9.3 Pesticide Resistance Management

Pesticide resistance is a heritable decrease in susceptibility of a pest population to a pesticide. Individuals that survive a dose that used to control the population pass that trait to offspring. 40 CFR 171.103(c)(5)(v) requires Core candidates to know factors that influence effectiveness or lead to problems such as pesticide resistance. Chapter 5 introduced IRAC (Insecticide Resistance Action Committee), HRAC (Herbicide Resistance Action Committee), and FRAC (Fungicide Resistance Action Committee) group codes. This section is management: how selection happens, how mechanisms differ, and what you do on an Oklahoma job.

Resistance is not a plant or insect “getting used to” a chemical in one lifetime the way a person builds a caffeine habit. It is population genetics. You apply selection pressure. Survivors reproduce. The next generation starts less susceptible.

Selection pressure

Every time you use a single site of action on a large fraction of the population, you kill susceptibles and leave resistant individuals. Repeated same-site-of-action applications—glyphosate, then glyphosate, then another company’s glyphosate—are one pressure. Residual products that sit on a surface or in soil for weeks extend that pressure. Treating the entire habitat so no untreated individuals breed can raise pressure still further, which is why some insect programs keep refugia (below).

Palmer amaranth in Oklahoma cotton and soybean is the agricultural poster. Oklahoma State University Extension (PSS-2783) reports glyphosate-resistant pigweed species, particularly Palmer amaranth and waterhemp, becoming more common in those crops, and glyphosate-resistant marestail confirmed in several Oklahoma counties. The weed did not study the label. The field selected plants that already carried resistance alleles. A female Palmer plant that escapes can return hundreds of thousands of seeds. That is why a few green survivors after a Group 9 postemergence pass are not a cosmetic issue; they are next year’s population.

Low rates and poor coverage select too, often for metabolic resistance that later chews through more than one group. “Just enough to burn them” is not conservative. Use labeled rates on the right size of weed or the right insect instar. Oversized Palmer amaranth is already a coverage and rate problem before genetics enter. The same logic hits 7A: a faint film of pyrethroid residual on a greasy tile selects cockroaches that tolerate that dose.

Mechanisms Core expects you to tell apart

You will not sequence a gene on the exam. You should recognize three stories:

Category 7A German cockroaches illustrate why “I rotated baits” can be false. Populations can show physiological resistance to pyrethroid residuals and to bait actives in several IRAC groups, and they can refuse a bait matrix. Rotating syringes by color is useless if the IRAC group and the matrix both stay the same. Rotate unrelated sites of action, change matrices, and keep sanitation so the bait is the best food in the account.

Armyworm, bollworm, and pecan weevil programs fail the same exam logic when every pass is one IRAC group timed to the wrong stage. A larva inside a pecan kernel is not “immune”; it is not exposed.

Resistance, tolerance, and immunity — use the words correctly

Resistance is an inherited shift in a population that used to be susceptible.

Tolerance is a natural lack of susceptibility that was always there. Grasses are generally tolerant of many Group 4 auxin herbicides such as 2,4-D at pasture rates; that is not “johnsongrass evolved 2,4-D resistance” unless a previously susceptible johnsongrass population actually shifted. Broadleaf weeds are naturally not controlled by many Group 1 acetyl-CoA carboxylase (ACCase) grass herbicides. Palmer amaranth is a broadleaf; a johnsongrass-only grass herbicide missing Palmer is tolerance of a non-target group, not proof Palmer is glyphosate-resistant. Decide which story you are in before you change products.

Immunity is a sloppy word on pesticide exams. People use it to mean “the spray did nothing.” It is not the resistance-management term of art. Prefer not susceptible, tolerant, or resistant after you know which one you mean. Poor coverage, rain-fast failures, treating the wrong stage, and an unlabeled pest are not immunity.

IRAC, HRAC, and FRAC rotation

Rotation means switching sites of action, not brands and not the color of the jug.

Do not “rotate” two Group 9 glyphosate products. Glyphosate inhibits EPSPS. A Roundup-brand rate in fluid ounces and a generic rate that delivers the same acid equivalent are the same site of action. OSU has noted that dozens of glyphosate labels have been registered for Oklahoma crop use; that variety is salt, surfactant, and concentration, not a resistance rotation. The same trap exists for two IRAC Group 3 pyrethroids on cotton stink bugs or German cockroach residuals, two IRAC Group 4A neonicotinoid cockroach baits, or two FRAC 11 strobilurins in a peanut leaf-spot or turf program.

Use multiple effective sites of action across the season: a preemergence herbicide from a different HRAC group than your postemergence product; overlapping residuals so Palmer is not raised on Group 9 alone; a cotton insecticide sequence that does not live on one IRAC number; peanut and turf fungicide programs that sequence FRAC groups the label allows. Cultural tactics from Section 9.2—rotation, hand-roguing Palmer, sanitation in 7A—cut the fraction of the population that chemistry must kill, which cuts selection pressure.

Mixtures

Tank-mixing two modes is not automatically smarter. Mix sites of action only when scientifically justified and labeled. Justification means each component still controls the target at an effective labeled rate, so you are not hiding a free-rider dash of a second herbicide under a full Group 9 rate. The mixture must be allowed on both labels for that crop, pest, adjuvant, and water-quality restriction. Mixing two Group 9s, or mixing a full Group 9 with a decorative amount of another group below its effective rate, is not a resistance strategy. Mixing incompatible formulations is a Chapter 5 jar-test problem, not IPM.

Refugia and untreated areas

Some insect programs, especially those involving Bt crops or area-wide insecticidal pressure, use refugia: host plants that remain untreated or are treated differently so susceptible insects continue to breed and resistance alleles stay rare. When a seed or insecticide label requires a refuge, that instruction is part of the labeling you already treat as law. Do not invent a refuge percentage that is not on the product in the hopper.

For weeds, leaving a Palmer amaranth strip to go to seed is not a refuge program; it is a seed-bank donation. The weed analog is diversity: effective PRE/POST groups, tillage or harvest weed-seed tactics where they fit, and pulling escapes—not a sanctuary of pigweed.

Mosquito Category 8 work and 7A accounts should not paint every square foot with the same residual every week. Use monitoring zones, rotate IRAC groups in baits and residuals, and let sanitation carry part of mortality so the entire population is not sitting on one chemistry.

Labeled rates, IPM, and the two Oklahoma posters

Resistance management is IPM with genetics in mind:

• Identify the pest (Palmer versus a grass; German cockroach versus a wood cockroach; armyworm larvae versus adults).
• Use cultural and mechanical tools so chemistry is not 100 percent of mortality.
• Apply labeled rates to the correct stage.
• Rotate HRAC / IRAC / FRAC groups, not brands that share a group.
• Do not cut rates to stretch a jug.
• Record the group you used.

Palmer amaranth is Oklahoma’s herbicide-resistance classroom: glyphosate (Group 9) overuse in cotton and soybean, confirmed Group 9 marestail in the state, and a seed producer that punishes survivors. German cockroach bait aversion and residual resistance are the 7A classroom. Both fail the same way: one site of action, over and over, at a dose the survivors can live with.