Several rows of American elm trees that the National Park Service (NPS) first planted during the 1930s line much of the 1.9 miles (3.0 km) length of the National Mall in Washington, D.C. DED first appeared on the trees during the 1950s and reached a peak in the 1970s. The NPS used a number of methods to control the epidemic, including sanitation, pruning, injecting trees with fungicide and replanting with DED-resistant cultivars. The NPS combated the disease's local insect vector, the smaller European elm bark beetle ("Scolytus multistriatus"), by trapping and by spraying with insecticides. As a result, the population of American elms planted on the Mall and its surrounding areas has remained intact for more than 80 years.