Spinetoram remains highly effective against fall armyworms, killing 95% of tested pests and reducing egg production to just 2.93 eggs per female per day, while thiodicarb has become nearly useless with resistance ratios up to 97.88. According to Gram Research analysis of this 2026 study, farmers should rotate between effective insecticides like spinetoram and discontinue thiodicarb use to prevent further resistance development.
Fall armyworms are becoming resistant to many pesticides used to protect crops, making pest control harder for farmers. Researchers tested seven common insecticides on six different armyworm populations to see which ones still work best. They found that one pesticide called thiodicarb barely works anymore, while spinetoram remains highly effective at killing the pests and preventing them from reproducing. According to Gram Research analysis, rotating between effective insecticides like spinetoram could help farmers manage resistance and keep their crops protected longer.
Key Statistics
A 2026 research article testing seven insecticides on six fall armyworm populations found that spinetoram achieved 95.13% mortality while reducing fecundity to 2.93 eggs per female per day, making it the most effective option tested.
The same 2026 study confirmed that thiodicarb showed extremely high resistance in field populations, with one population displaying a resistance ratio of 97.88, meaning nearly 100 times more pesticide would be needed to achieve the same killing effect as in susceptible worms.
According to the 2026 research, most other commonly used insecticides showed only low to moderate resistance levels with resistance ratios between 6.56 and 8.75, indicating they remain partially effective but require higher doses than previously needed.
The 2026 study found that spinetoram significantly delayed crucial developmental stages including egg hatching, larval growth, and pupation, disrupting the entire armyworm population’s growth cycle beyond just killing individual insects.
The Quick Take
- What they studied: How well seven different pest-killing chemicals work against fall armyworms, and whether the worms have developed resistance to any of them.
- Who participated: Six different populations of fall armyworms collected from the field, tested in controlled laboratory conditions with a special diet.
- Key finding: Spinetoram killed 95% of armyworms and reduced egg production dramatically, while thiodicarb barely worked at all, with armyworms showing up to 98 times more resistance to it.
- What it means for you: If you’re a farmer, switching between different pesticides instead of using the same one repeatedly can help keep pest control effective. For consumers, this research supports sustainable farming practices that protect crop yields.
The Research Details
Scientists collected fall armyworms from six different farm locations and tested how resistant they were to seven commonly used pesticides. They used a special method where they mixed each pesticide into the worms’ food at different concentrations and measured how many worms died and how many eggs the surviving worms produced. This approach mimics real-world conditions where worms eat treated crops. The researchers measured resistance by comparing how much pesticide was needed to kill the resistant worms versus normal worms—a number called the resistance ratio (RR). A higher number means the worms are more resistant.
Testing pesticides this way is important because it shows which chemicals still work in real farming situations. Laboratory tests that use direct contact with pesticides don’t always match what happens when worms eat treated plants. By testing six different worm populations, the researchers could see if resistance was widespread or just in certain areas.
The study used a standardized bioassay method, which is a reliable way to test pesticide effectiveness. Testing six field populations strengthens the findings by showing patterns across different locations. However, the study doesn’t specify the exact number of individual worms tested, which would help readers understand the precision of the results.
What the Results Show
Spinetoram was by far the most effective pesticide, killing 95% of the armyworms tested. It also dramatically reduced reproduction, with surviving females producing only 2.93 eggs per day compared to normal production rates. Spinetoram also slowed down important life stages like egg hatching, larval growth, and pupation, which weakens the pest population over time. Most other insecticides showed low to moderate resistance levels (resistance ratios between 6.56 and 8.75), meaning they still work reasonably well but require slightly higher doses than they used to. Thiodicarb, however, showed catastrophic failure, with one population showing a resistance ratio of 97.88—meaning it would take nearly 100 times more pesticide to kill resistant worms compared to normal ones.
