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Look under your plants for armyworms and/or signs of their frass (droppings). You may also find larvae hiding under plant debris. If you are growing barley or wheat, you may find worms inside of the heads. Newly hatched armyworms may have no distinguishing marks, but older armyworms will be either brown with yellow stripes or green with light stripes. Armyworm droppings resemble black pepper. If armyworms have infested your hay field, mowing the field is the first course of action. As the hay dries, the armyworms lose interest in it as a food source and move on. You can apply insecticides to the field using either ground or aerial equipment. The following insecticides have been proven effective on controlling armyworms: Asana XL should be used only on corn crops and should not be applied within 21 days of harvest. Permethrin is also only for use on corn and should not be used within 30 days of harvest. Carbaryl (Sevin) may be applied either to corn or wheat. Do not do more than two applications and do not it apply within 21 days of harvest. Ethyl is useful for corn, sorghum and all small grains but can only be applied with an aerial application. Do not apply ethyl within 12 days of harvesting corn or sorghum, and within 15 days of harvesting small grains. After you apply the ethyl, post notices that the field is being treated, and stay out of it for three days. Lorsban can be applied to corn and sorghum. Do not allow livestock to graze in the field for at least 15 days after you apply lorsban. Do not feed meat or dairy animals grains treated with Lorsban until at least 35 days have passed. Lannate or Malathion can be used on all crops. Do not spray these insecticides within seven days of harvest and stay out of the field for two days after treatment. Methyl is used for aerial application on corn and small grains only. Do not apply it within 12 days of a corn harvest, or 15 days of a small grain harvest. Post notices in the fields, and stay out of them for two days. Warrior can be used in corn, sorghum or wheat fields. Do not apply it within 20 days of corn harvest or 30 days of sorghum or wheat harvest.

Summary:
Look for armyworms and/or droppings. Mow your hay fields. Use insecticide.