Summer is in full swing and pests are abundant, as usual. I see a lot of growers who find an effective pest control product and then use it solely, because it works so well. When we repeatedly expose a pest population to the same mode of action (MoA), it selects for individuals that can survive that chemistry. Survival leads to resistance over time as the survivors pass on resistance to their offspring. Here are some basics to keep in mind to minimize the chance that pests become resistant:
Don’t spray if you can avoid it
My favorite alternative to spraying is slapping Colorado Potato Beetles into a bucket with a few inches of water with a little dish soap. I've had success with this method on plantings up to 4-acres in size. Walk the row and strike the top of each plant against the bucket to dislodge larvae into the soapy water — expect to capture roughly 90% of them. 90% is a great success rate and may exceed that of many chemical sprays these days. Colorado Potato Beetles have become resistant to many pesticides and organic producers are seeing their main weapon, Spinosad, lose effectiveness in Iowa.
Another approach shared by Footprint Farm in Vermont involves dislodging larvae into the pathway between with badminton or tennis rackets, then flaming them with a flame weeder. I’m not sure how effective this was, but it seems very satisfying! The physical removal bandwagon is gaining traction, equipment manufacturers in the Netherlands are marketing a $15,000 tractor implement that slaps the beetles into collection bags.
Other growers have had success with applying deep straw or hay mulch. The Midwest Vegetable Guide cites a study that showed, “deep mulching can reduce populations by 50-75% by hindering their movement.”
Rotate Modes of Action, Not Just Products
The key when resistance management is to rotate by how the product kills the pest not simply by brand name or product. For example, rotating Pyganic (pyrethrins) with Azera (pyrethrins plus azadirachtin) would not be effective rotation since they both contain pyrethrins. Effective rotation might look like applying Bt to kill cabbage worms 2-3 times in a row, then switching to a product with a different mode of action, like Spinosad. Rotating Bt products, such as DiPel followed by Xentari or Deliver may be effective, but would not be truly effective rotation since each product merely contains a different Bt formulation.
Using 2 MoA is good, but Michigan State University’s Greenhouse Insect Pest Management Guide for 2026 encourages growers to rotate with a minimum of 3 Modes of Action.
For Pests with Distinct Generations (e.g., Colorado Potato Beetle)
When a pest has two clearly separated generations per season, use one mode of action on the first generation and switch to a different one for the second. For example:
- First generation: Physical Removal, or Entrust (Spinosad)
- Second generation: Physical Removal, or Azera (azadirachtin + pyrethrins), or a tank mix of AzaGuard and Pyganic.
Source : iastate.edu