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Table 3.--Lymantria monacha preference for new needles of spruce, Picea abies.
Instar Preference index n P
I 1.00 + 0.00 20 < 0.001
II 1.00 + 0.00 17 < 0.001
III 0.84 + 0.32 9 < 0.020
IV 0.63 + 0.15 21 < 0.020
V 0.54 + 0.26 21 n.s.
However, as indicated in Table 2, some species actually prefer the old needles, and to such an extent that they will die
if fed the tender, nutritious new needles. An example of this is the sawfly, Gilpinia hercyniae, known in North America as
the European Spruce Sawfly. It is a former pest in Canada, but not in Europe. When offered only new foliage, this defoliato'r
dies without eating (Jensen 1988). Experiments have shown that one possible deterring compound to G. hercyniae could be
quinic acid, which together with shikimic acid is responsible for much of the acidity of these new needles. Quinic acid is
found in huge amounts in the new needles, but in low concentrations in the old ones. When quinic acid was added to old
(preferred) needles in an experiment (Jensen 1988), G. hercyniae refused to eat after concentrations were increased to the
levels of new needles.
With respect to the phenological window, insects like G. hercyniae, feeding on old needles have no problems. In
contrast, the flush-feeders must find the phenological window. If they hatch very early, even dispersal will not help if the
food resource is not yet present. The only way to survive is to find an alternative food. In the dense monocultures of
European spruce forests it is not easy to find other host species. However, occasionally, i.e., with 2-7 year intervals flowering
buds might offer this alternative. The pollen buds are prime food, being soft, with high nutritional content and rather low
content of secondary compounds (Table 1). Laboratory experiments on newly emerged L. monacha showed that these larvae
are very fond of male flowers. When they hatch early and are reared on branches with only old needles, they die. If male
inflorescences are present they survive and grow well (Fig. 1). Once started on flowers, they can eat new and old needles.
Later in the season the larvae may struggle for some time until vegetative buds burst.
0.5 - Weight (g)
0.3 - ""
0.4 - o,,o
0.2 - ,," __"
0.1 - _o",, ,. "
0.02
/
0.04 0.00
_/_/ ,,"
16. May
o Male flowers ->New needles
® Male flowers ->Old needles
Old needles ->Old needles
Figure 1.--Lymantria monacha growth on male flowers and needles of Norway spruce (Picea abies).
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