Fall armyworms

Percent defoliation thresholds gain popularity over number of worms per square foot.

By Carroll Smith Editor

The fall armyworm, which doesn’t overwinter here, typically begins its migration in South America and moves up through Mexico. It usually appears in the Mid-South in late July and August. But in 2021, wind and rainfall patterns created the perfect storm for the caterpillars to reproduce quickly and build up large numbers. In mid-May, populations were spotted in Texas, Louisiana and south Mississippi.

“Around June 1, we started picking up some really large numbers that started in pastures and then moved into rice and soybeans,” says Nick Bateman, University of Arkansas Extension entomologist. Some of my counterparts who have been around awhile said it was the worst armyworm infestation they had ever seen.

“We also saw a lot of fluctuation in pyrethroid control. For example, in one of our trials we got 20% control and the next one that we sprayed seven days later about a mile down the road, we got 80% control. That’s not typical. The armyworms that get in rice are usually easy to control with Lambda insecticide. That’s one reason we sought the Section 18 last year on Intrepid 2F insecticide because we didn’t have any other options.”

Percent defoliation thresholds

When it comes to rice, armyworms can infest every growth stage. The traditional scouting procedure is to look at the number of worms per square foot. Although seeing a lot of worms out in the field can be scary and prompt a knee-jerk reaction to spray, Bateman said Arkansas researchers are now moving toward basing thresholds on percent defoliation. The new threshold says no treatment is needed between seedling and 2- to 3-tiller rice unless armyworms are feeding on the growing point. For May and June plantings, treat when defoliation exceeds 40% at 5- to 6-tiller rice. For all plantings, treat when defoliation exceeds 20% at green ring.

“Higher yield loss has been observed for later plantings,” Bateman said. “Armyworms should be treated if head feeding or clipping occurs.

“We don’t have a true threshold based on counts with a sweep net. I sweep for armyworms to get an idea of how big the population is and what size the caterpillars are. The fall armyworm has between six and seven instars. If I have quite a bit of defoliation and a bunch of small caterpillars out there, I know they have the potential to exceed the defoliation threshold because they have quite a bit more to eat.

“If I see a lot of large caterpillars and a small amount of defoliation, I’ll probably ride the population to see if they will naturally cycle out. In a year like this where it’s going to be more costly in general to produce the crop, I am going to let the armyworms prove to me they can get up to those defoliation thresholds before I spray for them.”

Bateman said one of his goals this year is to provide education at field days regarding what 10% versus 20% defoliation in rice looks like.

Row rice, flooded rice

There has been speculation about whether armyworms are more troublesome in row rice than in flooded rice.

“Last year, the first 25% to 30% of the phone calls we got about armyworms were in row rice,” Bateman said. “But they also lay eggs in flooded rice, so I wouldn’t say one is worse than the other although flooded rice does have some benefits, especially if it’s smaller. If armyworms get in the field just prior to flood, and we can flood a bit earlier, we can potentially run them out of there. Armyworms can’t swim, so if they eat everything they are feeding on, they can’t move on because of the flood. If they get in the water, they will drown.

“In row rice, it did appear that the armyworm damage went further out in the field than it did in flooded rice. In flooded rice, we saw pockets with quite a bit of defoliation, whereas in row rice they damaged sections of fields because they could pick up and move a lot easier.”

Bateman also points out that rice seed treatment insecticides, such as Dermacor X-100 and Fortenza, are a good option for armyworm control.

“If you have a seed treatment on the rice, odds are you won’t have to worry about armyworms at all,” he said.

VICKY BOYD Arkansas entomologist Nick Bateman sweeps for armyworms to get an idea of how big the population is and what size the caterpillars are.