Fall Armyworms: Round 2

About a month ago, we received a flurry of questions on fall armyworms in hayfields and some crop fields in southern Iowa. While inquires tapered off towards the end of August, we’ve started to see another wave of reports about fall armyworms in hayfields this week. This is a reminder to take some time to scout hayfields and pastures for fall armyworms. This is particularly important as many hay fields are regrowing from the final cutting of the season and could be particularly susceptible as the plant is preparing for winter.

Scouting:

The best time to scout is ideally at sunrise or sunset, when the larvae tend to be most active. If you go out in the middle of the day, fall armyworms are typically found closer to the soil or hiding under residue. 

When scouting, using a sweep net is the best way to confirm the presence of fall armyworm in a field as the net will collect small larvae. If you find fall armyworm larvae in the sweep net, determine how many caterpillars are present per square foot by looking on plants and in the litter on the soil. If you don’t have a sweep net, check areas that look dead, wilted or drought-stressed, or where birds are congregating.

Management:

What do you do if you find fall armyworms this time of year? Harvesting the field is not an option given the calendar date. Therefore, farmers have two choices – spray an insecticide or let Mother Nature take its course. An insecticide application could be warranted if the larvae are less than ¾ inch long and there are more than 2-3 per square foot. Pyrethroids are likely a good option, but other states have noted minimal efficacy in some fields. Use high volume and pressure to create small droplets that ensure contact with the larvae. Always read and follow label directions and don’t forget to check pre-harvest intervals.

If the larvae are over ¾ inch long, they become harder to kill with an insecticide and it may be best to let Mother Nature take its course. The good news is that this pest cannot overwinter here in Iowa, and their growth will slow down when temperatures dip below 50 degrees. The first frost should take care of any remaining fall armyworms this year.

Cover Crops and Fall Armyworms:

Fall armyworms may find their way to cover crops. Where it could make sense to spray a cover crop field for fall armyworms is if you are planning on grazing or harvesting the cover crop for forage either this fall or next spring. With spraying, it’s important to consider the size of the larvae. Follow the recommendations above regarding the size of the larvae and the threshold.

For cover crop fields with extensive damage and that have been literally chewed to the ground, the question becomes will the cover crop come back? The growing point of cereal grains would still be below the ground, especially if the field was drilled, as the growing point doesn’t move above the soil surface until it reaches the jointing stage, which occurs in the spring. This is good news because it means the plants will continue to grow. However, as long as armyworms are still present in the field they will continue to eat off the new growth. Stand loss may be affected by how much regrowth there is before winter dormancy is induced.

Read more on fall armyworms and cover crops in this ICM Blog.