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VARIATIONS IN SPRUCE NEEDLE CHEMISTRY AND IMPLICATIONS FOR
THE LITTLE SPRUCE SAWFLY, PRISTIPHORA ABIETINA
C. SCHAFELLNER, R. BERGER, J. MATTANOVICH, and E. FUHRER
Institute of Forest Entomology, Forest Pathology, and Forest Protection
Universit_it ftir Bodenkultur, Hasenauerstrasse 38, A 1190 Vienna, Austria
INTRODUCTION
The plant-herbivore system of Norway spruce, Picea abies Karst., and the little spruce Sawfly, Pristiphora abietina
Christ (Hymenoptera: Tenthredinidae), provides a good opportunity to study the synchronization between the host's bud burst
and the beginning of larval feeding. Adults of the sawfly emerge in spring at about the same time as spruce bud break and
oviposit in the soft, partially-expanded needles. Every single spruce bud is suitable for oviposition for only a few days. The
emerging larvae almost always remain on the expanding bud where they hatch and only under starving conditions do they
switch buds. After 2 or 3 weeks, depending on weather conditions, the larvae are fully grown and drop to the ground to form
cocoons. They overwinter in the litter until the next spring.
The larvae of the little spruce sawfly are early-spring feeders and restricted to feeding on expanding spruce needles.
Their success and survival depend on the exact phenological coincidence of bud break and oviposition, because larvae can
initiate feeding only in flushing buds. Seasonal variations in the timing of bud break can therefore be of defensive value for
the trees: trees that are genetically predisposed to flush needles earlier or later than the rest of the stand, thereby preceding or
following the sawfly swarming, may escape larval feeding.
During the last 3 decades the sawfly has become an important spruce pest in several parts of Austria. The most
severe attacks occur in lowland areas where the natural deciduous forests have been replaced by monocultures of Norway
spruce. As the larvae feed exclusively on current-year needles, the affected trees do not die, but repeated infestations lead to
deformed, bushy crowns and to reduced height and volume increments. In recent years, an increase in frequency and
duration of epidemics has been observed, particularly in areas exposed to air pollutants like sulfur and nitrogen (Sierpinski
1985, Berger 1992). There is good reason to believe that some air pollutants may favor the sawfly's success, either by
improving food quality due to changes in needle chemistry and/or by weakening the tree's defense system (Schafellner et al.
1993, Schafellner et al. 1994, Berger und Katzensteiner 1994).
This paper reports a study on the most important food quality parameters of newly emerging needles during the short
period of larval feeding, and the changes that occur during the weeks of rapid growth. Additionally, the effect of excess
nitrogen input (via fertilization) on needle chemistry and the nutritional value of the spruce needles for the feeding larvae is
demonstrated.
METHODS
Three even-aged stands of trees 16 years old, were selected for study to see if food quality parameters show significant
site-specific variation. Stand A is located at ca. 550 m elevation in Hochstrass, Lower Austria, and stands B and C are
located at ca. 720 m elevation in the Hausruck, Upper Austria. Moderate sawfly attack was observed at stand A during the
mid- 1980's. Throughout the 3 years of attack, tree flushing and sawfly attack for every individual tree was recorded (Holzer
t988). The stand was attacked irregularly so that affected and unaffected individuals often stood close together, although the
trees did not vary in the timing of bud break and should have been suitable for oviposition. From spring 1988 onwards, the
sawflies vanished from the area so that during the 1989 sampling year, no frass was collected. Based on the data collected
during the years of attack, 15 trees that had been affected once and 15 trees that remained unaffected, were selected for
Mattson, W.J., Niemel_i, P., and Rousi, M., eds. 1996. Dynamics of forest herbivory: quest for pattern and principle. USDA
For. Serv. Gen. Tech. Rep. NC-183, N.C. For. Exp. Sta., St. Paul, MN 55108.
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