What is killing our black walnut trees?
We are heartbroken over the death of an approximately 70-year-old black walnut tree that died in two years, starting in 2013. Our neighbor's black walnut tree died last year also. Also, we have two 70-year-old ash trees that we have been treating for the last four years with Bayer ash borer insecticide.
Many black walnuts in the Denver area have died from thousand cankers disease, introduced into the tree by walnut twig beetle. See Walnut Twig Beetle and Thousand Cankers Disease of Black Walnut for more information.
The Bayer insecticide (imidacloprid) will be effective on emerald ash borer if, and more likely when, it arrives in Jefferson County. Imidacloprid is not very effective on lilac-ash borer, which has been here for many years, attacking stressed ash trees. See Lilac/Ash Borer: A Common Wood Borer of Colorado’s Street Trees for more information.