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ŠUMARSKI LIST 3-4/1965 str. 10     <-- 10 -->        PDF

What would then account for the drop in the increment of Pedunculate Oak in
those years, and especially in 1960?


Taking into consideration the fact that in these forests there has not occurred
a Gipsy Moth outbreak since 1957 (when it was destroyed by warm aircraft mist-
spraying), it seems that some other pests would have caused the drop in the increment
of Pedunculate Oak. As a matter of fact this is borne out by our observations.


After the end of the Gipsy Moth outbreak in 1958 in the lowland Oak stands of
Slavonia, an abundancy of caterpillars and pseudocaterpillars of various other species
of Lepidoptera and Sawflies was noticed, which for the most part had not been
known earlier as especially harmful forest pests. This phenomenon was noticed also
in the Lipovljani forests, and it distinguished itself also by some special features.


In one earlier paper in which the control of Gipsy Moth in 1957 in the Lipovljani
forests (S p a i ć 1959) was described, it was reported that this operation was carried
out rather late. Gipsy Moth caterpillars, owing to an increased dosage of insecticide,
were completely destroyed, true, but together with them unfortunately also the whole
population of parasites died. Among the numerous species of parasites in the Lipovljani
forests there occurred in 1957 in great numbers especially the following: Apanteles
liparidis Bouche and A. solitarius Ratz. (Hymenoptera, Braconidae), Hyposoter
disparis Vier. (Hymenoptera, Ichneumonidae), Compsilura concinata Mg., and a species
of Pales (Diptera, Larvivoridae).


And then because of the rather late application of aerial mist-sprays some other
harmful insects — which besides the Gipsy Moth also occurred in the Lipovljani
forests — escaped from the control treatment. These were in the first place insects
whose caterpillars emerge earlier and also pupat earlier than those of the Gipsy
Moth, as for instance the Green Oak Leafroller (Tortrix viridana L.), and winter
moths Operophtera brumata L. and Hibernia defoliaria CI. The Oak Sawfly (Apethymus
abdominalis Lep.) is diapausing in the soil in a relatively high percentage
(as was established in these forest — Spaić 1959) so that it survived the control treatment
in large numbers. The Oak Processionary Moth (Cnethocampa processionea L.)
which occurred abundantly in these forests is very resistant even to somewhat increased
insecticidal doses which were applied here, so that it too survived the control
treatment to a great extent. A similar resistance to DDT is known also in the Brown-
Tail Moth (Euproctis chrysorrhoea L.), which was especially numerous along forest
borders.


All the mentioned parasites are polyphagous, especially Compsilura concinata.
As they were destroyed by insecticide, the survived insect pests developed in the
years following in more favourable conditions for them, so that they began to over-
populate. Besides the mentioned species of harmful insects, in 1959 the sawfly Periclista
albida KL, furthermore Euproctis similis Füssl., and some other species from
Geometridae and Noctuidae families were noticed in large numbers.


The autbreak of the majority of these insect pests, especially of the Oak Sawfly
(Apethymus abdominalis), the Oak Processionary Moth (Cnethocampa processionea),
and Green Oak Leafroller (Tortrix viridana) culminated in 1960. Then there occurred
also the greatest damages to Pedunculate Oak as can be seen from the enclosed tables
and graphs. It was shown that the concurrent action of various pests — otherwise
considered as less important or even unimportant — can cause equal damage as
defoliation by the Gipsy Moth.


In the Lipovljani forests Spai ć followed up especially the biology of the Oak
Sawfly (Apethymus abdominalis); he established that in 1960 it was in the culmination
of its overpopulation. In Tab. 2 are given data about the numbers of eonymphs
of this sawfly for the 1960—1963 period. The data were established by inspection of
the soil each year in September (in fact, the infestation for 1960 was assessed in the
autoumn of 1959, for 1961 in the autumn of 1960, etc.).


Tab. 2


Numbers of eonymphs in individual years


Compartment
No. 1960 1961
Year
1962 1963
20 1093 1555 240 142
21 636 194 173 154
28 529 79 236 181
Total 2258 1828 649 477