Insecticide and rodenticide resistance is a heritable reduction in susceptibility across a pest population, not a single insect or rodent becoming immune. It arises when repeated exposure at doses too low to kill leaves survivors to breed. A treatment that worked before can then fail on the same product at the same site.
Resistance is an inherited reduction in a pest population's susceptibility to a pesticide, passed from surviving parents to their offspring. The unit is the population, not the individual. What happens is that the few animals whose genetics already made them less susceptible are the ones that live to breed, and their share of the next generation is larger.
Two consequences follow from that definition. Resistance is specific: it attaches to an active substance or a chemical class, so a population resistant to one class can stay fully susceptible to another with a different mode of action, the biological route by which a product kills. And resistance is local: it exists in particular populations in particular places, and it is not a national uniform condition you can assume from a postcode.
| What resistance is | What it is not |
|---|---|
| A heritable reduction in susceptibility across a pest population | An individual insect or rodent getting used to a poison during its own lifetime |
| The outcome of selection, where survivors of a sub-lethal dose breed | Proof that the technician mixed or applied the product incorrectly |
| Specific to one active substance or chemical class | Blanket immunity to every treatment method |
| Detectable only at population level, by testing or by documented control failure | Something visible by looking at a single insect or rodent |
| A reason to change the active class, the formulation or the method | A reason to increase the dose of the product that failed |
| Present in some local populations and absent from others nearby | A permanent, uniform property of a species |
Resistance develops through repeated exposure at sub-lethal doses, which selects for survivors. A sub-lethal dose is any amount of active substance that a pest contacts or eats without dying: too little product, product in the wrong place, product that has degraded, or bait taken once and abandoned.
Three conditions have to line up. The population needs some existing genetic variation in susceptibility, which most large populations have. The exposure has to be repeated, so that selection happens generation after generation. And the survivors have to breed, which they do quickly: German cockroaches and house mice both produce several generations a year in a heated building.
Shop-bought products make resistance worse because they are formulated to be safe in untrained hands, which usually means weaker, and because they are used in the pattern that selects hardest: repeatedly, partially, and without ever confirming the population is dead.
Pesticide products are authorised for either amateur or professional use, and the two categories differ in concentration, in the formulations available and in what the label permits a user to do with them. A professional-use rodenticide, placed where the rodents actually feed and topped up until take stops, is far more likely to kill the whole population than a supermarket bait block put out for a fortnight, moved, then forgotten.
Scattering compounds the problem. A pyrethroid aerosol fired into a bed bug harbourage, the sheltered spot where a pest population lives and breeds, disperses insects into adjoining rooms, and every insect that walks through the residue and survives is a selected survivor.
Two cases are well established in UK pest management. Resistance to certain insecticide classes has been documented in bed bug populations, which is why professional bed bug work has moved towards heat, steam and combined approaches rather than a single spray. And anticoagulant resistance has been documented in some house mouse and rat populations in parts of the country, an anticoagulant being a rodenticide that kills by preventing blood clotting, which is why the choice of active substance for a rodent job is not interchangeable.
Neither case is universal, neither is measurable from the kerb, and resistance is only one candidate explanation for a control failure. The others are missed harbourage, an untreated adjoining property, an unrepaired entry point and preparation the occupant did not finish.
Non-chemical methods are the practical answer to a resistance risk you cannot test for on the day. 247 Pest Control lists heat and steam among its specialist treatments, and those methods matter here because they kill by temperature rather than by a chemical mode of action, so resistance to an insecticide class does not apply to them.
Professionals work around resistance in three ways: rotating active classes, adding non-chemical methods, and combining formulations so that a pest meets more than one mode of action. Each tactic attacks the same weakness: a population cannot be selected for tolerance of a pressure it has not met.
Rotation means not returning to the class that failed. Changing brand while staying inside the same chemical class achieves nothing, which is why the label matters more than the tin. Non-chemical methods such as heat, steam, physical removal, vacuuming and proofing have no mode of action for resistance to act on. Combining formulations means pairing a residual insecticide with steam and physical removal in a bed bug job, so insects that survive one element still meet another. Underneath all three sit dosing to label every time, because under-dosing is the mechanism that creates the problem, and monitoring afterwards, so a failure is identified rather than assumed to be a success.
The CRRU UK Code of Best Practice restricts permanent baiting partly because leaving rodenticide down indefinitely is continuous low-level exposure, which is the ideal condition for selecting resistant rodents. Bait left in place after activity has stopped is no longer treating anything: it is a standing dose that any surviving or incoming rodent can nibble and walk away from.
The Code's stewardship approach is built around the same logic that governs resistance management generally: use rodenticide as a defined campaign with a beginning and an end, record and check every bait point, and remove unused bait when the job finishes. Permanent baiting is allowed only in specific justified circumstances rather than as a default, and the environmental case for that restriction runs alongside the resistance case rather than replacing it.
A failed treatment is not always a failed technician. Resistance means that a correctly identified pest, treated with a correctly applied product at the correct dose, can still survive, and that the fix is a change of method rather than a repeat. Treating insecticide and rodenticide resistance as a possibility changes what you should ask for after a failure.
Ask three things: what active substance or class was used, what the monitoring showed between visits, and what different mode of action or non-chemical method comes next. A firm that answers all three is managing the problem; one that offers another dose of the identical product is not.
Where a treatment has failed, the next step is a survey rather than more of the same. 247 Pest Control states that same-day attendance is available across London and that it responds to emergency enquiries 24/7, while noting that attendance time cannot be identical for every call. An attending technician confirms the species, recommends the treatment the property needs, and gives the earliest date it can be booked. Turnaround carries extra weight after a failure, because a population that survived one product keeps breeding through the delay, and every generation that passes is another round of selection working against the next active class.
Your consumer position is separate from the science. A pest control service must be carried out with reasonable care and skill. Where it was not, you can require the firm to perform the work again at its own cost, and where performing it again is impossible or is not done within a reasonable time you can seek a price reduction instead. Resistance may explain a poor result. It does not excuse a job done without care and skill, and the two questions are worth keeping apart in any complaint you make.
No. Resistance reduces susceptibility to particular active substances, not to every method. Heat, steam, physical removal, vacuuming and proofing have no chemical mode of action for resistance to act against, and a population resistant to one insecticide class is often fully susceptible to another. The job becomes harder, longer and more expensive, not impossible.
Not on the day. Resistance is confirmed by laboratory testing of collected specimens or by documented control failure across a properly conducted treatment, so it is a conclusion drawn after the event rather than a reading taken during a survey. A technician who suspects it should say so as a hypothesis and change the mode of action, not assert it as a finding.
No, and it is usually an offence. A pesticide must be used in accordance with its authorised label conditions, which set the dose and the application method. Exceeding them is unlawful, raises the risk to occupants and pets, and does not overcome resistance, because resistance is a property of the population rather than a matter of quantity.
To avoid applying the same selection pressure twice. Repeating a class that has already left survivors selects the resistant part of the population again, while a different mode of action attacks it from a direction it was never pre-selected against. Control depends on correct species identification and locating the harbourage, and rotation only helps once both are right.
No. Stop, and leave everything in place. Continuing amateur treatment feeds the population a sub-lethal dose, scatters insects into adjoining rooms and destroys the evidence a technician needs to identify the species and find the harbourage. Note the rooms and times you have seen activity, seal food away, and leave droppings undisturbed.