Bryan Jensen, UW Department of Entomology and Division of Extension
A phone call from Shawn Conley indicated some possible armyworm problems in soybean after a rye cover crop. Sounds out of the ordinary?? Maybe not. Consider what is happening. The moths are attracted to the rye cover crop to lay eggs and when that is crimped or sprayed what is left for the larvae to feed on? Soybeans. Yes, some insects have a very narrow host range and will die before feeding on other plants. Armyworms are not one of them. Although they “prefer” grasses they will feed on many other plants.
In this situation use the common insect defoliation threshold of 30% feeding in vegetative stage soybean or 20% feeding in reproductive stage soybean. A few key things to consider before spraying.
Get an accurate assessment of defoliation over the whole plant, don’t just key on upper leaves. What size are the larvae? If they are early instars there is still a lot of “preventable yield loss” that may occur. If are advanced instars (>1.25-1.5 inches), although it may be hard to hold off spraying, they will be pupating soon. The larger larvae will also be difficult to kill. Many times, however, populations are of mixed age making it difficult to reach a treatment decision.
Carefully look for small white eggs on the outside of several armyworm. There is a tachinid parasitoid which can eventually slow feeding and kill the larvae. If abundant, consider not spraying and let nature takes its course.
Also, not to downplay the importance of scouting corn, now it is a good time to scout small grains. Armyworm have a habit of sometimes clipping heads just prior to harvest. Don’t automatically assume they will, but if they do, read the insecticide labels carefully before reaching a decision. Pre-harvest intervals may be long and not appropriate for the anticipated harvest schedule.