All the effective pesticides except thiodicarb significantly reduced key population growth measures, including the rate of population increase, generation time, and the number of female offspring. This is important because it means these pesticides don’t just kill individual worms—they disrupt the entire population’s ability to grow and reproduce. Thiodicarb failed to slow development or reduce population growth, confirming it’s no longer useful for controlling this pest.
This research confirms what farmers and agricultural scientists have suspected: fall armyworm resistance to certain pesticides is a growing problem. The extremely high resistance to thiodicarb suggests this pesticide should be phased out of use. The effectiveness of spinetoram aligns with other recent studies showing that newer insecticide classes remain effective when older ones fail.
The study doesn’t specify exactly how many individual worms were tested, which makes it harder to assess the statistical precision of the results. The research was conducted in laboratory conditions with a controlled diet, which may not perfectly match field conditions where worms face other stressors. The study also doesn’t evaluate the cost-effectiveness of different pesticides or their environmental impact beyond toxicity to the target pest.
The Bottom Line
Farmers should rotate between effective insecticides like spinetoram rather than repeatedly using the same pesticide. This rotation strategy is strongly supported by this research and can slow the development of resistance. Thiodicarb should be removed from pest management programs due to widespread resistance. These recommendations have high confidence because they’re based on direct testing of field populations.
Farmers growing crops vulnerable to fall armyworm damage (corn, cotton, sorghum, and others) should pay close attention to these findings. Agricultural extension agents and pest management professionals should use this research to update their recommendations. Consumers benefit indirectly through more sustainable and effective crop protection. This research is less relevant for home gardeners unless they grow large quantities of susceptible crops.
Switching pesticide rotation strategies can show benefits within a single growing season by maintaining pest control effectiveness. Long-term benefits appear over multiple seasons as resistance development slows. Farmers may notice reduced pesticide effectiveness within weeks if they continue using thiodicarb, so switching should happen immediately.
Frequently Asked Questions
What pesticide works best against fall armyworms right now?
Spinetoram is the most effective option, killing 95% of armyworms and reducing their reproduction to just 2.93 eggs per female per day. Most other common pesticides still work but require higher doses due to developing resistance.
Why do some pesticides stop working on fall armyworms?
Repeated use of the same pesticide allows resistant worms to survive and reproduce, passing resistance genes to offspring. Over time, the resistant population grows and the pesticide becomes ineffective. This is why rotating between different pesticides is important.
Is thiodicarb still useful for controlling fall armyworms?
No. A 2026 study found thiodicarb has become nearly useless, with some armyworm populations showing resistance ratios of 97.88, meaning it would take nearly 100 times more pesticide to achieve the same effect as in susceptible worms.
How can farmers prevent pesticide resistance in fall armyworms?
Rotate between different effective insecticides like spinetoram every 2-3 applications instead of repeatedly using the same pesticide. This prevents resistant worms from dominating the population and keeps pest control effective longer.
Does spinetoram affect how fast armyworms grow?
Yes. Research shows spinetoram significantly delays egg hatching, larval growth, and pupation, disrupting the entire population’s development cycle. This weakens the pest population beyond just killing individual insects.
Want to Apply This Research?
- Track which pesticides you use on which crops and when, noting the pest control effectiveness (percentage of crop damage prevented) for each application. Record this weekly during the growing season to identify patterns in pesticide performance.
- Set up a pesticide rotation schedule in your app before the growing season starts, alternating between spinetoram and other effective options. Set reminders to switch pesticides every 2-3 applications to prevent resistance buildup.
- Monitor pest population levels and crop damage weekly, comparing effectiveness across different pesticide rotations. Track whether damage increases over the season (suggesting developing resistance) or stays controlled. Compare your results year-over-year to identify if resistance is developing in your fields.
This research evaluates pesticide effectiveness in laboratory conditions and may not perfectly reflect real-world field performance. Pesticide selection and application should follow local agricultural extension recommendations and product labels. Consult with a licensed pest management professional or agricultural extension agent before making changes to your pest control strategy. This information is for educational purposes and should not replace professional agricultural advice. Always follow all safety guidelines and regulations when handling pesticides.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.