Why Are My Sprays No Longer Killing Pests? Understanding Pesticide Resistance

If you have been using the same spray product for several seasons and it seems to be less effective than it once was, pesticide resistance is likely the cause. Resistance occurs when repeated exposure to a pesticide creates selection pressure that favours individuals in the pest population with natural genetic traits that reduce their susceptibility. Over successive generations, resistant individuals become the majority — and the spray stops working.

Pesticide resistance is not hypothetical: aphid resistance to many synthetic insecticides is widespread in the UK. Spider mite resistance to several acaricides is well documented. Understanding resistance is essential for any IPM programme that includes chemical intervention.

How Resistance Develops

No pest population is genetically uniform. Some individuals carry gene variants that, by chance, make them slightly less susceptible to a given pesticide — perhaps a slightly different enzyme that metabolises the compound faster, or a receptor that binds to it less effectively. In a normal population, these individuals are a tiny minority. After a pesticide application that kills 95% of the population, those resistant survivors make up a much higher proportion of the remaining 5%. They breed, passing resistance traits to their offspring. After several generations and several spray applications, the resistant genotype dominates.

Pests Most at Risk of Resistance

Resistance develops fastest in species with rapid reproduction — many generations per year — and large population sizes. Aphids (up to 30 generations per year in summer), whitefly, thrips, and spider mites are the primary resistance concerns for home gardeners. These species can shift from susceptible to largely resistant populations within a single season under heavy spray pressure.

Resistance Management Through Rotation

The primary resistance management strategy in IPM is spray rotation — never using the same active ingredient or the same mode of action in consecutive applications. Alternate between soap (which has a purely physical mode of action and is essentially resistance-proof), neem (multiple modes of action), and pyrethrin. This approach ensures that any individual resistant to one product is still killed by the alternative, preventing the resistant genotype from spreading unchecked.

Why IPM Reduces Resistance Risk

IPM reduces resistance risk structurally. By spraying less frequently, applying only when thresholds are crossed, and relying primarily on biological and physical controls, IPM avoids the sustained spray pressure that drives resistance evolution. Maintaining refugia — areas of the garden where pest populations are untreated and susceptible individuals survive — allows re-mixing with the treated population, diluting resistant genotypes between spray events.