Pesticide resistance and its management
How pests become resistant to insecticides, fungicides and herbicides, the mechanisms, how fast it happens, and how rotation, refuges and IPM slow it.
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Every time a pesticide is used, it kills the individuals it can and leaves behind those it cannot. If the survivors survive because of their genes, their offspring inherit that ability, and the next spray works a little less well. Repeat often enough and the product fails. This is pesticide resistance, evolution by natural selection happening in a single field, in a few seasons.
More than 600 species of insects and mites, hundreds of weeds and many plant pathogens are now resistant to at least one pesticide. Managing resistance is one of the core reasons for integrated pest management, and a regular topic in entomology, plant pathology and weed science exams.
What counts as resistance
The Insecticide Resistance Action Committee (IRAC) defines resistance as a heritable change in the sensitivity of a pest population that is reflected in the repeated failure of a product to achieve the expected level of control when used according to the label recommendation for that pest.
Three points in that definition matter:
- Heritable: it is genetic, not just a pest that was missed or a poor spray.
- Population: individuals do not "become" resistant; the proportion of resistant individuals rises.
- Label use: failure because of the wrong dose, poor coverage or bad timing is not resistance.
The first documented case of insecticide resistance was described in 1914, when A. L. Melander reported San José scale surviving lime-sulphur sprays in Washington State. Resistance became widespread after the arrival of synthetic insecticides in the 1940s. Some pests are notorious: the Colorado potato beetle, the diamondback moth, the cotton bollworm, the whitefly Bemisia tabaci and the two-spotted spider mite have each evolved resistance to dozens of active substances.
How pests resist
| Mechanism | How it works | Example |
|---|---|---|
| Target-site resistance | the protein the pesticide binds to is altered, so the pesticide no longer fits | "knockdown resistance" (kdr) mutations in the sodium channel against pyrethroids; altered acetylcholinesterase against organophosphates and carbamates; altered target enzymes in herbicide-resistant weeds |
| Metabolic resistance | the pest breaks down or sequesters the pesticide faster, using enzymes such as cytochrome P450s, esterases and glutathione S-transferases | widespread in aphids, whiteflies, mosquitoes, many weeds |
| Reduced penetration | a thicker or modified cuticle slows uptake | often adds to other mechanisms |
| Behavioural resistance | the pest avoids treated surfaces or feeds in protected places | some insects avoid treated leaf surfaces |
Two related terms are examined often:
- Cross-resistance: one mechanism protects against several products, usually those sharing a mode of action. Resistance to one pyrethroid usually means resistance to others.
- Multiple resistance: a pest carries several mechanisms and resists products with different modes of action.
What makes resistance develop faster
- Genetic factors: the starting frequency of resistance genes, and whether resistance is dominant (heterozygotes survive) or recessive (only homozygotes survive).
- Biological factors: many generations a year, high reproduction, little movement between treated and untreated populations, and a narrow host range so that the whole population is exposed.
- Operational factors: frequent applications, long-lasting residues, applying the same mode of action generation after generation, treating large areas uniformly, and doses that kill only the most susceptible individuals.
How quickly selection works, and what a refuge does
Take a deliberately simple model: resistance is fully dominant, resistant insects survive spraying, and each spray kills 95 per cent of susceptible insects, so 5 per cent survive. The resistant fraction starts at 1 in 10,000.
Each generation, the ratio of resistant to susceptible insects is multiplied by 1 ⁄ 0.05 = 20.
- Start: ratio 1 : 10,000 (0.0001).
- After n generations: 0.0001 × 20n. Resistant insects become half the population when this ratio reaches 1, that is when 20n = 10,000: n = 4 ⁄ log10 20 = 4 ⁄ 1.30 ≈ 3 generations.
Now leave 20 per cent of the area untreated as a refuge. Susceptible insects then survive at 0.20 + 0.80 × 0.05 = 0.24, and the ratio grows by 1 ⁄ 0.24 ≈ 4.2 per generation:
- 4.2n = 10,000 → n = 4 ⁄ log10 4.2 = 4 ⁄ 0.62 ≈ 6.5 generations, more than twice as long.
