SALEM - A limited number of trees should die as a result of the pine butterfly defoliation that was very evident in this area last year.
The eastern Oregon outbreak, last summer in its third year, is thought to be the largest documented in the state, said Oregon Department of Forestry entomologist Rob Flowers.
Flowers recently presented a report and results of a cooperative 2011 annual aerial survey to the Western North America Defoliator Working Group.
The issue is covered on Oregon Public Broadcasting's "Oregon Field Guide," scheduled for 8:30 p.m. Thursday, Feb. 2, and at 6:30 p.m. Sunday, Feb. 5.
"Aerial detection of defoliation by pine butterfly increased significantly in 2011 to an estimated 250,000 acres, up from an estimated 24,000 acres in 2010, and 4,000 acres in 2009," Flowers said. "The current outbreak appears to be the first confirmed case and to our knowledge the largest ever documented in Oregon."
"Defoliation intensity was deemed moderate-to-high over the majority of the affected area, but tree mortality to this point has been rarely observed," he noted.
Multiple ownerships have been affected by the outbreak, including a large area of the Malheur National Forest, as well as adjacent Bureau of Land Management and private lands northeast of the city of Burns, said Flowers.
"While the population dynamics of pine butterfly are not well-understood, natural enemies and larval starvation due to localized host depletion have historically been credited with the termination of outbreaks," he said. "These occurrences were observed to a much greater degree this year, to the extent that the outbreaks are expected to collapse by 2012, with limited tree mortality."
Natural enemies of the pine butterfly include a particular wasp species, which parasitizes the larvae and has been credited with population declines in the past. Usually the parasite's populations lag 1-2 years behind the pine butterfly, notes the Oregon State University Extension.
Other Extension information on the pine butterfly:
The larvae feed upon pine needles. When they are depleted, larvae die.
Insect populations are also affected by weather and disease.
Most outbreaks are generally short lived, but can lead to growth loss and tree death, and are often associated with other factors such as fire, bark beetle outbreaks and drought.
Flowers noted that cooperative research is ongoing and has provided additional insight into pine butterfly biology, ecology, and outbreak dynamics. Post-outbreak evaluations of radial growth loss and tree mortality are also planned.
Until 2009, the most recent defoliation by the pine butterfly in Oregon was in 1982, a small area of the Ochoco National Forest.
The recent outbreak also coincides with booms of other insects, including Western spruce budworm and pine sawflies.
Flowers noted that pine sawflies co-occurred with the pine butterfly in some areas. Defoliation from pine sawflies is conservatively estimated from aerial detection surveys at 11,500 acres, he said.
Western spruce budworm, which takes place in Douglas- and true fir trees, also increased significantly in 2011. "There are no signs that the activity has, or will diminish in the near future," said Flowers.