Insecticide Resistance: How It Happens & How To Prevent It In Your Fields

DR. FELIPE C. BATISTA AND DR. RAUL T. VILLANUEVA

PRINCETON, KENTUCKY

Currently, we have been evaluating two insect species that are occurring in western Kentucky and have developed resistance to pyrethroids. The fall armyworm, Spodoptera frugiperda(Lepidoptera: Noctuidae), and the alfalfa weevil, Hypera postica (Coleoptera: Curculionidae). Briefly, we detected an outbreak of a fall armyworm population in October 2024 (Figures 1A and 1B) and conducted laboratory tests that subsequently revealed the resistance of this wild armyworm population to several classes of pyrethroid insecticides. In the spring of 2025, we found and received multiple reports of alfalfa fields infested with alfalfa weevils in western Kentucky (Figures 2A and 2B) after applications of pyrethroids. Resistance of alfalfa weevils to pyrethroids has been known for a while in many states. These findings prompted us to prepare this publication, aiming to inform local growers and agricultural professionals about the development of resistance in insects and strategies to manage it effectively and mitigate further resistance development.

 How Insecticide Resistance Develops

Insects can develop resistance to insecticides in three main ways:

1. Having a thicker cuticle (insect “skin”) that reduces the penetration of the insecticide,
2. Breaking the insecticide down faster by having higher levels of specific enzymes, or
3. Having small changes at the target site, preventing the insecticide from binding effectively in the insect body.

Similar to what happens to humans and other living beings, individuals within a population of insects are slightly different as a result of a mix of genes from their parents. Thus, a population of insects sometimes has few individuals with one or more of these natural traits that make them less affected by certain insecticides.

Insecticide resistance is a gradual process of selection that happens over time if a population is repeatedly exposed to the same pesticide. A good insecticide is expected to kill at least 80% of the insects, which means even an effective product might leave behind some survivors after spraying (Figure 3). Those survivors go on to reproduce, and their offspring inherit the traits that allow them to survive. So, those few survivors can lead to thousands of resistant insects in the field after only one generation (Figure 3). If this process continues, and the resistant insects keep surviving and reproducing, it only takes a few generations before the entire population becomes resistant and the insecticide becomes ineffective.

  The same idea applies when insects are resistant because the insecticide does not penetrate their bodies well due to a thicker cuticle. In that case, you can use a product that penetrates better, or one that gets into the insect in a different way (such as through ingestion, when the insect eats a treated plant) or interferes with their metamorphosis. It is also similar to resistance caused by degradation. Some insects produce enzymes that break downinsecticide A, but might not affectinsecticide B.That’s why rotating insecticides with different modes of action is so important. It helps prevent resistance from building up and keeps your insecticides working, so your field stays protected!

Take-home Message

One of the main reasons for the repeated use of pyrethroids is their lower cost compared with other insecticides. However, it is important to consider that the insecticide itself represents only a fraction of the total application cost, which also includes equipment, fuel, and labor expenses. If cheaper insecticides are overused and pests become resistant, growers will end up needing extra sprays to control them. When considering all associated costs, relying on the same insecticides can be both less efficient and more expensive in the long term. Sometimes, spending a little more up front on a different insecticide can save money overall.

Our laboratory tests with the resistant fall armyworm (FAW) population revealed resistance to pyrethroids. However, other insecticides, such as chlorantraniliprole and indoxacarb, provided 100% control. Rotating insecticides with different modes of action after initial treatments helps manage both susceptible and resistant insect populations. This strategy slows the development of resistance and helps preserve the long-term effectiveness of available products – including the cheaper ones we want to protect.   ∆

DR. FELIPE C. BATISTA AND DR. RAUL T. VILLANUEVA: University of Kentucky