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1992). However, several authors have recorded higher levels of phenolics in immature than in rnature leaves (Coley 1983,
Puttick 1986, Mauffette and Oechel 1989, Hatcher 1990), as did the present study, but finally a gradual increase in concentration
occurred with needle maturation (Schafellner et al. 1993). The decline of phenolics during the first month of needle
expansion may be attributed to a dilution effect resulting from the addition of other leaf constituents and/or as phenolics are
important precursors to lignin, to an incorporation into the cell wall.
Flushing needles of Norway spruce have the highest concentrations of key nutrients (total nitrogen, amino acids,
water), however, they also contain the highest concentrations of secondary compounds (quinic acid, phenolics). Although
nitrogen is an important factor for growth and survival and positive correlations between needle carbohydrates and insect
performance have been demonstrated (Schopf 1986, Jensen 1988), we propose that between-tree differences in needle
phenolics caused the observed pattern of sawfly attack. The amount of phenolics in the newly emerging needles may be
crucial for the feeding larvae. High phenolic levels suggest a potential defense mechanism of the host tree and seems to be
determined more by intraspecific genetic differences than by site-dependent or seasonal factors (Lunderstfidt 1980). Additionally,
we found no hint that needle quality deteriorated for the years after sawfly attack: nitrogen and carbohydrates did
not decline and phenolics did not increase.
Fertilization with N increased the nutritional value of spruce needles for P. abietina larvae. This increase in nutritional
value is based on an increase in the concentration of needle nitrogen and carbohydrates and a reduction in needle
phenolics. Our results are consistent with several reports that N fertilization increases the concentration of nitrogen and
reduces the concentrations of carbon-based secondary compounds in leaves of woody plants (Waring et al. 1985, Haukioja et
al. 1985, Glyphis and Puttick 1989) and that such changes in leaf chemistry are causally related to changes in leaf nutritional
value for immature insects (Bryant et al. 1987).
An oversupply of N tends to change the metabolism of carbohydrates and nitrogen. Probably because of a higher
demand for carbohydrates during N-absorption and N-assimilation, the phenolic content of spruce needles decreases (Tuomi
et al. 1990), whereas the concentration of total nitrogen and free amino acids increase with N fertilization or N deposition
(N_isholm and Ericsson 1990). The low concentrations of phenolics in the leaves of fertilized trees may explain the increased
insect susceptibility of trees exposed to N deposition and excess N (Berger and Katzensteiner 1994).
In our study N fertilization also caused a significant increase in the needle concentrations of soluble carbohydrates
(sugars, cyclitols), probably due to a stimulated growth of photosynthetic tissue, and a slight decrease in starch. N fertilization
did not cause changes in beech foliar concentrations of total sugars, starch or soluble protein, but total phenolic compounds
decreased by about a third (Balsber-Pahlsson 1992). Changes in total sugar content were found in the current-year
needles of Scots pine (Ericsson 1979).
Our results indicate that a shift in the carbon/nutrient balance of spruce by N fertilization increases the nutritional
value of the needles for P. abietina larvae in two ways: by increasing the concentration of nutritional chemicals (carbohydrates,
nitrogen), and by reducing the concentrations of toxic and/or digestion-inhibiting chemicals (phenolics) in the needles.
The experiment demonstrated that nitrogen fertilization enhanced larval growth. Low needle nitrogen concentrations and
high concentrations of phenolics have been correlated with reduced larval weights of P. abietina feeding upon fertilized and
unfertilized spruce trees. This supports the idea that phenolics are specific agents for protein deactivation.
It has often been shown that the effect of air pollution may be favorable to insect herbivores (Holopainen et al. 1991 ).
Excess nitrogen supply (via atmospheric N-input) may promote the sawfly's success by improving its food quality: stimulated
tree growth with high carbohydrate and nitrogen levels at the cost of limited allocation of resources for the synthesis of
defensive compounds (phenolics).
SUMMARY
The present study on the plant-herbivore system Norway spruce, Picea abies, and the little spruce sawfly, Pristiphora
abietina, illustrates variations in spruce needle chemistry as a potential factor for successful insect attacks. While nitrogen,
carbohydrates, starch, organic acids, fiber, and water content of the new emerging needles showed no significant betweentree
or site-specific differences during the time of larval feeding, the levels of the needle phenolics were significantly lower in
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