True Armyworm Larvae (Photo by Adam Varenhorst)
- True armyworm moths arrive every year and lay eggs, typically on grassy species
- Pastures, cover crops, small grains, weedy no till fields, and corn planted adjacent to the aforementioned crops may be at increased risk
- True armyworms typically are detected at dusk or dawn on field edges
- Application of insecticides should only be considered if larvae are less than one inch in length and applied at dusk or dawn when larvae are exposed and most active
Armyworm reports have been on the increase in several parts of Western Illinois and Iowa in recent days. This article will discuss these insects and their management.
General information: Armyworm moths fly in the spring and look for preferred egg-laying sites. These include pastures, cereals and weeds in no-till or reduced till fields. In wheat, leaf feeding will occur and head clipping may be observed. Eggs hatch and grow into caterpillers that feed, preferable on grasses. Corn fields should be scouted for armyworm damage especially if planted in no-till ground or neighboring cereal cover crops where armyworms are present. As the cover crop is killed, the larvae will start looking for other food sources and corn is a suitable meal for armyworms. Damage will occur mostly at night and will start on the leaf margins toward the midrib. Under heavy pressure leaves may be completely stripped off with only the midribs left. Although not the most preferred crop for armyworms, soybeans planted into wheat stubble or dying rye with heavy armyworm pressure could potentially be at risk. If the larvae are starving and soybeans are the only food available, some damage may occur. Scouting crops planted into or next to cereal crops with heavy armyworm pressure is the only way to assess damage and make insecticide decisions if necessary.
Scouting: Armyworms can be hard to find during the day as they are most active when it is cool and humid. While scouting, make sure to check under crop residue. Examine 20 plants in 5 areas of the field and record the number of plants showing signs of armyworm feeding. You may start by examining the areas adjacent to small grains or rye.
Control: Control may be warranted if 25% of more plants show feeding from VE-V4 and larvae are smaller than ¾ inch OR larvae are smaller than 1 inch and 75% of plants show feeding and plants are between V5-V8. Larvae larger than 1 inch are too big to be controlled by insecticides and are close to pupation, and hence will soon cease feeding. Thus, insecticide applications to larvae larger than 1 inch are ineffective and unnecessary. Due to the pest starting feeding on field edges adjacent to grasses, spot treatments can be effective if the damage and armyworm presence is concentrated on the edge or defined areas of the field. Occasionally, if planting into a killed rye crop with heavy armyworm pressure, tank mixing an insecticide with the rye burndown products (i.e. glyphosate) may be considered. There are numerous products labeled for armyworm control. In general, time is of the essence, so consideration for what is available and can be applied quickly should be a major consideration if thresholds are met.