Real resistance is often partly recessive, which slows selection further and makes refuges even more effective, because rare resistant survivors mostly mate with susceptible insects from the refuge. This logic is the basis of the high-dose/refuge strategy for insect-resistant (Bt) crops.
Managing resistance
The aim is to reduce the selection pressure of any one mode of action and keep susceptible genes in the population.
Know the mode of action
Products are grouped by mode of action in international classifications: IRAC for insecticides and acaricides, FRAC for fungicides and HRAC (with WSSA) for herbicides. The group number is printed on labels in many countries. Rotating between brand names achieves nothing if the products share a group.
Rotate by window
IRAC recommends treating a pest generation, or a window of roughly 30 days, with one mode of action (one or more sprays), then switching to a different group for the next window. FRAC and HRAC give equivalent guidance, including limits on the number of applications of high-risk fungicide groups per season and the use of mixtures with multi-site fungicides.
Use mixtures carefully
Mixtures of two effective modes of action can delay resistance if each component is used at an effective dose and the pest is not already resistant to one partner. Mixtures of products the pest already resists add cost and residues without benefit.
Keep refuges and susceptible pests
For Bt crops, a refuge of non-Bt plants lets susceptible insects survive and mate with any resistant survivors. Untreated borders and wild hosts play a similar role for other pests.
Spray only when needed, and properly
Use economic thresholds and monitoring, the full label rate and good coverage, and do not use long-residual products more than necessary. Every spray that is not needed is selection without benefit.
Combine with non-chemical control
Resistant varieties, crop rotation, sanitation, biological control and cultural practices reduce the number of sprays and the size of the population under selection, and cannot be "resisted" in the same way.
Monitor
Bioassays and molecular tests for resistance genes detect shifts in susceptibility before control fails, guiding product choice in a region.
Key points
- Resistance is a heritable decrease in a population's sensitivity, shown by repeated failure at label use; over 600 arthropod species are resistant to at least one product.
- Mechanisms: target-site changes (for example kdr), metabolic detoxification (P450s, esterases, GSTs), reduced penetration and behaviour.
- Cross-resistance spans products with the same mode of action; multiple resistance combines mechanisms.
- Resistance develops fastest with many generations, frequent use of one mode of action, persistent residues and no refuges.
- Manage by rotating IRAC, FRAC or HRAC groups by window, using effective mixtures, keeping refuges, spraying only above thresholds at full rate, and combining with non-chemical methods.
What the exams ask
- School-leaving and entrance papers: the meaning of resistance and simple ways to delay it.
- Degree and research-fellowship entrances: mechanisms with examples, cross- and multiple resistance, factors affecting the rate of resistance, and the mode-of-action classifications.
- Plant protection examinations: designing a resistance management programme for a crop, the high-dose/refuge strategy for Bt crops, and resistance monitoring.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
If a product stops working, should I double the dose?
No. Higher doses increase cost, residues and risk, and usually accelerate resistance. Switch to an effective product from a different mode-of-action group and combine it with non-chemical measures.
Can resistance disappear if a product is not used?
Sometimes partly, if resistance carries a fitness cost and susceptible insects are more competitive without the pesticide. But resistance genes often persist at a low level and return quickly when the product is used again.
Do organic and biological products avoid resistance?
They reduce it but are not immune. Insects have developed resistance to some Bacillus thuringiensis toxins and to spinosyns, so these too should be rotated and used within IPM.
Sources
- IRAC. IRAC Mode of Action Classification Scheme and Resistance Management for Sustainable Agriculture and Improved Public Health. Insecticide Resistance Action Committee, updated regularly.
- FRAC. FRAC Code List and Fungicide Resistance Management Guidelines. Fungicide Resistance Action Committee.
- HRAC. HRAC Mode of Action Classification. Herbicide Resistance Action Committee, 2024 update.
- Mota-Sanchez, D. and Wise, J. C. The Arthropod Pesticide Resistance Database. Michigan State University, online.
- Tabashnik, B. E., Brévault, T. and Carrière, Y. "Insect resistance to Bt crops: lessons from the first billion acres." Nature Biotechnology 31, 510–521 (2013).
- Melander, A. L. "Can insects become resistant to sprays?" Journal of Economic Entomology 7, 167–173 (1914).
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