This year we have seen a tremendous outbreak of armyworms affecting pastures and hay meadows. This incredibly rainy weather that we have had in the first half of the year has really provided ideal conditions for their development and expansion.
Looking around my area in East Texas, Longview and Tyler area ranchers report they saw the caterpillars in early July. The earliest I’ve heard about this year was near Palestine in early June and in Panola County in late May. Armyworm outbreaks can occur anytime from early summer to the fall, and follow periods of rain which create favorable conditions for eggs and small larvae to survive.
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, it 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 often first 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.
Homeowners can typically choose from an array of insecticides that are labeled to kill insects in the lawn. The key is to make contact with the insect, regardless of the product.
For commercial forage producers, there are several insecticides labeled for armyworm control in pastures and hay meadows. Some of the more common ones that do not require a pesticide license include Intrepid 2F, Baythroid XL, Sevin variants (to include Sevin 4F, Sevin XLR, Sevin 80S, & other generics), Malathion 57%, Tracer, Prev-Am, and Vantacor (which has the same active ingredient as Prevathon.) Others that you can purchase with a Private Pesticide License in Texas are Mustang Maxx, Lambda Cy, Dimilin 2L, Besiege, and Tombstone.
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 larger agricultural operations, the costs per acre vary, but more important is the restrictions of certain products related to when you can graze or bale hay after each 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.
