Lilac-ash borer, which flies from April through June, is the number one problem-borer of ash. Small trees seem to be most susceptible, along with under-watered and trunk-damaged trees. It's also likely that trees in poor condition have less capacity to recover from borer attack.
Another borer, the banded-ash clearwing, is found in Colorado's eastern plains. This borer flies in mid- to late summer.
The best way to control borers is to apply insecticide to the bark. Applications are most effective when adult borers are flying. Permethrin (Astro) is the most widely labeled insecticide for borer management. Sevin has some labeled uses for borers.
Another way to help prevent damage is to maintain a tree's vigor through consistent, year-round watering and to avoid bark injury.
For more information, see the following Colorado State Extension fact sheet(s).
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Updated Friday, April 19, 2013