Ag Line: Armyworms in Pastures
By Wade Parker County Extension Coordinator

Almost every year in late summer, armyworms and corn earworm caterpillars invade pastures, hay fields, and turfgrass throughout the state. In one year, an armyworm infestation created a greased spot in the middle of a paved road as they attempted to move from pasture to pasture! As armyworms invade pastures, damage could be severe before the worms are noticed. The grass does not die, but hay and forage yield will be drastically reduced to almost nothing if left untreated.

During this time of year, most of the worms are fall army- worms. The adult moths are active at night and females lay eggs in batches of 50 to several hundred. Eggs hatch in 2- 10 days, and the young larvae begin to feed on leaf tissue. Damage from small larvae may at first skeletonize the leaf, but as the worms grow, the entire plant is consumed. Armyworms are most active early and late in the day, spending the hotter hours down near the soil in the shade. Larvae feed for 2-3 weeks before pupating in the soil. Moths emerge 10-14 days later.

Control options have changed some in the last couple of years. Along with the stand-bys, Sevin and Lannate, we also have Tracer, Dimilin, and now Mustang Max available. Dimilin and Mustang have no grazing restrictions and can be applied by ground equipment, with the cows in the field. Tracer has no PHI for grazing, but the label says not to let the cows graze "until sprays have dried." Keep in mind that Tracer at 1-2 oz is easy on beneficial insects and does provide a two-week residual. However, at least 1.5 oz/ acre may be needed to control large worms (3/4" long). The PHI for grazing after Sevin and Lannate remains at 14 and 7 days, respectively. There are also restrictions for hay or silage harvest and total amount per season for all products. Refer to the labels for those restrictions. In any case, if the hay is ready to be cut, cut it before treating. It also helps to increase your spray volume.

Controlling caterpillar pests is not very difficult. The key to control is detecting the pest early and treating as soon as possible. As always, please call (982-4408) if you have any questions!

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