I’ve gotten a few calls on fall armyworms (FAW) infesting cover crop wheat and production wheat. FAW, in large enough numbers, will defoliate a wheat field if left uncontrolled and UT’s FAW threshold is four worms per square foot. To date, pyrethroids have been providing satisfactory control of FAW in soybeans and pastures and the same can be assumed for wheat. Pyrethroids are the most economic option but also provide the shortest residual (3-5 days). Other options include Vantacor, Besiege and Radiant/Blackhawk. Insecticide seed treatments for Hessian Fly and/or aphids won’t control FAW and if numbers exceed threshold a foliar spray may be required. Another option is to delay wheat planting until after October 15th (Hessian Fly Free Date) as infestations of fall armyworm almost always occur in early planted wheat, especially for wheat planted in September.
Bollworm egg lay across West TN is rapidly increasing. Our trap catches across the survey area are steadily increasing and I’ve started receiving several calls about large numbers of eggs . Fortunately, the vast majority of our cotton is Widestrike 3 (WS3) or Bollgard 3 (BG3). To date, I’ve had no reports of slippage through any three gene cotton in Tennessee. Going forward, our threshold for foliar applications targeting bollworms occurs when a combination of square and boll sampling shows 6 percent or more injury (e.g., 3 percent square injury and 3 percent boll injury, 4 percent square injury and 2 percent boll injury, etc.) and/or 4 or more larvae are present per 100 plants. Treatments based on egg lay is not recommended in 3 gene cottons. My colleague at Mississippi State, Dr. Tyler Towles, summarized our regional data (Fig. 1) demonstrating no economic benefit to making a diamide application to 3 gene cottons. Unless you’re at threshold, save the money on the diamide spray in 3 gene cotton.
Fig 1. Diamide Sprays by Bt Technology
One final note, as we are nearing the end of the 2024 growing season below are insect termination timings based on DD60s.
I’ve received several reports, over the last two days, on fall armyworms (FAW) infesting double cropped soybeans. Almost every one of these calls were incidents of grass escapes and the lone exception was FAW migrating from a grass turn-row into v-stage beans. Somethings to keep in mind, vegetative soybeans can withstand 30% defoliation before yield loss and reproductive stage soybeans can withstand 20% defoliation before yield loss. Below is guide to help gauge defoliation levels.
Visual Guide to Soybean Defoliation
If you do see armyworms it shouldn’t be automatically assumed that pyrethroids won’t be effective. We don’t fully understand why pyrethroids provided inconsistent control in 2021. Resistance is a possibility but overlapping generations causing mixing of various worm sizes resulting the appearance of poor control is another possibility. Also, since armyworms don’t overwinter in Tennessee and there is no evidence of reverse migration, the armyworms we experienced in 2021 won’t be the same ones we see in subsequent years. However, if pyrethroids do fail to control armyworms please contact your local county agent.
The FAW we are currently facing are the rice/grass strain, not the corn strain. They are impossible to distinguish visually but they are very host specific. Grass strain FAW almost always colonize grass, not soybeans, corn or cotton. Corn strain FAW almost always infest corn, soybeans or cotton not grass or rice. Another key difference is grass strain FAW are typically very susceptible to pyrethroids while corn strain are resistant. What happens when grass escapes occur in soybeans is the grass is controlled and FAW are forced off of a dying host on to soybeans. They don’t necessarily want to eat the soybeans but have no other options. If you have FAW in your fields and haven’t controlled grass yet, adding a pyrethroid would be a prudent step to save another potential trip. If you want other options outside of pyrethroids or are concerned they won’t work that list is here:
Acephate: 0.75 lb ai/ac,
Diamond: 6.0 fl oz/a,
Intrepid 4.0 fl oz/a,
Intrepid Edge 4.0 fl oz/a,
Vantacor 1.2 fl oz/a,
Elevest 5.6 fl oz/a,
Besiege 7.0 fl oz/a
Don’t automatically reach for the diamides (Vantacor, Elevest, Besiege) these products provide excellent FAW control but are costly. All of the above products will provide satisfactory control of FAW in soybeans.
