Suspected Resistance in a Mite Population
A greenhouse grower has treated a spider mite infestation on ornamental foliage plants every 7 days for eight weeks with the same miticide, rotating only between two products that share the same mode-of-action group. Control was good for the first three weeks, then declined steadily. The grower has now increased the rate above the labeled rate and shortened the interval to 5 days. Mites are still present in patches, and beneficial predatory mites that were released early in the season are no longer found. The grower wants to add a third product from the same MOA group and spray hotter to 'burn them out.'
1. Assess whether this is a resistance problem and explain the evidence for and against that conclusion.
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Resistance is the most likely explanation, supported by:
- Repeated use of a single MOA group for eight weeks; rotating between two products in the same group is effectively no rotation and selects for resistance.
- Progressive loss of control after initial success - the classic resistance signature.
- Surviving patches rather than uniform failure, consistent with resistant subpopulations.
- Loss of beneficials, which removes natural control and lets survivors rebound faster, mimicking or amplifying resistance.
Alternative or contributing causes to rule out:
- Application problems: poor coverage, wrong pH, expired product, or inadequate volume can reduce efficacy without resistance.
- Re-infestation from other plants or vents.
- Life-stage timing: miticides often miss eggs, so a 5-day interval may still allow rebound.
A definitive answer may require a resistance bioassay, but the management decision should proceed as if resistance is present.
2. What resistance-management strategy should replace the current program, and why is the grower's plan to add a third same-group product and spray 'hotter' a bad idea?
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Why the grower's plan fails:
- A third product in the same MOA group applies the same selection pressure and accelerates resistance. Rotation must be to a different MOA group, ideally with a different target site.
- Spraying 'hotter' (higher concentration) increases selection for resistant individuals and risks phytotoxicity and label violations; it does not solve resistance.
- Shortening intervals and exceeding labeled rates are label violations and increase selection pressure.
Better strategy:
- Rotate to a different MOA group and confirm the product is labeled for the crop and pest.
- Use the labeled rate and interval, not more.
- Integrate nonchemical controls: scouting, sanitation, humidity/temperature management, and physical removal of heavily infested plants.
- Reintroduce or conserve beneficials once residues allow, since the current program eliminated them.
- Avoid consecutive applications of the same MOA; keep records of which groups were used.
- Consider a resistance bioassay to confirm and guide choices.
Spraying more of the same chemistry is the opposite of resistance management.
3. How should the grower document and monitor this situation going forward?
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The grower should:
- Keep application records by date, product, EPA reg number, MOA group, rate, and interval, so rotation history is visible.
- Scout on a fixed schedule (e.g., weekly) and record mite counts per leaf or per plant on a map of the greenhouse, tracking whether patches shrink or spread.
- Record beneficial counts to confirm whether natural enemies recover.
- Note environmental conditions (temperature, humidity) that affect mite reproduction.
- Evaluate each treatment after a set interval and document efficacy; if control fails again, reassess rather than escalating rate.
- Share records with an extension specialist or crop consultant if resistance is suspected, and consider a bioassay.
Good documentation both guides decisions and demonstrates responsible pest management if questions arise about the earlier over-label rate increase.