Fall armyworm on seedling alfalfa

ISU field crop specialist Mark Carlton (south central Iowa) reported fall armyworm infestations on oats seeded with alfalfa and also on rye. We also got a call last Friday from a crop consultant reporting seedling alfalfa damage. It seemed that the oats (rye in the Friday call) were being eaten first, but that the alfalfa seedlings were taking a beating in spots. This type of damage is unusual, and we have no set thresholds for fall armyworm on seedling alfalfa.

Early-instar fall armyworm.

Fall armyworms feed on a wider range of plants than do true armyworms, and fall armyworms, true to their name, are most active in the fall. Fall armyworms feed more in the daylight hours than other armyworms and feeding by large populations of fall armyworms can rapidly lead to severe damage.

Fall armyworm larvae are identified by the following characteristics:

  • Dark brown to black chevron-shaped spots on each of the four prolegs (the four pair of fleshy rear legs).
  • Yellowish-white upside-down Y-shaped mark on the head capsule that frames the eyes.
  • Four prominent black spots that form a square on the top rear of the body.

A producer or field agronomist needs to get into the field and honestly assess the amount and potential for further damage. With no thresholds available, the decision to spray or not to spray is difficult. If the stand of alfalfa is being taken and larvae are under 1 1/4 inch in length, spot spraying is possibly warranted, but the producer's economic situation is a big factor. At least one producer has treated a whole field. Pounce at about 4 oz/acre is a reasonable choice.

We have reports of significant fall armyworm damage to wheat in Oklahoma, so small grains in southern Iowa should be checked in addition to the alfalfa seedings. If oats or rye used as a cover crop is damaged by armyworm activity, however, it is difficult to justify treatment.

This article originally appeared on page 181 of the IC-480(23) -- October 12, 1998 issue.

Updated 10/11/1998 - 1:00pm