If you love trees in Colorado, the news for you is awful, heartening or mixed, depending on which tree species and pest you’re talking about.
Even as experts and urban foresters watch for the ash-decimating exotic insect found in Boulder in 2013, another chronic pest of elms has developed resistance to a type of pesticide widely used to control it.
is a barely-moving, fast-reproducing insect that drips “honeydew” — a polite term for insect excrement — on parked cars. In elm trees, it produces stunted foliage, leaf drop and branch dieback.
“This is nothing to be happy about,” told about 200 people Wednesday at a major annual horticultural trade show and education session in Denver. “This is a poster-child example of how to make an insect resistant.”
Nearly all scale-infested elms over large areas of Colorado were treated for years with neonicotinoids — the controversial pesticides that have spurred protests in Canada and some parts of Europe — because the chemicals were so effective at controlling scale.
Cranshaw said the large elms on the CSU campus oval, infested with resistant scale, are now part of a trial to begin finding control methods that will work. In some trees, neonic pesticides have actually worsened infestation by killing off insects that prey on the scale, he said.
While this scale isn’t a rapid and certain tree-killer like emerald ash borer, it’s still a big problem, said Steve Geist, plant pathologist with Swingle Tree Care, where documents dating back 60 years chart the company’s fight against this single elm pest.
“It’s not suitable for a landscape tree. It causes dripping and dead branches; worst-case, it does the tree in,” Geist said.
And it’s widespread. Every elm tree likely harbors some European elm scale, though a few, especially varieties from Asia, seem to have some natural resistance, Geist said. “I’d kind of like to solve this one, because elm would be a nice replacement for ash.”
On the up side for elms, the European elm flea weevil, another important pest, is now being controlled by a parasitic wasp no bigger than Lincoln’s ear on a penny. Cranshaw said the wasp is attacking and killing more than 80 percent of the weevils, which chew shotholes in leaves, defoliating the trees.
Finally, Thousand Cankers disease, an insect-transmitted fungal disorder that kills walnut trees, has spread east in Colorado as far as Fort Morgan and Eads — and leaped across the ocean to the Veneto region of Northern Italy.
But in Boulder and Rocky Ford, where the disease has been around for roughly a decade, a few walnut trees that should have died are surviving or unaffected, Cranshaw said.
He drives by a single walnut tree near the library in the famous melon-growing town whenever he’s in the area to check on it. The tree was once surrounded by 11 other walnuts, eight of which were killed by Thousand Cankers.
Experts don’t know why these trees are surviving, but they’re keenly interested, especially if the reason is a genetic resistance that could repopulate the species. So they’re collecting seed samples from them.
If you know of a healthy black walnut tree in a Thousand Cankers-affected area — Denver, Boulder, Colorado Springs, Fort Collins and Pueblo are among them — contact your CSU extension office. A list of county offices is available at www.ext.colostate.edu/cedirectory/countylist.cfm.
Susan Clotfelter: 303-954-1078, sclotfelter@denverpost.com or twitter.com/susandigsin
