Corn fields in western and west central Wisconsin are exhibiting advanced armyworm larval stages and extreme corn leaf feeding to the midrib. Some of the reports, as detailed below, are related to grassy weed presence and/or later than optimal grassy weed control. However, more extensive field areas are being defoliated due to heavy populations, independent of weed management issues.
Advanced Armyworm and Outbreak Pockets in Western Wisconsin
Eileen Cullen, Extension Entomologist, UW Entomology Department
Corn fields in western and west central Wisconsin are exhibiting advanced armyworm larval stages and extreme corn leaf feeding to the midrib. Some of the reports, as detailed below, are related to grassy weed presence and/or later than optimal grassy weed control. However, more extensive field areas are being defoliated due to heavy populations, independent of weed management issues.
True armyworm has three generations per season. Larvae of 2nd generation are now being reported. Grasses and small grains are attractive oviposition sites for armyworm moths. There are two types of infestations that can occur in corn fields. In the first, infestation may occur throughout a corn field in July if grassy weeds (foxtail, quackgrass) or nutsedge were present in the field during armyworm moth flight and oviposition. As a result, plants in scattered areas of the field will have ragged leaves from larval feeding. In the second type of infestation armyworms migrate from pastures, oats, or grassy alfalfa fields and damage corn plants in outside/edge rows of field.
Edge of contour strip with Armywormleaf feeding to midrib.
Photo Courtesy of Craig Saxe, UWEXJuneauCounty.
Reports from Ryan Tichich (UWEXPolkCounty, 7/22) and Craig Saxe (UWEXJuneauCounty, 7/26) detailed cases where grassy weed control (in both cases by herbicide application) was thought to be later than optimal. The late weed control for grassy weeds likely played a part in these problem field areas, providing oviposition sites and attractive host plant material from which armyworms migrated to corn plants following weed control. However, larval damage is not only associated with troublesome grassy weed control.
Kevin Schoessow (UWEX Burnett, Sawyer and WashburnCounties) reported Wednesday evening 7/27 pockets of extremely high armyworm reports and firsthand field visits in Burnnett, Washburn, Sawyer, Polk and RuskCounties. Kevin has also received calls from home owners due to this outbreak situation in the western part of the state (larvae eating “literally every blade of grass”). Similarly, Pete Oberlin, Pioneer Hi-Bred International, reported (7/26) heavy 2nd generation armyworm pressure in fields he has walked or received calls on in Polk, St. Croix, Jackson, Pepin, Chippewa and Dunn Counties.
Late instar true armyworm larva.
© 2004 Kevin Black, Growmark
The general rule of thumb and UW recommendation, similar to other states, is that when Armyworms have reached a body length exceeding 3/4-inch, insecticide treatment becomes increasingly non-economical because most of the damage has been done. For example, 3/4inch larvae will only feed another few days and 1-1/2 inch larvae will soon pupate. If you have earlier instar larvae (3/4-inch and under) still present, or migrating into fields along edges, and two or more armyworms per plant on 25% of the plants, or one armyworm per plant on 75% of the plants, then treatment is still economic and will prevent further feeding damage.
Reports received 7/22-7/26 are of advanced larval instars nearing 1-inch or longer. Kevin Schoessow reported Wed. evening (7/27) that there are a range of sizes, some still near the ¾ -inch smaller size. Local coops in the outbreak areas are treating the infestation to retain existing crop and prevent further damage from the relatively smaller larvae (closer to 3/4 –inch). In these areas the situation is beyond threshold, and the 2nd generation will subside soon as the larvae reach full size and pupate. With corn height 5 to 6 ft., aerial or hi-boy applications can be logistically complicated (e.g. running down corn with a ground rig, some hi-boy sprayers already at maximum height, insufficient acreage to justify an air application). These are factors to be aware of when considering treatment in heavy pockets to prevent the final days of feeding, and may be worth the drawbacks in situations of extremely heavy feeding outbreak.
The third (fall) generation of armyworm is typically not injurious to corn. Kevin Schoessow asked a couple of good questions. First, “how confident are we that the third generation will not cause problems?”. With the outbreak populations of 2nd generation that we are seeing, this will certainly lay the foundation for higher than typical fall generation. However, the fall generation is often heavily parasitized by beneficial insects, fungi and viruses.
Kevin also asked “do the caterpillars eat each other so that there is only one or two/corn plant?? From what I've seen there can be 10+/plant feeding.” True Armyworm does not cannibalize to reduce numbers to single larvae per plant. We are seeing true armyworm in the current outbreak areas. The behavior that Kevin asked about is associated with Fall Armyworm, where typically only one larva is found per whorl in corn due to cannibalism.
This and other information, can be found at a Fact Sheet link to the Minnesota Department of Agriculture titled “Armyworm and Fall Armyworm”.