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Fall Armyworm and Stem Maggot Management in Hay

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This year, the Fall Armyworm made is presence a little earlier than what we typically see in our area. This pest is one that hay producers usually have on their radar in late August but reports of damage came in during July. The Fall Armyworm is a common pest in hay, especially bermudagrass, and can decimate a crop if left unchecked. Another insect that often goes unnoticed in hayfields and has become a common visitor over the last few years is the Bermudagrass Stem Maggot (BSM). Although there are several types of “worms” that can be found in a hayfield. The fall armyworm and stem maggot are the most important to identify and control.

Fall Armyworm Identification: To determine if a field has an infestation, look for caterpillars with dark heads that are usually marked with a distinct, pale, inverted “Y” on top. Typically, you will find a black stripe down each side of their body and a yellowish-gray stripe down their back. Fall Armyworms come in a variety of colors including, green, brown or black which can make identification difficult.

Before managing Fall Armyworms, it’s important that we understand this pest’s lifecycle. Fall armyworms are the larvae (or caterpillars) of the Ash-gray moth. Like butterflies, the Ash-grArmywormay moth starts out as a caterpillar before going through metamorphosis. This moth has white wings with light gray spots. Female moths lay eggs at night and lay up to several hundred that hatch within 2 to 4 days. What hatches from these eggs are what we call fall armyworms. Development from egg to fully grown Fall Armyworm requires about 2 to 3 weeks. At this point, armyworms burrow down into the soil and form pupae. In about 10 to 14 days, the moths emerge.

While this article concentrates on hay fields, note that Fall Armyworms will attack centipede and bermudagrass lawns. Armyworms feed just about any time, day or night, but are most active early in the morning or late in the evening. Because they are active in the morning, this is a great time to scout your fields. These caterpillars will march like an army across your fields eating plant matter along the way. They tend to start from a field edge and work their way across to adjacent farms. In severe infestations, leaves will be completely eaten with only stems left behind. This highlights the importance of scouting your fields regularly so that you can implement a control measure in a timely manner.

Stem Maggot Identification:  The bermudagrass stem maggot (BSM) is a relatively new pest of bermudagrass grown for hay. This pest is native to south Asia and was first reported in the United States in Georgia in 2010. This pest is only known to infest bermudagrasses. The adult stage of the BSM is a small, yellow fly, and lays its eggs on bermudagrass plants. Once the egg hatches, the maggot moves to the top of the stem, burrows into the shoot, and consumes the plant tissue in the stem. This stem damage results in the death of the top 1-2 leaves while the rest of the plant remains green. The damaged leaves can be easily pulled from the stem. Grass growth will be stunted and damage can appear similar to frost, drought stress.

If you cut the stem below the dead leaves you can find the tunnel created by the maggot and occasionally you can see the maggot itself. The full-grown maggot is yellowish and about 1/8 inch long. Once the maggot completes feeding, it drops to the ground and enters the pupa stage. The adult fly later emerges from the pupae. The life cycle from egg to adult fly requires about 3-4 weeks, and there are several generations a year.

Should I spray? Most research has shown that an average of 3 medium to large armyworms (about ¾ of an inch) per square foot is enough to cause significant damage to your hay crop or pasture and justifies a control measure, such as an insecticide treatment. For stem maggots, scout your fields for frost or a burn appearance. Before spraying an insecticide, you should consider the height of your grass. For both pests, if the grass is longer, usually within a week or so of harvest, it’s usually more cost-effective to mow and bale early instead of making an insecticide application.

What can I spray? If you do choose to apply an insecticide, read the label carefully to ensure it is safe to apply on hay! There are numerous insecticide options available so choosing the right product can be a bit overwhelming. Several pyrethroid insecticides, such as Mustang Maxx and Karate are effective against Fall Armyworm and stem maggot but are restricted use and can only be purchased with a pesticide license. Other products, such as Dipel and Intrepid Edge are worm-specific insecticides and are not restricted use but may be higher in cost. Regardless of what product you choose, be sure to abide by all label specifications, calibrate your sprayer and pay attention to grazing or haying restrictions!

If you have any questions concerning hay or pasture management please contact the N.C. Cooperative Extension of Richmond County office at (910) 997-8255.