Resistance management with IRAC groups: rotating actives so sprays keep working
Two products with unrelated names can share an IRAC mode-of-action group, so rotating brands doesn't rotate resistance risk. How to build one that does.
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Two miticides sold under different names, made by different companies, can still be the identical mode of action as far as a mite is concerned. Rotate between them every ten days for a season and the mite population has faced one continuous selection pressure, not four separate ones. A spray sheet that looks compliant on paper, four products, four labels, four different-sounding actives, can still fail in the field if none of those choices actually moved to a different target site.
Resistance develops against a specific biochemical target, not against a brand. The Insecticide Resistance Action Committee (IRAC) groups every insecticide and acaricide by that target, its mode of action (MoA), with a number, and sometimes a letter, that has nothing to do with chemical family names or marketing copy. Two products from unrelated chemical classes can share a group number because they hit the same site in the pest's nervous system, growth pathway or respiration chain. Two products with near-identical-sounding chemistry can sit in different groups because they don't [1].
The number is the target site, not the chemical family
IRAC group 20, for example, covers compounds that block mitochondrial complex III at the same electron-transport site, but its four lettered subgroups (20A–20D) are chemically distinct: hydramethylnon, acequinocyl, fluacrypyrim and bifenazate look nothing alike on a structure diagram, and are sold under names that share no root word, yet a mite resistant to one starts from a head start against the others [1]. Group 21 works the other way: fenpyroximate, pyridaben, tebufenpyrad and a handful of other "METI" acaricides (mitochondrial electron transport inhibitors) are close chemical relatives that all block complex I, so a rotation that alternates between them is, functionally, no rotation at all [1].
The letter matters less than the number for rotation planning. A and B within the same group usually still share the same primary target site; treat the numbered group as the unit you rotate against unless a specific product's data sheet or a resistance report for your pest says otherwise.
Finding a product's group before it goes in the tank
IRAC group numbers increasingly appear directly on the product label, because a growing number of registration authorities now require it. Check there first. When the label doesn't show it, or a product is unfamiliar, IRAC maintains the definitive classification, searchable by active ingredient, at its Mode of Action Classification page [1]. Build the lookup into the routine rather than the memory: before any new product joins a rotation, find its group and write it down next to the product name on the spray log, before the application, not after three sprays have gone out on an assumption.
The table below covers the IRAC groups a cannabis IPM programme meets most often. It is a starting reference, not the full scheme. IRAC adds and revises groups as new actives are registered, so treat the source page as the current word, not this table [1].
| IRAC group | Target site | Example actives | Usually used against |
|---|---|---|---|
| 4 | Nicotinic acetylcholine receptor (nAChR), competitive modulator | Imidacloprid, thiamethoxam, dinotefuran | Aphids, whitefly, thrips |
| 5 | nAChR allosteric modulator, site I | Spinosad, spinetoram | Thrips, caterpillars |
| 10 | Chitin synthesis inhibitor (mite growth inhibitor) | Hexythiazox, etoxazole | Mite eggs and immatures |
| 11 | Midgut membrane disruptor | Bacillus thuringiensis (var. kurstaki, aizawai) | Caterpillars |
| 15 | Chitin biosynthesis inhibitor, type 0 | Diflubenzuron, lufenuron | Caterpillars |
| 20 | Mitochondrial complex III inhibitor (Qo site) | Bifenazate, acequinocyl | Mites |
| 21 | Mitochondrial complex I inhibitor (METI) | Fenpyroximate, pyridaben, tebufenpyrad | Mites |
| 23 | Acetyl-CoA carboxylase inhibitor (lipid biosynthesis) | Spiromesifen, spirotetramat | Mites, whitefly |
| 28 | Ryanodine receptor modulator (diamide) | Chlorantraniliprole, cyantraniliprole | Caterpillars |
Source: IRAC Mode of Action Classification, accessed 2026-09-26 [1].
Building a rotation that survives an audit
IRAC's own principle is generation-based, not calendar-based: successive generations of the pest should not meet the same MoA group, with applications grouped into "spray windows" set by the pest's biology and the crop stage rather than a fixed number of sprays [1]. In practice that means spacing switches roughly a generation apart, commonly every 7–14 days for a fast-cycling mite or aphid population in a warm room, though the real figure depends on the species and temperature (check the pest's own profile for its development time), and returning to a group only once you've cycled through the others at least once.
