Skógræktarritið - 15.05.2001, Blaðsíða 181

but see Hunter and Elkinton
1999).
As regards mortality of young
larvae, in a greenhouse experi-
ment where natural enemies and
abiotic conditions were under
control, the biggest difference in
survival to the end of second
instar was found between trees
with low contents of both pro-
teins and hydrolizable tannins (96
% of survivors) and trees with
high levels of both proteins and
hydrolizable tannins (70 %).
Although the causes of this pat-
tern are uncertain, likely, bound-
ing of proteins was more effective
when concentration of both pro-
teins and tannins were in was
high concentration (see Zucker
1983 for further discussion), this
would decrease larval growth and
subsequently the probabilities of
dying increased. Interestingly,
individuals who died were those
recording lightest weight at the
beginning of the instar. This last
finding is particularly relevant
when we try to extrapolate the
results to natural conditions.
Mortality in the greenhouse was
unusually low in first instar com-
pared to natural patterns (person-
al observation) and causes of
death during the second instar
were presumably not the same as
in the field. However, results sug-
gest that the main effect of a poor
food quality food is to weaken
individuals feeding on it and this
will probably interact with other
abiotic factors such as tempera-
ture and natural enemies
(Virtanen and Neuvonen 1999).
Acknowledgements
I am grateful to the staff of Kevo
Subarctic Institute Station and
Satakunta Environmental Re-
search Centre for their assistance
during the experiments, to Erkki
Haukioja for introducing me to the
study system, to Seppo Neuvonen
and Kai Ruohomaki for discussing
SKÓGRÆKTARRITIÐ 2001 l.tbl.
with me about ballooning experi-
ments, to Marianna Riipi for her
assistance during the greenhouse
experiment, and to the organizers
of the NSSE workshop for provid-
ing me the opportunity to discuss
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