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investigation. Sampling was done on 3 occasions at 2-week-intervals during needle expansion in spring: needles were taken
from twigs of the upper crown region, where the sawflies prefer to oviposit. To record differences between the physiologically
older needles at the bottom and the physiologically younger needles at the top of a given shoot, on the second and third
occasion needles were separated according to their location on the expanding bud: top (t), middle (m), and bottom (b)
needles.
In contrast to stand A, the trees on stand B were undergoing an actual heavy sawfly attack during the sampling period
in spring 1989. Additionally, large variation in the timing of bud burst of the trees was found: the early flushing individuals
flushed 4 weeks before the late flushing ones. Sawfly attack, however, occurred only on the late flushing trees. The early
flushing trees (n = 10) were harvested 3 times (1,2,3), the late flushing trees (n = 10) only twice (la, 2a). After the second
sampling of the attacked, late flushing trees (2a), the larvae were fully grown and were no longer feeding. Although the
sampling on early and late flushing trees differed by more than 4 weeks, needles from sampling date 1 corresponded phenologically
with 1a and needles from sampling date 2 corresponded with 2a.
A fertilization experiment was done on stand C with late flushing, attacked trees. The stand was fertilized with 130
kg nitrogen per ha/year, applied on 4 occasions: in summer 1989, in spring and summer 1990, and in spring 1991. Sampling
was done during needle expansion in June, 1991, from 15 fertilized and 17 unfertilized (i.e., control) trees.
Samples for chemical analyses were put into liquid nitrogen, then freeze-dried and milled. Nitrogen (total and
soluble) was determined by micro-Kjeldahl technique with selenium as catalyst. Soluble carbohydrates (sugars, cyclitols) as
well as the organic acids (quinic acid, shikimic acid) were determined by GLC, starch content was estimated enzymatically as
glucose-equivalents. Needle phenolics were defined and determined as the potential of the needle extract (done with boiling
50% aqueous methanol) to precipitate bovine serum albumin (BSA). Absorption was measured at 590 nm. Fiber (cellulose,
hemicellulose, lignin) was estimated after a method to determine the fiber content of cattle food: after hydrolyzation with
strong acids and alkalines and heating up to 550 degrees, the remaining ash residue was weighed. All values are expressed on
a dry weight basis.
Larval experiments were performed at stand C with late flushing, fertilized, and unfertilized trees in spring 1991.
Larvae from naturally attacked buds as well as larvae (lst to 3rd instar) placed on expanding buds and enclosed in cages were
studied. After cocoon formation, larvae were collected, sexed, and weighed. Parasitized larvae were excluded from the
analyses.
RESULTS
During initial feeding in mid-May (bud length 4 cm) the young sawfly larvae are confronted with both high total and
soluble nitrogen (2% N tot. and 0.2% N sol.) and organic acid concentrations (12%). On the other hand, carbohydrates
(sugars and cyclitols) and starch are at rather low levels (8% and 2%). Fiber was approximately 10% of the needle dry
weight (dw). For all these traits, damaged and undamaged trees did not differ (Fig. 1, upper graph). The crucial difference,
however, was the phenolics: concentrations were significantly lower in the attacked trees than in the unattacked trees
(p