Dutch elm disease has been reported back in Denver after more than a decade of its absence.
The Denver forestry office discovered the disease last month in the old American elms along 17th Avenue Parkway in South Park Hill.
Prior to the introduction of Dutch elm disease, the native elm bark beetle was not a major pest. The two non-native species are more aggressive with drought-stressed American elms. Older elms or stressed ones are the preferred hosts for bark beetles, as are other stressed trees. A female beetle finds a weak spot on the top of an elm and then tunnels a straight chamber, parallel to the grain, to lay eggs. The larvae nestle under the bark over winter. In early spring, the larvae eat channels that spread out from the egg chamber feeding on the phloem and damaging the xylem. These leave the tree with marks under the bark that are centipede-shaped.
An iconic example of American elms in Colorado is the Oval on Colorado State University’s campus, home to 99 elms. Insecticide use would do more harm than good in this case. It would have to be sprayed in the crown to kill beetles. However, the beetles would be under the bark protected, so it would kill other beneficial insects, too.
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