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Fertilized and control trees all grew at essentially the same rates during the 16-year period prior to fertilization (Fig.
5). The downward trend starting in 1986, probably induced by precipitation deficits, was sharply reversed in 1990 for
fertilized trees. Those trees eventually resumed their downward trend, but the spurt of growth in 1990 gave them a noticeable
advantage over the untreated controls.
3
2.5
Control
--Fertilized .
_ ,, \
Treatment
Application
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E / X...
t -.
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(_ _ / " [] Precip.departures frommean \\
(b
+10 cm
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1972 1976 1980 1984 1988 1992
Year
Figure 5._Trends of precipitation and radial growth of ponderosa pine at dbh. Plotted points are averages of 10 plot means.
DISCUSSION
All stages of the pandora moth are large and easily seen, and tree defoliation is spectacular during an outbreak. Such
episodes, however, occur at intervals of 20 or 30 years, and tree mortality as a result of feeding is usually negligible. Even
though the pandora moth makes an interesting test insect for herbivore studies, its irregular, boom or bust dynamics limit its
consistent availability. Because of life history differences between western spruce budworm and pandora moth, comparisons
of fertilizer effects are tenuous. The pandora moth has a 2-year life cycle; therefore, larvae, on alternate years, feed mostly
on older needles rather than emerging new foliage so that the tree has a constant supply of new foliage. Conversely, the
budworm has a l=year life cycle and feeds on new needles annually and can repeatedly remove new foliage for 7 to 10 years.
We found that larvae responded negatively in 1990, at least in terms of individual weights, to the fertilizer treatment.
This occurred even though available N measured in 1989 foliage was almost 50% higher in fertilized plots. The opposite
effects on larval weights were reported for a similar study of western spruce budworm in Oregon (Mason et al. 1992, Waring
et aL 1992). Available N in 1990 insect frass was over 6 times higher on fertilized plots. A possible explanation is that
pandora moth larvae may be differentially absorbing proteins or amino acids from fertilized compared to unfertilized foliage
and concentrating N in the frass. Why the increased nutrient resource did not result in larger individuals is an interesting
question. Perhaps nutrients were imbalanced.
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