They don't attack every year. But when they do, fall armyworms can cause major boll damage — after they've chomped on your corn.

To make it worse, Bt cotton and corn have failed to provide full fall armyworm control in many areas, says Ryan Jackson, entomologist for USDA-ARS, Stoneville, MS.

“With the large increase in corn production in the South, corn is likely to be the driver for fall armyworm infestations,” says Jackson. “Bt cotton and corn have never provided complete control of fall armyworms. They will infest the corn first. Once that dries, they will move into cotton and other host plants.”

Fall armyworms are sporadic pests. Infestations may be absent for several years before new strikes occur. Unlike bollworms, their larvae have an inverted “Y” on their head, according to insect descriptions from Texas A&M University.

Most fall armyworm larvae are tan in color with some stripes. Bollworms can be tan also, but are more solid in color and often pink to yellow to black in color, as well. Fall armyworm larvae can grow to sizes similar to bollworms, up to 1½ in. long.

“It's hard to detect fall armyworm infestations at early stages,” says Jackson. “Egg masses are low in the canopy and difficult to locate. Economic damage is caused by the larger larvae.”

In observing how the worms react to Bt cotton, such as Bollgard, Bollgard II and Widestrike, Jackson says the insects can cause boll damage in all three.

“Studies have shown a 30% reduction of damage with Bollgard cotton, 30-50% with Bollgard II and 50% with Widestrike,” he says. “So these are not stand-alone products, and supplemental insecticide applications will be necessary in some situations.”

Treatment may be justified when counts average 10,000-20,000 small worms or more per acre. Texas A&M entomologists indicate that insecticide application may be justified at first bloom when 30% of the green squares are worm-damaged.

If bolls are present, application may be warranted when 15 or more small larvae are present per 100 plant terminals and 10-20% of the squares or bolls are worm-damaged. If worm numbers are high, it may not be appropriate to wait until the damage threshold of 5-15% square-damage is reached.

Treatment of fall armyworm can be made with several insecticides. “Intrepid works well,” says Jackson. “So do Tracer and Steward. But depending where you are, the most common treatments have been high rates of a pyrethroid or Orthene, depending on whether there are plant bugs or bollworms in the mix.

“In the last couple of years, we've also seen good control with Diamond. Regardless of insecticide, it is recommended that spray volume be increased to increase control,” Jackson says.

He says Diamond has worked well in controlling tarnished plant bugs. “Because of recent problems with resistance development to some of the organophosphate standards, such as Orthene, Diamond should be a good fit into insecticide chemistry rotations for plant bugs,” says Jackson. “We've seen some good residual control of fall armyworm following Diamond applications for plant bugs.”

Movement of fall armyworms typically takes place from south to north. “Our problems are normally later in the year in later-planted cotton or in later-maturing varieties,” says Jackson. “Our treatable levels vary from year to year.”

With the projected increase in corn acreage, it's likely there will be an increase in whorl stage applications directed at fall armyworms. “Pyrethroids are a common choice for fall armyworms at this stage,” he says. “These sprays are economical and usually provide good control because it's easy to deliver the insecticide to the insect in the whorl.

“Other insecticide options are available, and we encourage producers and consultants to contact their local or regional Extension entomologist for recommendations,” Jackson says.