True Armyworm moth captures have increased in black light traps in Wisconsin over the past two weeks, and are high in neighboring states MN and IL. Here is an excerpt from the WI DATCP black light trap report for true armyworm for the week ending June 15th, by Clarissa Hammond: “Elevated numbers of true armyworm moths were captured for the second week in black light traps around the state. A high count of 257 moths was registered at the Marshfield trap location. Other counts were as follows: Chippewa Falls 7, Sparta 19, Janesville 78, Lancaster 32, Manitowoc 43, and Mazomanie 22. This flight is comprised of the adult progeny of migratory moths that arrived from reservoirs in the Gulf States last month.”

 

Over the last week, I’ve received a couple of reports of armyworm feeding on corn, from northwestern Wisconsin. Jerry Vacek, CCA, Advantage Ag, in the LaCrosse and Monroe County areas reported one V4-V5 corn field following winter rye with threshold feeding damage. June 15th, Rob Shields, CCA, Wisconsin River Agronomy, Mauston, WI found armyworm damage in corn, but was unable to find the larvae. With high black light trap captures in WI and regionally, and WI detection of feeding – this is a reminder to scout corn for first generation true armyworm larvae and feeding damage. Small grains and pastures can also be affected this time of year.  

Armyworm larvae are brownish green, hairless, and have alternate dark and light stripes down their backs. There are six larval instars, reaching approximately 1-1/2 to 2-inches when full grown. After larvae reach approximately 1-inch, they are nearly full grown and will stop feeding and pupate. Therefore control recommendations are based not only on the number of larvae but also their size (smaller larvae will continue to feed for a longer time). Armyworm economic thresholds for corn (gathered from several areas in the field, checking 5 sets of 20 plants) are 2 or more armyworms at ¾-inch length or smaller per plant on 25% of the plants, OR, 1 armyworm (3/4-inch or smaller) per plant on 75% of the plants.

 

Late instar true armyworm larvae

Photo Credit: Kevin Black, Growmark, Inc., Bloomington, IL  

Grasses and small grains are the preferred egg-laying sites for female moths. The greenish white eggs are laid in rows or clusters on leaves. One week to 10 days after the eggs are laid, larvae begin to emerge and feed. After 3-4 weeks, full grown larvae pupate for 2 weeks and emerge as adults. There are three generations per season, each generation lasting approximately 5-6 weeks. The success of the current first generation will set the stage for 2nd generation later in July. Second generation is typically the larger population and potentially most damaging. In July 2005, NW Wisconsin in particular experienced a 2nd generation true armyworm outbreak. (The third, fall generation is typically not injurious and is often heavily parasitized by beneficial insects, fungi and viruses).  

 

True armyworm feeding on corn, ragged from outer leaf margins in toward midrib

Photo Credit: Matt Hanson, Crops/Soils Agent, UW-Extension Dodge County (2006)

 

 

 

True armyworm feeding on corn, June 2004

Photo Credit: Joe Lauer, Corn Agronomist/Extension Specialist UW Madison, Agronomy.

 

 

For first generation true armyworm, corn fields with crop residue, weeds or dead grass should be watched closely. Presence of grass weeds in corn fields will attract moths for egg-laying. No-till corn fields may be particularly suited for egg-laying if soil is covered with crop residue, weeds, grass. Corn fields preceded by a winter rye crop may also attract female moths. As small grains are cut or grass weed hosts dry down, armyworm larvae can move quickly to corn, this can happen following a herbicide burn down in no-till or conventional tillage fields.

 

Larvae tend to feed at night or on cloudy days and hide in soil or under foliage during the day. Leaf feeding begins from the outer leaf margins, inward toward the leaf midrib – giving corn leaves a ragged appearance.

 

In wheat and pasture grasses, examine the soil between two rows at several points in the field and determine the number of larvae per square foot, populations at 3 larvae per square foot may justify treatment. I haven’t received reports to date of wheat or pasture grass infestations, but will forward information as/if reports come in.

 

Consult UW-Extension bulletin A3646 Pest Management in Wisconsin Field Crops for insecticide label information for armyworm in corn, small grains and pasture if economic thresholds are reached.