ARROW ROCK, Missouri — It only took two days for caterpillars to destroy 10 acres of Duane Brune’s alfalfa. Brune, who runs Pin Oak Farms in New Haven, said he was stepping on four or five every time he put his foot down. “It was literally an army of worms crawling across the ground,” he said. Indeed. The caterpillars are the larvae of the fall armyworm moth, a planetary crop invader. The annual toll of their attacks is at least $300 million for farmers in the U.S., and billions of dollars around the globe. But now scientists from the University of Missouri are on the edge of a new frontier in…