In October 2024, we received a report of a fall armyworm (Spodoptera frugiperda) (Lepidoptera: Noctuidae) outbreak in a wheat field in Daviess County, western Kentucky (Figure 1). After visiting the field, we collected 82 live caterpillars after the application of a pyrethroid insecticide. The caterpillars were taken to the entomology laboratory at the UK Research & Education Center in Princeton. Mortality was recorded at only 14.6% and 15.8% at one and three days after the spray, respectively. 

How Insecticide Resistance is Developed in Insect Populations

One of the reasons for such low insecticide efficacy is resistance. An insect can be resistant to insecticides through three main mechanisms: 

• Having a thicker cuticle (“skin”) that reduces the penetration of the insecticide;
• Having higher levels of specific enzymes to break insecticides down faster;
• Having small changes at the target site that prevent the insecticide from binding in their body and stop it from working properly

Some insects in a population may already have one or more of these natural traits that make them less affected by some insecticides (i.e., resistant). When the same insecticide is used repeatedly, these resistant insects survive and reproduce, while the susceptible ones die. As a result, the few initial survivors can give rise to thousands of resistant individuals in just one generation. If this process continues over multiple generations, then most of the insect population will be composed of resistant individuals and that specific insecticide will no longer be effective.

Insecticide Resistance - Prevention and Management

However, this problem can be managed by rotating insecticides with different modes of action. While most insecticides target the insect's nervous system, they bind to different sites within it. Therefore, if an insect is resistant to one insecticide because it has a small change at the target site that prevents binding, it will still be susceptible to any other insecticide that binds on a different and unaffected part of its body.

We have conducted laboratory tests with this resistant fall armyworm population, and the results confirmed resistance to several pyrethroid insecticides. However, other insecticides—such as chlorantraniliprole and indoxacarb—provided 100% control. Using insecticides with different modes of action helps eliminate resistant individuals within a population, thereby preserving the effectiveness of available products. This practice also maintains access to a broader range of control options, including more affordable alternatives.

Further Recommended Reading

For more detailed information on the referred fall armyworm outbreak and insecticide resistance management, the following articles are recommended:

• Villanueva, R. T., & Batista, F. C. 2024. Warmer temperatures & tail winds of Helene might cause an outbreak of fall armyworm in cover crops in central KY. Kentucky Pest News. 
  https://kentuckypestnews.wordpress.com/2024/10/15/warmer-temperatures-tail-winds-of-helene-might-cause-an-outbreak-of-fall-armyworm-in-cover-crops-in-central-ky
• Batista, F. C., & Villanueva, R. T. 2025. Insecticide resistance: How it happens & how to prevent it in your fields. Kentucky Pest News. 
  https://kentuckypestnews.wordpress.com/2025/04/29/insecticide-resistance-how-it-happens-how-to-prevent-it-in-your-fields