What resistance means
Resistance is a change in a population, not in a single bug. Survivors of a spray pass their traits to their young. Over time, more of the population can shrug off the same chemical.
Bed bug numbers began to rise almost together in Europe, the US and Australia in the late 1990s. A UK-based review says resistance made a significant contribution. It also notes that pyrethroids became the mainstay of control after older chemicals were withdrawn. The review comes from Rothamsted Research in Harpenden.
What the evidence shows
In 2007, US researchers found extremely high resistance to two pyrethroids in bed bugs from Kentucky and Ohio. Tests on bugs from across the US suggested the problem was already widespread. Young from resistant and susceptible parents had a middle level of resistance.
A later study checked 110 US populations for two nerve-target mutations. It found them in 88% of the populations it sampled. These figures come from the US, although the UK-based review above describes the same trend worldwide.
Researchers have also named other mechanisms, such as a thicker outer layer on the bug. One laboratory study suggests this may weaken dusts that act on that layer. Those dusts still killed the resistant bugs, however. So no single product removes the problem.
What resistance does not mean
Resistance does not mean every product fails. It differs between populations, so a product can work well in one home and poorly in the next. A sighting cannot tell you which case you have.
It also does not mean chemicals are useless. Instead, it means they work best as one part of a plan. For example, a residual insecticide may still help when heat or steam has dealt with the bugs you can reach.
Nor does resistance mean your home is dirty. The NHS says bed bugs live in both clean and dirty places. Resistance describes the bugs, so it says nothing about your housekeeping.
Why stronger is not the answer
It is tempting to use more product or to mix two. That is unsafe, and it breaks the label. The HSE says you must not use a biocide for any purpose other than the one on its label.
Reviewers also warn that misusing chemicals can harm human health. Therefore ask what the label allows. See the safety and labels guide for the checks to make.
Combine methods to reduce reliance
Reviewers say that, because bugs hide well, control needs a mix of non-chemical methods and careful insecticide use. In practice, that means inspection, vacuuming, mattress covers, heat or steam and checking. Residuals or desiccant dusts come in where the evidence supports them.
Our post on how long heat treatment takes explains what a visit involves. The heat treatment guide covers the method itself. Foggers fail partly because of resistance, as we explain in why foggers fail.
After any treatment, give it time while you keep monitoring. Read how long bugs take to die after treatment for what to expect. Then use signs a treatment has worked to check progress. If activity continues, review the plan.
Questions to ask a professional
You do not need to know how tough your bugs are, yet you can still ask good questions. Which methods will you use, and why those?
Also ask how they will monitor the visit and when they will review the plan. Then ask what happens if activity continues. A clear answer shows that the plan does not rest on a single product.
Sources and further checks
Open each source before you act on anything that can change, such as council services, prices, tenancy rules and product labels. Labels and official guidance are revised from time to time, so the source always wins over our summary.