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Bill Starr: Army worms on the march

Back in the late 1800s there was an infamous Union General (Sherman) that made a 285-mile march across Georgia pretty much destroying everything in his path. Well, here we are in 2016, and it seems another army is marching into our area. If you listen carefully I will bet you can hear them marching, or probably more appropriately eating,
No, I am not talking about some invading army, at least not an army of people any way; I am talking about army worms. I have had several reports this week of army worms in our area. Army worms are a good example of a pest that can sneak up on you. When it does, the results can be disastrous. Usually later in the summer army worms invade pastures, hayfields and to a lesser extent, turf grass throughout our area. Particularly in hayfields and pastures, damage may be severe before worms are noticed. The grass is not killed, but hay and forage yield can be reduced to almost nothing over whole fields in extreme cases.
Army worms get their name from their feeding habits, Army worms will eat everything in an area and once the food supply is exhausted the entire “army” will move to the next available food source. The army worm’s diet consists of mainly grasses and small grain crops. An infestation is difficult to detect as the caterpillars migrate to new feeding areas during the cool of the night. Army worm damage seems to appear almost “overnight.”
Generally the young, small army worms don’t eat that much; it is the larger, older caterpillars that eat more than all the other ages put together. So probably an infestation may have been present but not detected due to the small size of the caterpillars. Large army worms frequently seem to disappear almost as suddenly as they appeared, either burrowing into the ground to pupate or moving in search of food.
Scouting pasture and hayfields can help detect army worm infestations before they cause economic damage. An easily detectable sign of army worms is the presence of flocks of birds (especially cattle egrets or cowbirds) feeding in pastures or hayfields. Closely examine areas where birds are congregating. When scouting, if no caterpillars are found on the grass, look carefully at the base of the plants for larvae and caterpillar “pellets.”
The decision to treat for army worms will depend on the stage of the army worms and the intended use of the forage. A population of three or more per square foot is a reasonable threshold. Timing is critica;l if infestations are detected too late the damage may have already been done. If necessary,  treat with insecticides at the right time. Small army worms are much easier to kill than large ones. Quite often mowing is the best option for salvaging a hay crop. Mowing may make it possible to avoid using an insecticide. If an insecticide is needed there are several choices available, Carbaryl (Sevin), Lannate, Karate and a few others. Dimlin is a growth regulator and can be used as a preventative if you have had issues in the past. As always please read and follow all label instructions; each of these products have specific instructions regarding their use.
If you have questions please don’t hesitate to call our office. Apply insecticides either early in the day or late because army worms are most active at these times. Control of larvae greater than three-quarters of an inch may be poor; control of armyworms in tall thick stands of grass may also be poor. So if you are cruising by your pasture and you hear marching, you may want to take a closer look to see if you have the “army” in your pasture.

Bill Starr is Sumter County Extension agent, University of Georgia Cooperative Extension Service. Contact him at 229-924-4476.