Pine Butterfly Update

by Rob Flowers, Oregon Department of Forestry Entomologist

Map of Pine butterfly aerial survey results for 2012 and 2011

If you live in, or have traveled around Central Oregon, you have no doubt seen the white butterflies that fly around Ponderosa Pine trees. These are Pine butterflies, which are native to the region. The adult butterflies have a wing span that is approximately 2-inches and are mostly white with black streaking through their wings. They begin flying in August, lay eggs in August and September which overwinter and hatch into caterpillars in the spring. The caterpillars are the pine needle eaters.

Pine butterfly caterpillars feed on ponderosa pine, white pines, and lodgepole pine. Young caterpillars feed in clusters on individual needles early in the season. Later, they feed singly, consuming entire needles. Older needles are eaten first but new needles may also be fed upon when populations are high.  A variety of impacts can occur as some trees may only suffer growth loss, while other individual trees without starch reserves or many live buds may suffer branch die back, top-kill or even outright mortality.  While the health and vigor of the tree prior to defoliation plays a role in determining whether a tree will survive an attack, the percent of canopy actually defoliated is one of the most important factors, along with drought, or other pests such as mountain pine beetle, western pine beetle and Ips engraver beetle.  

2010 and 2011 saw large outbreaks of Pine butterfly. Recall Dave Shaw and Paul Oester’s article in the 2010 MWM Gazette:

The following provides an update on the situation based on aerial and ground surveys completed in 2012.

Provisional aerial survey data detected 104,633 acres of pine butterfly defoliation in 2012, down 58% from 250,325 acres mapped in 2011.  Areas described as having “heavy” defoliation declined significantly, while small increases in the overall area with “light and moderate” defoliation were observed.
These findings suggest that the outbreak is collapsing and the effects of the pine butterfly are declining.  Trees in many areas suffered less extensive and severe defoliation this year relative to last year.  Population declines appear to be due to increasing levels of natural predators and parasites among other factors. We expect some degree of defoliation to continue in the next year or two, but overall declines in pine butterfly populations should continue until they return to normal, endemic levels.
A cooperative research project, funded by the US Forest Service, was begun this year to assess the effects of the defoliation within forest stands.  Within the core area of defoliation, 15 stands were selected and 45 permanent plots have been installed.  These will be re-measured annually over the next 3-4 years with a focus on documenting the degree and causes of tree mortality.  While analyses are ongoing, initial findings indicate low levels of tree mortality to this point.  Where it did occur, it was most often attributed to attacks by bark beetles and other wood boring insects. Tree mortality most often occurred as small patches scattered widely across the landscape.  Additional examinations of radial growth loss as well as the effects of nutrient input (resulting from the defoliation) and effects of the recent fire that occurred within the area are also planned in coming years.
Despite the extensive defoliation that has occurred, it would appear that the vast majority of trees examined to this point have survived.  Some defoliation is expected next year in those areas where the pine butterflies were observed in high numbers this year, but the activity of their natural enemies is expected to continue to reduce their numbers until they reach low endemic levels within the next few years.
Thank you for your interest and we will provide additional information as it becomes available.

For more information:
Malheur National Forest - Pine Butterfly Update, August 2012:
ODF Forest Health Note, Pine Butterfly:

2012 Draft Aerial Survey Data for Oregon and Washington:

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