IRAC groups in this figure
- 21AMitochondrial complex I inhibitor (METI)Fenpyroximate, tebufenpyrad
- 10AMite growth inhibitor (chitin synthase 1)Hexythiazox
- 23Acetyl-CoA carboxylase inhibitorSpiromesifen
- 20DMitochondrial complex III inhibitor (Qo site)Bifenazate
Tag colours only tell this figure’s four groups apart; they are not an IRAC colour code. Rotate on the number, not the letter: hexythiazox (10A) and etoxazole (10B) count as one group.
Groups and target sites: IRAC Mode of Action Classification, accessed 26 Sep 2026 [1]. Spacing is a generation-length illustration, not a fixed interval: the real gap depends on the pest and temperature.
The table below works through one flowering-room season, four applications against a mite population, run two ways.
| Application | Correct rotation: active (IRAC group) | Common mistake: active (IRAC group) |
|---|---|---|
| 1 (day 0) | Fenpyroximate (21A) | Fenpyroximate (21A) |
| 2 (~7–14 days later) | Hexythiazox (10A) | Tebufenpyrad (21A) |
| 3 (~7–14 days later) | Spiromesifen (23) | Hexythiazox (10A) |
| 4 (~7–14 days later) | Bifenazate (20D) | Spiromesifen (23) |
Both plans use four different-sounding products across four applications. Only the left column is a real rotation: it never repeats a group across the season, so a fifth application could safely return to group 21 or 10, because at least one full cycle through the other groups has happened since either was last used. The right column repeats group 21A at applications 1 and 2 under two unrelated product names, the classic "I rotated" mistake, and only starts genuine rotation from application 3, a full spray late.
Why the sprayer should be the last tool reaching for a mode of action
Every chemical application is a selection event: it kills the susceptible majority and leaves a disproportionately resistant minority to breed. Cultural controls, sanitation and biological control agents don't carry that cost, because they don't select for a specific detoxification or target-site mutation — they simply reduce the population that ever needs a chemical MoA group at all. Extension guidance frames this plainly: apply a pesticide only when monitoring and an action threshold say it's needed, at the life stage the product actually controls, rather than on a calendar [2]. Fewer, better-timed sprays mean fewer selection events per season, which slows the pace at which any single group gets used up. A general IPM programme built around thresholds and biocontrol is what gives a chemical rotation room to work; see also checking spray compatibility with released beneficials before a chemical application follows a biocontrol release.
When two different group numbers still add up to the same risk
Group separation is a starting assumption, not a guarantee. IRAC's own guidance flags that metabolic resistance mechanisms can produce cross-resistance between MoA groups that are classified separately, and that rotation advice should be adjusted where this is documented for the pest in question [1]. This usually runs through a shared detoxification pathway rather than the target site itself: a pest population that evolves a faster way to break down or excrete one class of chemical can end up partly protected against an unrelated class that happens to be metabolised the same way, regardless of which IRAC number either was filed under. Before assuming that switching group numbers is enough, check whether cross-resistance has been reported for your specific pest and the products in your rotation. This is exactly the kind of detail a pest-specific resistance page, rather than a general framework, is built to hold.
The record that makes a rotation defensible, not just remembered
A rotation plan is only as good as the log behind it. Every application record needs the product name, the active ingredient, the IRAC group, the target pest, the dose and area treated, the applicator, and the date, with the IRAC group as its own column, not folded into a free-text product name field. Product name alone is what let the mistake column above happen in the first place; the group column is what an audit, or a resistance investigation six months later, actually needs. Tie each entry to its pre-harvest interval clock and to what's actually permitted on a flowering crop, and keep the whole thing in the same place as your scouting and monitoring records. A rotation and a monitoring log that live in different systems stop telling you anything useful the day efficacy starts to slip.
None of this changes with the pest: the mechanism is identical whether the population under pressure is a two-spotted spider mite, a cannabis aphid or a budworm. What changes is the generation time you're rotating against and which groups are actually registered where you grow. The spider mite resistance case works through that pest's own numbers in detail; start the group column in the spray log before the next application goes out, not after the third one stops working.