True Armyworms

True armyworms are showing up in SW Wisconsin and NE Iowa. Armyworms are an occasional pest in the area, but seem to me more numerous than normal is year. Bob Rose and Chris McLimans are both reporting fields with high enough pressures that growers need to take action to prevent stand and yield losses.

This pest will rarely cause trouble in conventional-tilled fields, but growers should be checking reduced and no-till fields. Another likely area of infestation would be fields that had wheat stubble or rye planted as a cover crop that was taken off in the spring. Any kind of ground cover makes a great place for the moths to lay their eggs as they fly up from the south. When the eggs hatch, usually about 7 to 10 days, they begin to feed and will continue for 5 to 6 weeks. At the end of the feeding period they will pupate and reemerge as moths to lay eggs and start the cycle again, we usually see up to 3 generations per year. The second generation will usually appear in July, and is the most damaging.

This insect will usually feed on the leaf margins and will stop at the midrib. Young corn plants have a remarkable ability to recover from the feeding, an ISU study showed that corn in the V7 to V9 stage with 50% defoliation recovered after treatment and only showed a 2-6% yield loss.
Treatment should be considered in V7-V8 stage if larvae are less than 3/4 inch in length, the population is larger than eight larvae per plant, and there is 25% or more defoliation. If armyworms are less than 3/4 inch, they still have about a week to feed before they pupate. At 1 ½ inches, the larvae are about done feeding and treatment is not going to give any economic benefit. There are many common insecticides labeled for armyworm control, you will need to check with your local chemical supplier to see what pricing and availability is for your area.

Sidedressing NH3

Anhydrous Ammonia is a very good source of Nitrogen and also usually the cheapest form available. It is important to make sure you apply the NH3 properly to avoid injury issues.


NH3 is injected into the soil, and usually is the most concentrated within 4” or so of the knife track. In sandy soils or very dry field conditions the concentrated band will be larger because NH3 has a huge attraction to water, and it will move to find available soil water. If you run the knife too close to the rows, you could see root injury from the applicator knife, but there is an even greater risk of burning the roots of the plant from the NH3 itself. See the picture below of root “burn” from NH3.

Once the NH3 ties up with soil water, it moves with soil water to the plant roots and can be taken up and used without causing any injury problems.


Another way NH3 can cause injury is by burning the leaves above ground. In very wet soils the knife will seal the sidewalls so the NH3 cannot move into the soil profile. This will cause the NH3 to move back up the slot made by the knife and escape into the air. Not only will you loose the NH3 that you are applying, it will burn the leaves of the young corn plants. Usually the corn will recover, but the Nitrogen that escapes is lost and does not available for the plant later in the season when it needs it most. It’s always good to make sure soil conditions are good and that you are getting a good seal in the knife tracks when applying NH3.