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Armyworms

A striped armyworm caterpillar wrapped around a young corn plant, eating the leaf whorl.

As of writing this article, both Fillmore and Houston counties, along with many other parts of the state, are in D0 (abnormally dry) or D1 (moderate drought) conditions. Given the current stage and conditions of our crops, there are many things that we need to be thinking about. Today’s article is going to focus on armyworms. If you’d like to stay up to date with other crop conditions throughout the year please check out the UMN Extension Strategic Farming: Field Notes webinars.

Before any application of insecticide for armyworms it is important to scout for economic thresholds. If economic thresholds are not being reached then the population is likely being kept in check by natural enemies. If that is the case you should hold off on spraying, but continue scouting to ensure that remains. Lush grasses are the preferred sites for egg laying. These can include stands of small grains, cover crops, and grass field borders. Armyworms seldom feed on broadleaf crops. The risk is increased in fields where a live grass cover crop or grass weeds are present at the time when moths are laying eggs.

Since true armyworms cannot overwinter in Minnesota they migrate into the state each spring. The larvae are active at night or on cloudy days. Scout using a sweep net in grassy borders and in small grains. Also, shake the small grain canopy and search the ground for larvae. The economic threshold for small grains is 4-5 larvae per square foot and/or once larvae begin to clip grain heads. The threshold in whorl stage corn is 25% of plants with 2 larvae per plant or 75% of plants with 1 larvae per plant.

Cultural control can be effective for armyworm management. Maintaining good, early-season weed control and avoiding late termination of grass cover crops can reduce the attractiveness of fields to egg-laying moths. 

Saw fly larva, wheat head armyworm, and cut worms are all look-a-likes to true armyworms. While scouting, ensure that you know the key identifying factors of armyworms. If you are unsure if what you found is a true armyworm you are welcome to submit photos to the UMN Extension. Residents in Fillmore and Houston counties can send photos by text message to 507-951-6609 or by email to wins0115@umn.edu

Additional information about armyworms can be found at www.extension.umn.edu by searching the page for armyworms. If you have questions about this or any other agriculture, horticulture or natural resources topic please reach out to your local Extension educator. Residents in Fillmore and Houston counties can call 507-765-3896 or 507-725-5807.

Author: Katie Drewitz, Extension educator

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