A stout brown wasp called the sawfly, eating needles off ponderosa pines across 7,430 acres southeast of Denver, is expected to spread — turning more pines into Charlie Brown Christmas trees.
Afflicted ponderosas change from puffy green pines to defoliated spines. The outbreak of sawfly wasps is the latest of multiple insect epidemics degrading Colorado forests including mountain pine bark beetles and spruce beetles.
State entomologist Dan West said Tuesday that forest crews will monitor ponderosas this summer to track sawflies’ spread northward from a stronghold in Elbert County.
“Three years of this insect feeding on ponderosa pine trees can cause the trees to die,” West said. “We really don’t know why this organism is in this tremendous boom now. That’s the big question.
“We’re still in the phase of trying to figure out if we’ll see a couple years of the boom or whether these are likely to become persistent and move to other locations to the east and north. You do start to worry how far north this outbreak could go, meaning through Parker and into Denver.”
Stingless and silent, sawflies lay eggs directly on ponderosa pines. Orange, black-spotted infant wasps crawl out and feed in groups on pine needles, eating from the bases of needles to the tips. As the wasps grow, they drop from trees to soil and form silky white cocoons.
They emerge from the cocoons after a few weeks as oval-shaped adults, about 10 millimeters long and five wide, brown with an orange tint. Males use feather-like antennae growing from the front of their heads to find fresh ponderosas to attack.
Ponderosa pines are a dominant pine tree along Colorado’s Front Range. The area where the sawflies are spreading, north of the Black Forest Fire burn zone, lies at the eastern edge of ponderosa terrain in the western United States.
Sawflies have multiplied around Colorado in recent years, targeting single-species forests. Moderate to heavy loss of trees is likely this summer, according to a Colorado State Forest Service report.
Federal and state foresters detected the outbreak reaching epidemic levels in July and conducted an aerial survey. It found sawflies defoliated ponderosas across 7,200 acres in Elbert County with the hardest-hit areas north and west of Kiowa. The sawflies have spread into Douglas County, where 20 acres of trees lost needles, and El Paso County, with 210 acres afflicted.
State foresters, teaming with Fort Collins-based forest health specialists, conducted an egg survey last fall and in April and are trying to predict damage.
“This is perhaps the largest outbreak seen in this area,” said Forest Health Management International entomologist Bill Ciesla.
Chemical insecticides can be effective in June, when eggs begin hatching. Natural control agents include virus diseases, parasitic wasps and other insect predators.
Looking at potential causes of insect outbreaks, West pointed to the tree-killing emerald ash borer in Boulder County, which entered the area on imported firewood.
Factors include drought, pesticide disruption of ecosystems, wildfire and changing temperatures, he said. “And part of it is that we are becoming more of a global economy: people are moving stuff everywhere.”
