Ole Schell remembers seeing giant clusters of monarchs in a eucalyptus grove on his family’s ranch in Bolinas. As a child, he said, “It used to be you could shake a tree and thousands would come off.” But in recent years, Mr. Schell has found only two or three monarchs in an entire season. He began searching for answers. Western monarchs have undergone a significant decline along the Pacific coast, losing more than 95 percent…