We had them here back in the summer of 2001. They strip the grass and leave the legumes alone from my experience. My Mom was on the news, I was working about 2.5 hours North at a large dairy farm that summer. They harvested all their barley silage a little early, the worms were coating the fenders. I never did find out how the CP tested. Only time in my wife I saw an Army Worm invasion. I guess a while back up in Aroostook County (Far North in Maine) it was so bad that the worms would cross roads after stripping a field and cars would slide off the road.
Sweet corn growers deal with them all the time in the tips, they are a bit more intesive scouting and deal with them early, us grass farmers tend to wait until things are bad and aren't used to using pesticides on grass. As with any IPM, it comes down to the economic threshold, but you gotta scout and find them before it's too late.