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PONDEROSA PINE RESPONSE TO NITROGEN FERTILIZATION AND
DEFOLIATION BY THE PANDORA MOTH_
COLORADIA PANDORA BLAKE
B.E. WICKMAN I, R.R. MASON z, and H.G. PAUL 2
_Silviculture Laboratory, 1027 NW Trenton Avenue, Bend, OR 97701, USA
2Forestry and Range Sciences Laboratory, 1401 Gekeler Lane, La Grande, OR 97850, USA
INTRODUCTION
The pandora moth, Coloradia pandora Blake (Lepidoptera: Saturniidae), is a well-known outbreak species on
ponderosa, Pinus ponderosa Laws, and lodgepole pines, Pinus contorta Dougl., in the western United States (Mattson et al.
1991). For example, it has periodically caused severe defoliation in the pineries of central Oregon. Historically, such
outbreaks have been short-lived and caused little impact. But, in the 1920's, an outbreak in the Klamath Falls region did
significant damage to the old-growth ponderosa pines so that they became highly susceptible to bark beetle attacks (Patterson
1929).
The insect has a 2-year life cycle (Carolin and Knopf 1968). Moths flights normally occur in June and July of evennumbered
years (those near Bend, OR occur in odd-numbered years)(Patterson 1929, Carolin and Knopf 1968, Schmid et al.
1982). Eggs are deposited in clusters on both needles and bark. Hatch takes place in August and the young larvae feed
gregariously in small colonies on current-year needles until limited by cold weather in late fall. Feeding continues on warm
winter days on the south side of trees and on trees highly exposed to sunshine. Larvae eventually disperse and feed solitari|y.
Feeding intensity increases sharply with the onset of warm spring weather, and defoliation eventually becomes noticeable in
early June as the caterpillars grow larger. By early July, fully grown larvae drop to the forest floor and pupate in the upper
several centimeters of soil where they spend nearly a full year.
The current outbreak in Central Oregon (first seen in 1988) has spread south of as well as north to the city of Bend
because of massive moth flights in 1991 and 1993. In May 1992, we collected larvae in the original epicenter that were
infected with a polyhedrosus virus. This usually portends the collapse of an outbreak, so 1994 could have been the year of a
widespread virus epizootic.
Because pandora moth outbreaks occur only every 20-30 years, the present infestation afforded an opportunity to test
the effects of fertilization on the trees and the insects in the nutrient deficient soils in the central Oregon pineries. We
hypothesized that increasing nutrient availability might improve canopy growth, and net photosynthesis and thereby lead to
elevated carbon-based defenses against herbivory. Increased tree growth might also offset the effects of defoliation.
In another forest-insect system, we found that fertilization may temporarily reduce the effects of defoliation (Mason
et al. 1992, Wickman et al. 1992). During an outbreak of the western spruce budworm, Choristoneura occidentalis Freeman,
both trees and insects were enhanced by fertilization, but trees more than budworms because they produced more new foliage
than the budworm could eat. Consequently, fertilized stands suffered less growth impact from defoliation than did untreated
stands. We tested this conclusion further in the ponderosa pine-pandora moth system.
The exact test was to determine if a single treatment with nitrogen in the form of urea would significantly reduce the
impact of pandora moth defoliation in a thinned second-growth ponderosa pine stand. A secondary objective was to determine
the effect of the treatment on growth and feeding behavior of pandora moth larvae and evaluate the chemical composition
of foliage and larval frass.
Mattson, W.J., Niemel_i, P., and Rousi, M., eds. 1996. Dynamics of forest herbivory: quest for pattern and principle. USDA
For. Serv. Gen. Tech. Rep. NC-183, N.C. For. Exp. Sta., St. Paul, MN 55108.
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