Managing Fall Armyworms in Alfalfa
“The alfalfa is alive with crawlers!” A remark I heard last week as I scouted an alfalfa field heavily infested with Fall Armyworms. 2021 threw some curveballs this year, but I certainly could not have predicted a widespread Fall Armyworm outbreak with much of the Midwest under drought conditions.
Spodoptera frugiperda, are a southern species of Armyworm that do not overwinter in the Midwest. As populations build during the summer, moths fly north often reaching the Midwest later in the summer or early fall. Hence the common name “Fall Armyworm”.
Fall Armyworms can feed on alfalfa, pastures and newly seeded cover crops and small grains. This year there have even been reports of them taking out entire lawns in search of lush plant material to feed on in dry conditions.
Life Cycle and Feeding Patterns
The quick lifecycle of the Fall Armyworm is what makes their feeding pattern so impactful to alfalfa stands. Eggs are laid by moths in clusters of 50-200 and hatch roughly four days later. Larvae grow quickly and move widely. Days 14-30 when larvae are growing to sixth instar are their most destructive stage as many growers notice a small, stressed patch one day with complete defoliation in as little as 48 hours.
Threshhold and Treatment
One to two worms per square foot can destroy seedling alfalfa and populations of 10-15 per square foot have been observed to destroy 12 to 14-inch alfalfa. Some experts advise not treating unless the majority of larvae are less than 3/4” long since more advanced larvae are nearing the end of their lifecycle. I agree with that line of thinking, however in alfalfa this must be heavily weighed against survivability of your alfalfa or cover crop stand. The short lifespan of armyworms means eggs can be laid at different times and all stages (instars) can be present in your field. With the aggressive and destructive nature of the insect, chemical treatment may be best way to control. It’s tricky, as we are in that time frame of “no cut” for alfalfa, a time which we need every single leaf to generate and build our root reserves for the winter months.
Bottom line: scout early, often, and plan quickly.