Pine sawfly outbreak emerges in ponderosa pine forests of Elbert, El Paso counties

Pine sawfly, a native insect that defoliates Ponderosa pines, has rapidly reached epidemic levels locally. First detected just a few weeks ago, the outbreak is believed to have started near the border of El Paso and Elbert counties, and has now been detected on the northeastern fringes of the Douglas County border. Courtesy Colorado State Forest Service
Staff report
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Pine sawfly, a native insect that defoliates ponderosa pines, has rapidly reached epidemic levels south of Denver. First detected just a few weeks ago, the outbreak is believed to have started near the border of El Paso and Elbert counties, and has now been detected on the northeastern fringes of the Douglas County border.

Residents in the area can expect to see heavy defoliation of impacted pine trees and large numbers of visible sawfly larvae in the trees themselves and on the ground beneath them.

Because of the significant larvae population, experts with the Colorado State Forest Service and USDA Forest Service are actively monitoring the outbreak. They believe that many of the larvae are depleting their food source and dropping to the ground early, before they mature, and are expected to die before they can pupate. Mature larvae, which hatch in spring and feed on living needles until late summer, soon will drop from trees to pupate in the upper soil layer of the forest floor.

Meg Halford, assistant district forester for the CSFS Franktown District, says that sawfly activity has been common and cyclic in the area for years. However, this outbreak is exceptional, possibly because of an unusually large number of eggs deposited on pine needles by adult sawflies last fall. What environmental factors may have accelerated this spike in the sawfly population is unknown.

“This is perhaps the largest outbreak seen in this area,” said Bill Ciesla, an entomologist with Forest Health Management International, who has worked closely with the CSFS to monitor the outbreak.

Natural control agents, including virus diseases, parasitic wasps and other predators, can combat sawflies. Direct control measures, in the form of chemical insecticides, also are effective in spring when the eggs begin to hatch. However, the most effective time for direct control this year has passed. The only areas where chemicals may be effective now are where trees are showing only partial defoliation and the risk of larvae developing into adults is high, Ciesla said.

Halford says that measures will be taken to monitor sawfly egg deposits this fall. This will help determine the potential for additional damage in 2015, so that landowners can take appropriate measures to control them next spring.

Aerial surveys also are being conducted to map visible defoliation and assess the extent of the area that has been impacted.

For more information, visit csfs.colostate.edu or contact the CSFS Franktown District at 303-660-9625.