TEN SLEEP, Wyo.—The alfalfa weevil, scourge of Western ranchers, appears when the frost melts, skeletonizing leaves and profits. There are ways to limit its damage—early harvest, livestock grazing, and intercropping alfalfa with grass—but most growers opt for insecticides.

R.C. Carter, a third-generation rancher in Northern Wyoming, recalled a realization he had while using a 1.5-gallon container of concentrated pesticide to spray a 60-acre alfalfa pasture.