Major Armyworm Flight Detected in Blacklight trap
Captures at Crookston

The blacklight trap at Crookston maintained by Mike Hanson, a summer technician in Ian McRae’s entomology project, recorded the following armyworm captures:

June 23 – 27 moths
June 26 – 81 moths

In the past, blacklight trap catches this high have been an indication of a major flight creating a high risk of an armyworm outbreak. Though low numbers of armyworm moths have been captured in this trap since the last week in May, Ardell Knudsvig, MDA Plant Health Specialist, reports finding low numbers of small armyworm larvae in his sweep net samples from field margins of crops in the northern part of the region.

Is a high blacklight count a guarantee of an outbreak? Of course not. You also need favorable environmental conditions for egg laying and for larval development. It is wise to monitor armyworm in grain fields. This situation is similar to events in 2001, when a late flight led to armyworm infestations that were detected well after heading, making treatment decisions difficult due to post harvest waiting intervals (PHI) for insecticide options.

Dave Noetzel, emeritus extension entomologist, cautioned that armyworm infestations can be detected about three weeks after the peak moth flight is recorded by a blacklight trap. Three weeks after peak moth capture is approximately the 5th instar based on the life table information (Table 2). This is the stage when larvae dramatically increase their consumption of foliage and begin to cause noticeable leaf loss in small grains.

If you are going to make a fungicide application on your wheat or barley this week, should you automatically include an insecticide? It probably isn’t the best strategy. Armyworm adults are laying eggs and none of the labeled insecticides in cereals specifically target the egg stage. You also can’t count on the residual activity of the insecticide to control larvae that may hatch up to a week later. You will be much better off financially by scouting and making a field by field decision. You can see in Table 2 that very little leaf material is consumed by armyworm larvae in the first two weeks after hatching. Scout fields now to try and find small larvae. If not detected now, there is still time to continue scouting before significant feeding occurs.

Scouting, Economic Thresholds, and Control

It is important to detect and control armyworms while they are small and before extensive damage is done.

First scout for armyworms in field margins, low areas with rank plant growth, and areas where plants have lodged. Look for leaf feeding, worm frass (droppings) around the base of plants, and severed leaf material that has fallen to the ground. Larvae hide beneath plant debris around the base of plants and in heads of wheat and barley.

Consider treatment if armyworms number 4 to 5 or more per square foot, are ¾ to 1¼ inches long, leaf feeding or head clipping is evident, and most larvae do not exhibit signs of parasitization (white eggs behind the head or small brown cocoons attached to the body). If armyworms are more than 1½ inches long, control is not likely to provide economic return.

Carlyle Holen U of M IPM Specialist
and
Phillip Glogoza, Regional Extension Educator