Sep 21, 2021 9:58 AM

CAMPBELL: Fall armyworms are abundant in farmer’s fields

Posted Sep 21, 2021 9:58 AM

Cottonwood Extension District

Armyworms are so-called because they can march across fields or landscapes in large groups. That seems to be happening this year as there are reports of armyworms feeding on tilled volunteer wheat fields, to the point they have taken out a lot of the volunteer wheat, even turning it white in some fields.

That brings up the question from farmers: Is this something I should be worried about, and possibly spray an insecticide for? The short answer is no. 

I visited with J.P. Michaud, entomologist at the Ag Research Center in Hays. Most fall armyworms should be nearing the end of their last generation this year, meaning once they reach maturity, they will pupate and then emerge as a moth. Then, due to shorter day length (winter coming), they will pack-up and head south for the winter. 

However, this could take a few more weeks, possibly more. Primarily, this is due to the sporadic hatching of the last generation of armyworms. The first generation is pretty synchronized, but after that, the following generations develop asynchronously, meaning some larvae are still small while others have finished feeding. 

Insecticide seed treatments do not work well against armyworms and other lepidopteran larvae.

The bottom line is early-planted wheat is at the greatest risk. Early-planted fields should be
inspected frequently during the first few weeks following emergence. Unless you are planting early for some grazing, it is preferable in our area to plant towards the end of September and into early October. Historically, yield does not decline unless planting occurs after the second week of October.

Early sowing of wheat can lead to several problems, including an increased chance of insect- or mite-transmitted viral diseases, or decreased emergence due to high temperatures. Ideally, growers should consider planting within the optimum window, but, if planting early due to moisture availability or a dual-purpose system, growers should consider selecting wheat varieties with tolerance to the major yield-reducing factors in their respective region. Growers might also consider a seed treatment with both fungicides and insecticides if planting wheat early in Kansas. 

Young fall armyworms are ½ to 3/4 inch long, and are 1 ½ inches long at maturity. Body color may vary from green to almost black, but light stripes will be visible along the length of the body. Look for a whitish inverted "Y" on the face. It normally takes 2 to 3 weeks to develop from egg to pupa. The adult is a moth.

Another question that may arise is, do I still need to spray or till under the volunteer wheat that had heavy armyworm feeding? I would err on the side of caution and still do so, as I doubt that every single volunteer wheat plant is completely dead. Also, please keep in mind that all volunteer wheat should be killed within a mile of any new planting, ideally two weeks beforehand. 

If you have any questions give us a call or email--Stacy Campbell 785-628-9430, [email protected]  J.P. Michaud 785-625-3425, [email protected]