During warm weather, the caterpillar stage lasts about 14 days, but eats about 80 percent of its total intake during the last three or four days. That’s when the worms are at their largest and can do the most damage to a crop.
It’s also when huge numbers of them can transform a hay field from a thing of beauty to a severely damaged nightmare seemingly overnight.
The caterpillar has broad bands down its body and a light colored inverted “Y” on its head. It also has some raised dark spots along with a few black hairs down its back.
The adult moth has a wingspan of about an inch and a half, with mottled gray forewings and light colored, translucent hind wings.
Unlike the seemingly indiscriminate diet, of the caterpillars, the female moths can be pretty selective about where they lay their eggs, opting for only the best, most productive pastures and crops for their future brood to flourish in.
However, wet weather can bring on exploding populations like we are currently experiencing. During these outbreaks, ‘housing shortages’ can occur so moths may lay their eggs on just about any available host plant.
The eggs are laid in clusters and covered with light-colored wing scales. Adult moths typically live 10-21 days, during which females lay 1,500-2,000 eggs — mostly in the first half of her adult life.
It’s also important to note that fall armyworms are great travelers. Their long-range flight takes many well into Canada each year where they feast on crops, grasses and weeds. But whether they feast in Canada or Texas, cold weather eventually zaps them as they can’t survive latitudes through the winter that are much farther north than South Texas.
Hay fields and pastures should be judiciously scouted for fall armyworms early in the morning or late in the evening. That’s because they hide under clods or dead vegetation to escape the heat of midday.
When two to three armyworms longer than half an inch are found per square foot, it’s time to take action as forage losses can be prevented by using an insecticide. As with scouting, insecticides will be most effective if applied when the worms are actively feeding on host plants in the early morning or late evening.
As far as what to spray, pyrethroid insecticides as they are relatively inexpensive and are normally an effective option for fall armyworm control in wheat, Coastal Bermuda grass and other hay and pasture crops.
Lonnie Jenschke is an Erath County extension agent.