By: Paul Oester and Dave Shaw, with contributions from Bob Parker, OSU Extension
This year in eastern Oregon, numerous landowners have been wondering what the little white butterflies are that flutter around the outer edge of ponderosa pines. Sometimes you can see up to a dozen or so, but sometimes clouds of many, many more. Oddly enough, after many decades of little activity and no visible defoliation, pine butterfly populations are on the rise in some parts of northeast Oregon. The areas showing visible defoliation are primarily on the Malheur National Forest with the largest portion currently about 25,000 to 30,000 acres, a10-fold increase from 2009.
These insects are native to the region and if outbreak history is a barometer to the present, they have the potential to become serious defoliators. Higher populations this year likely could mean more widespread noticeable defoliation next year, but we won’t know until next spring because insect populations are regulated by many factors, including weather, parasites, predators and disease. So any projection is speculation at this point. Although it’s really just too early to tell, entomologists working on this insect have seen a wider distribution of eggs this year compared to last year which likely will mean more widespread defoliation next year. Specialists also are predicting some mortality of affected pines, however most defoliated trees should recover.
The pine butterfly (Neophasia menapia), also known as the, “pine white”, or “white pine butterfly”, occurs in pine forests, as well as Douglas-fir, throughout the western United States and British Columbia. Robert Michael Pyle (Pyle 2002) notes that this is one of only three butterfly species in our region that have larvae that feed on conifers. Most conifer defoliators are larvae of moths. Although pine butterfly population levels are typically quite low, periodic large outbreaks are believed to have occurred in Oregon though none have been specifically documented until recently. Eastern Washington and Idaho have had large outbreaks reported many years ago. Most pine butterfly outbreaks are generally short lived, but can lead to growth loss and mortality of ponderosa pine across large areas, often associated with other factors such as fire, bark beetle outbreaks and drought. However, Western Forest Insects (Furniss and Carolin 1977) describes the pine butterfly as, “one of the most destructive insect enemies of ponderosa pine in the Northwest”.
Generally the larvae prefer to eat older pine needles, not the current year’s growth. Sometimes, older larvae will feed on new growth. This defoliation may have little to no effect on affected trees, but it can sometimes cause reduced growth and/or whole tree mortality, especially if they are under stress. While the health and vigor of a tree prior to defoliation plays a role in determining whether a tree will survive an attack, the percent of the canopy actually defoliated is also an important factor. Of course, outbreak duration also contributes to mortality rates.
The adult butterflies have an approximately 2 inch wingspan and are mostly white except for some black streaking through their wings. They look very much like the common cabbage butterfly. They fly in August and September, lay emerald colored eggs in September which overwinter, and hatch into larvae (caterpillars) in the late spring. These caterpillars only feed on the needles of conifers. Thus, look for defoliation to occur next spring/early summer, when the larvae are active.
Typically, outbreaks are short in duration. The population increases in size rapidly for a couple of years, and then natural enemies, environmental conditions and starvation usually lead to a dramatic crash in the population. An ichneumonid wasp that parasitizes the larvae has been credited with pine butterfly population declines in the past, but usually the parasite’s populations lag 1-2 years behind that of the pine butterfly. Other predators and parasites are also important population regulators, as well as winter egg mortality. However, it doesn’t seem like birds prey on them much because of nitrogenous wastes stored in their wings.
Silvicultural practices, such as thinning are best applied during the years between outbreaks. This approach provides for maintenance of high vigor trees that are better able to recover from defoliation. Favoring species diversity is another strategy that buffers stands from defoliation. There does not seem to be a relationship between tree size or crown class and tree mortality. Entomologists are recommending a cautious approach in terms of management since natural enemies are building up and should begin to impact the outbreak soon. Also, outbreaks have historically been short-lived so it’s difficult to respond in a timely way.
Insecticides can be effective in reducing defoliation, but need to be applied in early-late spring before pine butterfly larvae have grown large, depending on weather and geographic location. Past experience using insecticides in larger scale projects have been marginally successful because of re-invasion from surrounding areas. However, in high value areas, small scale projects or individual tree applications can be used to protect foliage or allow time to accomplish stand management activities, but cost can be high and annual applications may be necessary for several years. Bacillus thuringiensis (B.t.) is registered for aerial application to forest trees. Homeowners with high value ornamental conifer trees could consider a ground application of Bacillus thuringiensis (B.t.) or esfenvalerate in spring if larvae populations appear high enough to cause heavy damage. Waiting until next year to determine the amount of area-wide defoliation and the health status of affected stands will be critical to developing more specific control strategies or management recommendations.
For more information:
Pine Butterfly. 2010. Forest Health NoteOregon Department of Forestry. 3 p. http://www.oregon.gov/ODF/privateforests/fh.shtml. Also, here’s a link to “pest notes” page and direct to the pine butterfly note if interested. http://www.oregon.gov/ODF/privateforests/docs/fh/PineButterflyWeb.pdg
Western Forest Insects.
Furniss, R.L., and V.M. Carolin. 1977. USDA Forest Service, Misc. Pub. #1339. WA DC.
Field Guide to the Common Diseases and Insect Pests of Oregon and Washington Conifers Goheen, E.M., and E.A. Willhite. 2006. Field Guide to the Common Diseases and Insect Pests of Oregon and Washington Conifers. USDA Forest Service PNW Region, R6-NR-FID-PR-01-06. Portland, OR
The Butterflies of Cascadia
Pyle, R.M. 2002. Seattle Audubon Society. Seattle, WA.
Thanks to Lia Spiegel, entomologist, Blue Mountains Pest Management Center, LaGrande and Rob Flowers, entomologist, Oregon Department of Forestry, Salem for their review and comments.