An experiment in the fight against Dutch elm disease has been credited with eliminating nearly 6 million disease-carrying elm bark beetles from trees in Northwest Washington and Chevy Chase.
In August, about 100 Northwest-Chevy Chase elms, all hopelessly diseased and dying slowly, were killed quickly with a herbicide that causes rapid drying of the bark, on which the beetles feed.
The experiment, conducted under a grant to New York State University's College of Environmental Science, not only killed 6 million adult beetles but, in doing so, kept them from producing some 44 million offspring that otherwise would have attacked healthy elms next spring, according to project director Gerald Lanier.
The experiment could curb the spread of the tree disease in Northwest Washington, Lanier said, but he cautioned that "any advantage gained will quickly be lost if treatments are not continued or the District does not increase the speed at which infected trees are removed."
The beetles are carriers of a fungus that, since its accidental introduction into this country in the 1930s, has killed many of the nation's majestic American elms. Each year it kills 500 to 1,000 of the 18,000 elms still standing along Washington's streets.
Lanier recently examined samples from 16 of the 100 elms in the experiment and found beetle reproduction had been reduced by 91 percent in trees where half the crown was dead when the work was begun in August. He said the insects' reproduction was reduced 98 percent in trees where only one major limb system was infected. Even in trees that were nearly dead before the treatment, a 52 percent kill ratio was found, Lanier said.
Besides using the herbicide and cutting rings around the trunks to kill the infected trees, Lanier's group sprayed the elms with a chemical perfume, or pheromone, to attract the beetles to the dying trees, where they would be unable to feed or breed successfully.
Even though most of the beetles in the 100 experimental trees are dead, the city plans to remove the dead trees this winter. The bark beetles' eggs would hatch in April and May.
The "tree trap" experiment also is being tried in Syracuse, N.Y., and in Minnesota, where it has cut the rate of Dutch elm disease 50 percent.