I've been looking at some fields this week and I'm seeing high worm populations. These are fields where armyworms have been a problem in the past. I would say this year the numbers I am seeing are much higher than the last three years. In one field in particular, defoliation is close to threshold (25%).
I conducted some defoliation trials a few years back. We saw a yield reduction (about 4%) when defoliation was 50%. At 25% there was no yield reduction, but when the worms are hungry it does not take long to go from 25 to 50%.
It can be difficult to try and determine an exact percentage defoliation, but for the most part I think we all can tell when defoliation is getting close to severe. Also, consider that we still have one or two more weeks of worm activity ahead of us.
Why do armyworms seem to prefer some fields? I think one key factor is stand. Worms prefer thick stands because there they are protected from predators. If you have a field with a thick stand, make sure to open the canopy and inspect for defoliation and worms.