Contributed by Robbie Flowers, PhD. Forest Entomologist for the USDA Forest Service, Forest Health Protection. Central Oregon Forest Insect and Disease Service Center.
With paddleboard and mountain biking season in full swing, you may see a familiar site fluttering by you near the river or on the trail. This robust and colorful insect is known as the Pandora moth, Coloradia pandora (Lepidoptera: Saturniidae), and they have returned (again) to a Central Oregon forest near you. If you were in the Bend area in 2017, you’d remember a large number of moths were present in many areas of town and the surrounding forests. The Pandora moths were particularly abundant near outdoor lights of homes and businesses but also appeared in rather spectacular fashion on the foul ball nets at Vince Genna Stadium to watch the Bend Elks baseball club. We see the adults in odd calendar years during outbreak events, which give way to larvae in even calendar years. During the summer of 2018, we saw a significant amount of defoliation of pine-dominated areas as large numbers of hungry caterpillars ate their fill of pine foliage.
The Pandora moth is native to the western United States, and periodic outbreaks have occurred in many areas. The first recorded outbreak in Central Oregon was in the 1890s on the Klamath Indian Reservation. Pandora moth generally occurs only in areas with loose, granular, volcanic soils, which are needed for them to complete their life cycle. Tree-ring analyses of old-growth ponderosa pine suggest that over 22 outbreaks have occurred here during the past 600 years. While the adults tend to be primarily only a nuisance around homes and businesses, the larvae can cause extensive defoliation of pine-dominated forests. This defoliation can result in growth loss and even some tree mortality when coupled with other stress such as ongoing drought or bark beetle outbreaks.
During outbreaks, the defoliation occurs every other year because Pandora moth requires two years to complete one generation. The eggs that are laid by female moths hatch into larvae that feed in small colonies on the foliage that same year. These larvae overwinter on the tree, often at the base of the needles, and resume feeding the following spring and summer. Their feeding is usually completed by late June after which they move down from the trees and into the soil to pupate. It is during this exodus from the trees that they are most often observed by the public and may be a nuisance due to their sudden appearance along trails and roads or in camping and recreation sites. Those larvae that do not find the bottom of hiking boots or car and bike tires will transform into adults and the cycle continues.
However, outbreaks are usually short-lived and last for only 3-4 generations (6-8 years). It is suspected that we are now in the 6th year of the current outbreak cycle as increasing numbers of adults were observed in 2015, 2017 and now in 2019. Pandora moth larvae and pine defoliation were first observed in central Oregon in 2016 and this expanded to a much larger area with more severe defoliation in 2018. A special forest health aerial survey documented over 145,000 acres of defoliation last summer. Based on previous outbreaks, it appears we are nearing the end of this upswing in population. However, we still expect to see large numbers of moths this summer as well as some larvae and pine defoliation occurring in the summer of 2020. It is difficult to predict exactly when the outbreak will collapse, but we expect this will occur within the next 2 years. The good news is that since defoliation only occurs every-other-year during outbreaks, this usually allows time for affected forest areas to recover and we are not expecting any extensive tree mortality to occur.
As this is a native insect with a long presence and history in Central Oregon, Pandora moth outbreaks are allowed to subside naturally. They have a large number of natural enemies that help to regulate their population cycles and that contribute to the collapse of outbreaks when they occur. One of the most important is a disease caused by a virus, which infects larvae and can rapidly spread through the population. A large number of small mammals, birds and parasitic wasps also feed on various life-stages of the Pandora moth and have been reported as important natural controls. Periodic management of pine forests, which may include activities such as thinning, prescribed burning, or other treatments focused on restoring the appropriate tree species and densities to these areas are also an important factor in helping affected trees withstand the effects of and recover from defoliation by Pandora moth.
For more information, see:
Ciesla, W.M., A. Eglitis, and R. Hanavan. 2010. Pandora Moth. Forest Insect and Disease Leaflet #114. USDA Forest Service, 8 pp. https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fsbdev2_043666.pdf
Fitzgerald, S.A. 1992. Controlling Pandora moth in Central Oregon forests and landscapes. Oregon State University Extension Service, EC 1383. 4 pp.