Fall Armyworm Moth Numbers Increasing

Fall armyworm (FAW) is a migratory pest that re-infests Kentucky each summer and is killed by frost in the fall. It often shows up later in the season after corn has tassled and lost its attractiveness to this pest. But it has been a recurring pest of pastures, and there have been several outbreaks in pastures the past several years. Last week, we recorded a significant increase in moth captures in Princeton, which means we need to watch pastures and late-planted corn in the coming weeks for FAW. Kentucky counties affected by this pest so far include Ballard, Lyon, Caldwell, Trigg, Logan, and Warren. Numbers have remained low in traps in Lexington.

In pastures, pay particular attention to areas where the grass may seem to thin out or turn brown. FAW damage often resembles drought stress.

FAW doesn’t survive freezes in winter in Kentucky and must recolonize each year from southern areas in Florida and southern Texas.  While there can be three or four generations in the South, we typically have only one or two generations in Kentucky.

Figure 1. Fall armyworm can be recognized by the dark inverted ‘Y’ on its head and the four larger dark spots near the end of its abdomen. (Photo: Ric Bessin, UK)
Figure 2. A fall armyworm egg mass and the ‘window-pane’ damage caused by young larvae. (Photo: Ric Bessin, UK)

Corn is attractive to FAW egg laying while in the vegetative stages. As corn begins to tassel it loses its attractiveness for egg laying. So fields in western Kentucky that were late planted are more likely to have FAW.

There are two strains of fall armyworm: corn strain and rice strain, with important differences. The corn strain feeds most commonly on corn, sorghum, and cotton. The rice strain prefers rice, alfalfa, grasses in pastures, millet, and vegetables. However, these strains are indistinguishable based on appearance.

Scouting & Management

Figure 3. Young fall armyworm larvae and their damage to grass blades. (Photo: Ric Bessin, UK)

Catching fall armyworm in its early stages greatly reduces damage to pastures and hay. While damage may appear to happen overnight, feeding by young stages is minimal and often overlooked compared to losses by 5th and 6th instar larvae. Even though the time to reach the 5th instar is similar to the time spent as a 5th and 6th instar, these larger larvae consume 10 or more times the amount of food of the young stages.

While larvae may hide during the hottest part of the day, the best time to scout for fall armyworm in pastures is until late morning or in late afternoon. A sweep net can be used to locate early infestations of fall armyworm.

If you find fall armyworm, the next step would be to count the number per square foot. If more than two to three per square foot are found in a pasture, it would be time to control them with an insecticide or cut the field.

~ Ric Bessin, Entomology Extension Specialist, from Kentucky Pest News

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