Pest Resistance to Organic Sprays: Keeping Bt and Spinosad Working

There is a stubborn myth that pests cannot become resistant to natural products. Evolution does not check whether a spray is organic. If a material kills most of a pest population and a few individuals survive because of their genes, those survivors breed, and the next generation is harder to kill.

Spinosad shows this clearly. In a Canadian survey, organic potato farms in Quebec had higher Colorado potato beetle resistance than conventional ones, because spinosad was the only certified organic option for potatoes there. Western flower thrips populations in China and Spain have also shown resistance.

Organic growers have fewer materials to rotate than conventional growers, which makes each one more valuable. This post reviews the evidence on spinosad and Bt resistance and lays out a practical plan to keep both working.

Key takeaways

• Spinosad resistance is documented in Colorado potato beetle in parts of Canada and in western flower thrips in China and Spain.
• Resistance is regional. Czech field trials from 2007 to 2022 found no decline in spinosad efficacy against Colorado potato beetle.
• Repeated use drives it. In Canada, resistance correlated with the number of spinosyn applications.
• Bt crop resistance is well documented: 26 cases of practical resistance in 25 years. Organic Bt sprays share toxin families with Bt crops, so that trend matters to organic growers too.
• The defense is diversity: rotate materials where options exist and lean on non-chemical tactics such as exclusion, crop rotation and habitat.

The evidence on spinosad

Colorado potato beetle in Canada

A 2018–2022 national survey (Scott et al., 2023) reported spinosyn resistance rates by province:

The survey found a significant drop in beetle mortality at the diagnostic dose in year 5. Resistance correlated with the number of spinosyn applications. Organic farms in Quebec had higher resistance, which the authors linked to spinosad being the only certified organic option for potatoes there.

A counterpoint from Czechia

Resistance is not inevitable everywhere. Czech field trials published in PLOS ONE (2024) found spinosad efficacy against Colorado potato beetle "ranged between 84.8 and 100% without any incidence of a decrease in efficacy" from 2007 to 2022. Over the same period, efficacy of acetamiprid, a conventional neonicotinoid, fell from 90.2% in 2010 to 59.1% in 2022.

The lesson is that local history of use matters. A tool that is failing in one region may still work well in another.

Research into the mechanism

US researchers are also studying how resistance develops. A PLOS ONE study (2024) linked a gene, CYP9E2, and a long non-coding RNA to spinosad resistance in Colorado potato beetle. A CUNY Graduate Center dissertation examined the evolution of spinosad resistance in the same pest. We did not review either in full.

Western flower thrips

• China: Across 24 field populations in Shandong, resistance ratios reached 17.0 in Shouguang and 89.2–96.1 in Shenxian (2014–2015). Other populations ranged from 4.3 to 7.2. The authors described "a widespread reduction in spinosad efficacy" (Li et al., 2016).
• Spain: Greenhouse resistance was documented in southeastern Spain by 2007 (Bielza et al.).
• North Carolina: Field resistance to spinetoram, a related spinosyn, was documented in a 2022 study.

Spinetoram is not the same product as spinosad, but the two belong to the same chemical family. Resistance to one is a warning sign for the other. Ask your extension office whether cross-resistance has been seen in your area.

The evidence on Bt

Bt crops

The best-documented Bt resistance involves genetically modified Bt crops, which are not allowed in organic production. A 2023 review by Tabashnik and colleagues of the first 25 years of Bt crops found:

• 26 cases of practical resistance among 73 datasets;
• resistance involving 10 Bt toxins in 22 moth species and 2 beetle species;
• an average of 6.6 years until resistance developed;
• half of the cases in the United States;
• 30 cases with no significant resistance after 2–24 years.

Why organic growers should care

Bt sprays used by organic growers reportedly share toxin families with Bt crops. If pests such as corn earworm and fall armyworm evolve widespread resistance to Bt crops, organic Bt sprays could become less effective against those pests.

Field resistance to Bt sprays themselves has also been reported historically, for example in diamondback moth. We did not verify a source for that example, so treat it as something to confirm with your extension office.

