In Europe, the European white elm "(Ulmus laevis)" has received much attention. While this elm has little innate resistance to Dutch elm disease, it is not favoured by the vector bark beetles and thus only becomes colonized and infected when there are no other choices, a rare situation in western Europe. Research in Spain has suggested that it may be the presence of a triterpene, alnulin, which makes the tree bark unattractive to the beetle species that spread the disease. However this possibility has not been conclusively proven. More recently, field elms "Ulmus minor" highly resistant to DED have been discovered in Spain, and form the basis of a major breeding programme.