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attacked trees than in control trees. Successful larval feeding seems to be only possible on trees with low needle phenolics.
High phenolic concentrations during early larval feeding suggest an effective defense mechanism of the host tree and seems
to be genetically determined.
A significant negative correlation between needle nitrogen and phenolics in individual trees as well as the positive
effect of nitrogen and the negative effect of phenolics on larval weight indicate a causal relationship between nitrogen and
phenolics and larval growth.
We also demonstrated the effects of excess atmospheric nitrogen input via N fertilization upon different chemical
properties in spruce needles and the quality of these expanding needles as food for the larvae. Nitrogen fertilization resulted
in decreased phenolics, starch, organic acids, and fiber, increased nitrogen and carbohydrates, and in a better food value for
the sawfly larvae. Larval weight was negatively correlated with needle phenolic concentrations. Our findings indicate that
the quality of spruce needles as food for the P abietina larvae are influenced in two ways: by increasing the concentrations
of positive factors (e.g., nitrogen, carbohydrates) and decreasing the concentrations of negative factors (e.g., phenol ics, fiber,
organic acids).
ACKNOWLEDGEMENTS
This study was funded by the Austrian Fonds of Scientific Research, P 7093-CHE: Excess Nitrogen in Forest
Plant-Herbivore Associations. The authors wish to thank Ing. G. Mottlik, A. Stradner, and Mag. A. Straka for technical
assistance with the chemical analyses.
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