The 30 cases with no significant resistance are encouraging. They suggest resistance is not automatic when use is managed well.

Why organic growers are especially exposed

Conventional growers can rotate among many modes of action. Organic growers often cannot. When one material is the only effective option for a key pest, it gets used again and again, which is exactly what drove resistance on Quebec organic potato farms.

That makes non-chemical tactics central to organic resistance management, not optional extras.

Organic options to rotate

The research behind this article covers a limited set of materials. Rotation only helps if materials work differently, so confirm which products share a mode of action with your extension office or crop advisor.

Check that every product is allowed for your crop, registered in your state and approved by your certifier. The label is the law.

A resistance-management plan

Reduce the number of sprays

• Scout and use thresholds. Spray when the pest population justifies it, not on a calendar.
• Use exclusion where it fits. Mesotunnels in Cornell trials cut cucumber beetles about threefold and matched or beat organic spray rotations. Our post on exclusion netting and row covers has details.
• Rotate crops to break pest cycles, especially for pests like Colorado potato beetle.
• Try trap crops where extension guidance supports them for your pest.
• Build habitat. Flower strips increased pest control in adjacent fields by 16% on average in a research synthesis. NRCS offers support for pollinator habitat through CSP enhancement E420A.

When you do spray

• Rotate materials that work differently, rather than using the same product back to back.
• Count your applications of each material per season. The Canadian survey tied resistance to how often spinosyns were used.
• Follow label rates and intervals. Bt, for example, needs earlier coverage and shorter intervals in orchards.
• Target the right life stage. Bt and granulovirus act on larvae.
• Protect pollinators when using spinosad or pyrethrins. Our post on why organic-approved isn't the same as bee-safe explains the risks.

Watch for warning signs

• Record control results after each spray.
• Take reduced control seriously. Rule out coverage, timing and weather first, then contact extension about resistance testing.
• Share observations with neighbors and extension so regional patterns are caught early.

What to do next

1. List your key pests and the materials you rely on for each.
2. Flag any pest where one material does most of the work. That is your highest resistance risk.
3. Add one non-chemical tactic for that pest next season.
4. Ask your extension office about local resistance reports for spinosad, spinetoram and Bt.
5. Check with your certifier before adding new materials to your rotation.

Sources

• Regional differences in susceptibility to spinosyn insecticides registered for Colorado potato beetle management in Canada — Scott et al., Pesticide Biochemistry and Physiology (2023)
• Six-year monitoring of pesticide resistance in the Colorado potato beetle during a neonicotinoid restriction period — PLOS ONE (2024)
• The Evolution of Spinosad Resistance in Colorado Potato Beetles — CUNY Graduate Center Academic Works (accessed Sept 2026)
• Role of CYP9E2 and a long non-coding RNA gene in resistance to a spinosad insecticide in the Colorado potato beetle — PLOS ONE (2024)
• Field resistance to spinosad in western flower thrips — Li et al., Journal of Integrative Agriculture (2016)
• Resistance to spinosad in the western flower thrips in greenhouses of south-eastern Spain — Bielza et al., Pest Management Science (2007)
• WFT field resistance to spinetoram in North Carolina — Crop Protection (2022)
• The State of Insect Resistance to Transgenic Bt Crops — Entomology Today (Apr 18, 2023)
• Global Patterns of Insect Resistance to Transgenic Bt Crops: The First 25 Years — Tabashnik et al., Journal of Economic Entomology (2023)
• What are Biopesticides? — US EPA (accessed Sept 2026)
• New England Tree Fruit Management Guide: Insecticide Options in Organic Apple Production — UMass Amherst (accessed Sept 2026)
• Management of Key Insects and Insect-Vectored Diseases in Organic Cucurbits with Mesotunnel Row Covers — Cornell Vegetables (2025)
• The effectiveness of flower strips and hedgerows on pest control, pollination services and crop yield — Albrecht et al., Ecology Letters (2020)
• Establish pollinator habitat (E420A) — USDA NRCS (accessed Sept 2026)