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Some tree responses to fertilization were also different than in other recent studies. Additional N provided by
fertilization was expected to increase foliage production based on results of a study of fir infested with western spruce
budworm (Wickman et al. 1992). We found fertilized trees produced less tbliage per centimeter of twig length for the period
1989 through 1992 and buds also were smaller in all years except 1989.
Radial growth usually is related to foliage quantity and quality. Fertilized trees had less foliage production but more
radial growth than control trees, which suggested that resources may have been allocated to the tree bole at the expense of
foliage. Fertilization resulted in greater individual tree growth similar to findings of Cochran (1978). He fertilized thinned
ponderosa pine stands similar and close to our study area; however, his stands were not defoliated by pandora moth at that
time.
A study by Miller and Wagner (1989) of the effects of pandora moth defoliation on ponderosa pine growth in Arizona
found greater radial growth in heavily defoliated trees l year after the last defoliation. This is not the usual growth response
of trees defoliated by some other insects where growth is usually directly related to degree of defoliation and lags 1 year after
defoliation (Wickman 1979, Wickman etal. in press)
The study raised more questions than answers. Response of insects and foliage production of host trees was contrary
to findings from a recent study of western spruce budworm (Mason et al. 1992, Waring et al. 1992, Wickman et hi. 1992).
But radial growth seems to respond positively after fertilization on nutrient deficient soils irrespective of insect or host tree
species, and perhaps from a site productivity perspective that is what really matters.
It may be impossible to account for the smaller larvae from fertilized trees in this small study. Perhaps the added
nutrient resources (N) were only partially allocated for growth of trees and some increased defensive chemistry production in
fertilized trees resulted in smaller larvae. Maybe the larval growth was related to feeding efficiency determined by effects of
foliage phenolics or other foliage attributes, like needle toughness, on the previous generation of larvae that carried over to
the next generation. This explanation is difficult to pursue because we were able to measure larval weights from only one
generation of the three that affected the host tree through the outbreak period. Perhaps the insect frass with its high levels of
available nitrogen reacted synergistically with residues of N in the soil and this resulted in increased radial growth, even
while the trees were being defoliated. Miller and Wagner (1989) suggest that the heavily defoliated pines ability to compensate
for foliage loss involves certain compensatory growth mechanisms, perhaps accelerated nutrient cycling of insect frass.
We did find that pandora moth frass is rich in nutrients, but we had no way of accounting for interactions of frass and our N
treatments.
This study, though small and short on chemical analysis, does point out the vagaries associated with interactions
among fertilization, insects, defoliation, and the host tree. Our studies to date of budworm on fir and pandora moth on pine
do not indicate that fertilization enhances plant defenses against herbivores through the production of secondary resistance
compounds. One consistent result we have encountered is increased radial growth of fertilized trees.
SUMMARY
Responses of ponderosa pine and pandora moth to fertilization with N were studied for 4 years after treatment.
Fertilization had a negative effect on larval weights. The 1990 generation of treated larvae were significantly smaller than
control larvae, but there was no significant difference between pupal weights. Foliage and bud weights of fertilized trees
were significantly lighter than controls. Available nitrogen in both foliage and insect frass was higher in fertilized plots.
Radial growth at dbh of fertilized trees was significantly greater and almost double the growth of controls. The results
indicate that effects of fertilization differ with species of host tree and insect herbivore, except for increased radial growth
that has been consistently noted in all the recent fertilization studies in eastern Oregon. The highly complex interactions of
increased nutrient cycling from herbivore feeding and artificial application of N may help explain variable results found in
recent studies.
ACKNOWLEDGMENTS
Technical assistance was provided by Katie Bobowski, Ellen Stenard, and Wendy Sutton. We also thank Dr.
Russel Mitchell for collecting insect frass and Dr. Arthur Tiedemann for arranging for chemical analysis of foliage and frass.
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