Armyworms in Lawns and Pastures

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Cary Sims
Cary Sims is the County Extension Agent for agriculture and natural resources for Angelina County. His email address is cw-sims@tamu.edu Educational programs of the Texas AgriLife Extension Service are open to all people without regard to race, color, sex, disability, religion, age, or national origin.

This year we have seen a tremendous outbreak of armyworms affecting pastures and hay meadows. Even my own mom, who lives well north of me has been calling, complaining of armyworms in her lawn, and asking what to do.

Earlier this year, I was certain that winter storm Yuri was going to set these armyworms back with all of the cold, freezing weather we experienced. I was sure their emergence this year would be delayed and certainly diminished. But it was at a beef cattle field day on June 4th that I heard that we had already had some armyworms damaging hay meadows outside of Diboll!

Outbreaks can occur in summer and fall and follow periods of rain which create favorable conditions for eggs and small larvae to survive. Don’t let the proper name of this insect, the Fall Armyworm, trick you into thinking that they won’t show up until later in the year.

Armyworms hatch from egg masses laid by the adult moths and are very small at first, causing little plant damage, and as a result, infestations often go unnoticed early on. Larvae feed for 2-3 weeks and full-grown larvae are about 1 to 1 1/2 inches long.

Once larvae are greater than 3/4 inch, the quantity of leaves they eat increases dramatically. Armyworms consume 80% of their total food intake during the last few days of their life as a caterpillar. For this reason, extensive feeding damage can occur in just a few days when you are most likely to notice them due to their size.

Development from egg to adult requires about 3 to 4 weeks during the summer and is longer during cool weather. There are several generations a year. Development ends with cold weather in November.

Given their immense appetite, great numbers, and “marching” across land, armyworms can damage entire fields, pastures, or neighborhoods in a few days. Once the armyworm completes feeding, in tunnels into the soil and enters the pupal stage. In 7-10 days, the moth emerges from the pupa and starts its cycle anew.

The key to managing armyworms is to detect infestations before they have caused serious damage. Often, rural landowners will notice large numbers of cattle egrets on a hay meadow. Though the birds are easy to spot, this may be too late.

In residential areas, homeowners will notice the caterpillars crawling across walkways and onto the porch.

The best way I’ve heard to detect armyworms in an open pasture is to regularly scout pastures early in the morning while there is plenty of dew on the grass. Wear your black rubber boots and walk thru

fields, or drive thru them on your ATV with a foot dragging off to the side, looking for small caterpillars on your boot. Affected grass may have a ragged look, different from grazed or mowed grass.

Armyworm larvae feed primarily during the night and during cloudy weather. Later in the day, look for armyworms under loose soil and fallen leaves on the ground.

Hot, dry weather and natural enemies limit armyworm populations. Insect parasites such as wasps and flies, ground beetles, and other predators help suppress armyworm numbers. However, these natural factors can be overwhelmed when large numbers of migrating moths lay tens of thousands of eggs in a lawn or field.

There are several insecticides labeled for armyworm control in pastures. Remember that approved insecticides can kill non-target insects. Do not allow drift across fences to areas with blooming plants as beneficial pollinators will certainly be harmed.

For homeowners, it is very important to make contact with the worms by spraying in the morning when they are actively feeding.

For larger agricultural operations, costs vary, but more importantly is the restrictions of certain products related to having a pesticide license, grazing, and haying after application. Make sure to read the labels and apply correctly for best control. Always read and follow all label instructions and restrictions on any pesticide use.

When my mom asked what she needed to do for her lawn, I told her not to even worry about it. She can be thankful that she doesn’t need her grass to graze livestock, and, at worst, she’ll have a ragged looking lawn for a week or two until her turfgrass regrows.

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