 study in the Journal of Economic Entomology tested lures and carbon dioxide. It found a distinct positive relationship between carbon dioxide release rates and trap catches. The same paper reported that a lure mixture raised the catch of one passive interceptor by 2.2 times.

However, high cost and the need for a gas source have limited wide use. For most households, simple interceptors plus regular checks are enough. Ask a professional whether an active device suits a larger or shared building.

What a negative result does and does not prove

This is the most important point. A clear trap tells you that no bug fell into that trap in that time. It does not prove that the home is free of bed bugs.

Several things can explain an empty trap. The bugs may be few in number or hiding in another room. In addition, a trap in the wrong spot, or one clogged with dust, will catch less than it should.

A clear result is still useful. After treatment, an empty monitor over several weeks, together with clean visual checks, supports the view that the plan has worked. Our post on how long bed bug treatment takes explains how long to keep checking.

A positive result also has value. If a bug turns up, keep it in a sealed container or take a clear photo. Then follow the steps in our urgent bed bug help guide.

Where to place monitors in a flat

Start with the bed, since that is where bugs feed. Put an interceptor under each leg of the bed frame. Pull the bed away from the wall, and make sure the bedding does not touch the floor.

Next, cover the other places you sit or rest:

• under each leg of the sofa and any armchair
• along skirting near the head of the bed
• beside a bedside table or radiator close to the bed
• in the corners of the room, where bugs often travel

Check every trap weekly, and record what you find with a date. A simple note on your phone is enough. Moreover, a photo of each catch helps if you need to show a landlord or a technician.

Common mistakes

A few habits undermine otherwise good monitoring. Check for them before you decide a trap is not working.

• Leaving bedding or a duvet hanging to the floor, which gives bugs a bridge over the interceptor.
• Placing traps once and never looking at them again.
• Moving the bed without moving the traps with it.
• Treating a single empty week as a final answer.

Using monitors well

Monitors work best alongside other steps. Wash fabrics at 60C and tumble dry on a hot setting, as the NHS advises, and clear clutter from around the bed. Our guide to preparing for treatment explains these steps in order.

Monitors are one line in a wider budget. Our guide to bed bug treatment costs helps you see where a monitor fits in the overall picture. Ask any provider whether their plan includes monitors and who reads them.

Your layout shapes where you place traps. Start with bed bug guidance for Islington or what Tower Hamlets residents should check for local contact points. Meanwhile, the Enfield area guide, advice for Bromley households and the Hillingdon local guide serve readers in outer London.

Shared housing needs particular care, because bugs can pass between rooms. Our post on bed bugs in student accommodation in London covers halls and shared flats. To learn the signs a trap supports, read how to check a London flat for bed bugs.