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ŠUMARSKI LIST 3-4/1971 str. 33     <-- 33 -->        PDF

The results of the investigations can be summarized as follows:

1) The larvae fed on Euroamerican Poplar had a lower number of instars than the
larvae fed on other Poplars. The average number of moults on Euroamerican Poplar
was 3,94, on Populus nigra v. italica 4,05, on P. alba 4,50 and on P. tremula 4,51.

2) The shortest larval stage occurred on Euroamerican Poplar not only because
of the smallest number of larval instars but also because of the shortest duration of
individual instars. The development of larvae on Euroamerican Poplar was — according
to generations — for 0,6 — 2,3 days shorter than on Populus nigra v. italica, for
1,6 — 7,0 days shorter than on P. tremula and for 2,7 — 10,3 days shorter than on

P. alba.
3) The Euroamerican Poplar as food influences indirecly the diapause of larvae
in the manner that it accelerates their development. Therefore young larvae of the
third generation appear regularly under more favourable temperature conditions and
enter diapause in smaller number than those which were fed on other Poplar species.
Direct consequence of this is a larger number and increased activity of the older
(L3 — LÖ) larvae of this and following generations.

4) The mortality rate of larvae fed on the Euroamerican Poplar was the lowest,
amounting to 33%, on Populus nigra v .italica 41%, P. tremula 58%, and on P. alba
85% (two years´ averages). The mortality rate of pupae was insignificant for each
kind of food (less than 1%).

5) The moths reared on Euroamerican Poplar had the highest fecundity in all
generations except 3rd winter an 4th generations, when on the first place regarding
the fecundity were the moths from Populus nigra v .italica, whereas those from
Euroamerican Poplar were on the second place with somewhat lower fecundity. In
total average of all generations and the both years of experiment the number of
produced eggs by one female was on Euroamerican Poplar 877, on Popullus nigra v.
italica 779, on P. tremula 693 and on P. alba 567. The number of laid eggs per female
was on Euroamerican Poplar 595, on Populus nigra v. italica 510, on P. tremula 505,
and P. alba 326.

6) The sex ratio was on an average 0,50 on each kind of food, with slightly more
females on Euroamerican Poplar.

7) The reproductive potential was the greatest on Euroamerican Poplar (310),
which is mainly the result of higher fecundity and somewhat higher sex ratio on this
kind of food. After which follows Populus tremula (247), then P. nigra v. italica (230)
and P. alba (149).

Thus, the above mentioned results of investigations show that the foliage of Euroamerican
Poplar by its quality represents for Pygaera anastomosis L. a more fa^
vourable food than the foliage of native Poplar species: Populus nigra v. italica, P.
tremula and P. alba.

As the quality of Euroamerican Poplar as food is very favourable for Pygaera

anastomosis L., we may consider it as one of the causes of mass outbreaks of this pest

in Euroamerican Poplar plantations.