Contributed by Robbie Flowers, PhD, Forest Entomologist, USDA Forest Service.
This defoliation is caused by the Pandora moth, Coloradia pandora (Lepidoptera: Saturniidae), which is a native defoliator of pine forests in the western United States. Outbreaks have occurred commonly in Central Oregon.
Outbreak levels of Pandora moth occur intermittently. The first recorded outbreak in Central Oregon was in the 1890s on the Klamath Indian Reservation, and many other outbreaks have been documented here since that time. Outbreaks tend to occur in areas with loose, granular volcanic soils, which are needed for these insects to complete their life cycle. Tree ring analyses of old growth ponderosa pine suggest that up to 22 Pandora moth outbreaks have occurred here over the past 600 years.
Larvae feed on the foliage of several species of pines, with the primary hosts in Central Oregon being ponderosa and lodgepole pines. Outbreaks are typically observed in mature stands although light defoliation has also been reported in understory trees. Outbreaks can cause extensive defoliation, often leading to growth loss and some tree mortality. When tree mortality occurs, it is often associated with subsequent attacks from bark beetles or when prolonged drought conditions occur.
During outbreaks in a given area, pine defoliation occurs every other year because Pandora moth requires two years to complete one generation. In Central Oregon, larvae and defoliation are observed during even-numbered years and adult moths are seen during odd-numbered years. Adults are usually observed in late July to early August and during outbreaks, large numbers can be seen on trees or resting on homes or other structures nearby, especially near outdoor lights. The females lay eggs in the fall, which hatch into larvae that feed in small colonies on the current year’s foliage. The larvae overwinter at the base of needles and resume feeding the following spring; this is when the heaviest defoliation occurs. Larval feeding is usually completed by late June and larvae crawl down from the trees and burrow into the soil where they transform into pupae. This is when they are most often observed, especially along roads and trails within outbreak areas. Adult moths then emerge the following summer.
This appears to be the sixth year of a Pandora moth outbreak in Central Oregon; a large number of adult moths were observed in 2015, 2017 and again in 2019. Then, during the spring of 2016 and 2018, larvae and defoliation were seen over small areas to the south and west of Bend. Outbreaks typically last 3 to 4 generations (6-8 years), so we expect to again see increased numbers of larvae and defoliation during the spring of 2020. The Pandora moth’s two-year life cycle, with defoliation occurring every other year during outbreaks, usually allows for tree recovery.
Pandora moth outbreaks are usually allowed to subside naturally. A large number of natural enemies generally keep populations at low levels or contribute to the collapse of outbreaks. One of the most important is a disease caused by a virus, which rapidly infects larvae. Small mammals also feed on pupae in the soil and have been reported as an important natural control. Several birds are also predators, feeding on eggs and larvae, as well as parasitic wasps which also attack eggs and larvae.
Periodic management of pine stands, including thinning, prescribed burning, or other treatments focused on maintaining the appropriate species and densities for the site are an important factor in allowing affected trees to withstand the effects of defoliation and to recover after it occurs. Management efforts such as those recommended by the Deschutes Collaborative Forest Project in forest areas near Bend accomplish many objectives, including increasing space between trees and individual tree vigor, which help to mitigate the impacts of defoliators like Pandora moth and reduce tree mortality from other damaging insects like bark beetles.