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Remember to scout armyworms

Bruce Potter, IPM specialist, Dave Grafstrom, Researcher, Ian Macrae, IPM specialist, Ryan Miller, Extension educator - crops, and Fei Yang, Extension corn entomologist

It is important to scout grass crops such as corn and cereal crops this year. Currently, small armyworms have been observed in grass waterways, field borders, and grass hay ground with one report from corn. Migrating armyworm moths have been moving into Minnesota since early May. Not only did southerly weather systems in mid-May produce heavy rainfall in areas of southern Minnesota, but they also left behind large numbers of migrating true armyworm moths. Pheromone traps that were placed near northwestern MN grass seed fields detected a large armyworm flight on June 4-5.

The moths from these flights would have sought out and laid eggs in stands of lush, dense grasses, such as cereals, weedy grasses, roadsides, pastures, and hay ground. Rye cover crops are attractive egg-laying sites but differ in risk of armyworm infestation. Areas of dense, lush rye would be most attractive to moths. Rye that was killed before the moths arrived would not provide attractive egg-laying sites. True armyworms prefer to feed on grasses, but sometimes after rye or dense stands of grass weeds are killed, soybeans can be defoliated by starving armyworms.

Small armyworms and their feeding on grass hay. Photo: Simon Warmkagathje.

 Where to scout first

Check areas of dense, late-terminated rye, lush grass field borders, grass hay, or pastures first. Pay special attention to lodged areas of grasses or small grains, as they are particularly attractive to armyworms. Large groups of armyworms will move to new areas, such as nearby corn and small grain fields, if they run out of food sources where they hatched.

Armyworm larvae are most active at night and other low-light periods. The larval stage typically lasts 20-30 days depending on temperature and the mature larvae are 1 ¼ to 1 ½- inches long. Most vegetation is consumed during the last two instars and as a result, infestations often go unnoticed until it’s too late.

We could be seeing armyworm larvae well into July. Warm temperatures speed larval development. The presence of feeding injury to crops and frass means armyworms were there. Make sure armyworms are still present before you spray. A sweep net is a useful tool when scouting for the presence of small larvae. In corn, armyworm larvae may hide in the whorl or on the ground. In several areas within small grain or grass fields, vigorously shake the plants to dislodge larvae so they can be counted on the ground. In high populations, you may find pellet-like 'frass' (insect droppings) on the ground. Be sure to check under any dirt clods and plant residue.

Economic thresholds and treatment  

Economic thresholds for whorl-stage corn are 25% of the plants with two or more larvae/plant or 75% of the plants with one larva or more. For small grain and grass hay crops, treatment is recommended when there are 4 to 5 larvae/square foot. You may not need to treat the whole field if armyworm infestations are limited to a portion of the field but treat at least a swath or two ahead of the infestation.

In corn, only those Bt corn hybrids containing Vip3a protein are protected from true armyworm.  Depending on the crop, several pyrethroids are labeled for true armyworm control. In recent years, there have been some scattered reports of poor performance with pyrethroid insecticides on armyworms, but these are not necessarily due to insecticide resistance. Insecticides may not perform as well when applied under hot conditions and coverage is important.

Chlorantraniliprole, methoxyfenozide, spinosad, spinetoram, indoxacarb, carbaryl, and methomyl are also labeled for armyworm control on some crops. One site with a comprehensive list of insecticides and rates labeled for control of armyworms is the 2023 North Dakota field crop insect management guide.

Small larvae are typically easier to control and some of the labeled insecticides are only effective on small larvae. Read the label carefully for application rates, timing, re-entry interval and pre-harvest interval, herbicide interactions, and other restrictions. Always remember to evaluate the performance of your insecticide treatment.

More information on scouting tips and thresholds can be found at corn pest management - armyworm

Products are mentioned for illustrative purposes only. Their inclusion does not mean endorsement and their absence does not imply disapproval.

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