Fall Armyworms in Soybeans and Peanuts

We have gotten multiple calls over the last week about fall armyworms in soybeans and peanuts.  In every case, it has been a situation where people got behind on grass control when we were getting a lot of rain a couple weeks ago.  In those situations, they have sprayed the grass during the last two weeks with products such as Select or Roundup that are relatively slow acting. A lot of the grass is infested with grass-strain fall armyworm and big worms start moving over into the crop about a week to ten days after the application when the grass starts to die. The worms don’t really want to feed on the soybeans or peanuts, but when the grass dies, they are the only thing in the field that is still green. Because the worms are large at the time, they can completely defoliate small soybeans or peanuts in a short period of time.

Fortunately, the grass-stain of fall armyworm is very easy to control with insecticides, and a mid-rate of any pyrethroid usually does a very good job.  Also, because they are large, they will be cycling out pretty quick and may not need to be sprayed if defoliation is not too bad. Because it is the grass-strain, we don’t need to worry about controlling them to prevent the next generation from damaging the soybeans or peanuts.  As I mentioned earlier, they don’t want to feed on any broadleaf plant.  When the adults emerge, they will simply leave the field and find another grassy area to mate and lay eggs.

If fields have not been sprayed for the grass yet, you can check the grass for the presence of fall armyworms.  When the worms are small, they may be difficult to find, but the feeding shows up as window-panes on the grass leaves.  Large worms are fairly easy to find and you will see extensive feeding on the grass. In these situations, a pyrethroid can be tank mixed with the herbicide application to take out the worms before they have a chance to move into the crop.

If the grass has already been sprayed and the worms are starting to move into the crop, pay close attenetion to defoliation levels.  Both soybeans and peanuts can withstand significant defoliation during the early stages of growth without any yield loss. If a spray is needed, any of the insecticides labeled will provide very good control.  Pyrethroids are generally the most cost effective option, but any of the worm specific insecticides should provide excellent control if there is a threat of flaring other pests.

Grass-strain fall armyworm (Photo by Scott Akin)

Feeding injury (window-pane) from small armyworms in corn. Feeding will appear similar in other grasses.