Armyworm outbreaks can be difficult to predict but can be very devastating if unnoticed. Texas has four species of armyworms that can commonly be found. They include the yellow-striped armyworm, fall armyworm, beet armyworm and armyworm. The damage that we usually find this time of year is often from the fall armyworm.
Armyworms have four life stages which include the egg, larva, pupa and adult. The eggs are laid at the base of the plants in the field, and once they hatch, the larva is what causes the damage. They have 4 different instars or stages of growth that they go through that will last two to four weeks before they go into the pupa stage and go into the soil. Once they emerge as an adult, they breed, lay eggs and start the cycle over again. The fall armyworm larval color can vary from light tan to shades of green. The head is brown or black with a prominent white line between the eyes, which forms an inverted “Y.” The fall armyworm has four large spots on the upper surface of the last segment of its body. Along the middle of the larva’s back is a wide, yellowish-gray band with a dark, black stripe just below the yellowish-gray band.
In our area we often hear of the outbreaks occurring in bermudagrass and in small grain crops. The first three instars cause very little feeding damage, while the last two instars consume 85% of the total foliage consumed. I have heard reports of fall armyworms over the past month, but the much needed rain we received last week could increase the chances of outbreaks. Monitor your pastures and crops carefully. Control options should be considered when they are found in large numbers or plant damage is occurring. The best times to scout fields are early in the morning and/or late in the evening. Count the number of worms in a square foot area across many locations in the field and then determine an average per square foot. If two or three larvae per square foot are found in seedling small grains, then treatment may be warranted. For older small grains or improved pastures, three to four larva per square foot becomes the threshold.
Whit Weems is an Erath County extension agent. His column is published weekly and can be found online at yourstephenvilletx.com.