THE SIR EX WOOD-WASPS AND THEIR IMPORTANCE
IN FORESTRY [Reprinted from the Bulletin of Entomological Research, Vol., XIX., Pt. 3, December, 1928.]
(1) A complete review of the classification and status of the Siricid wood-wasps
occurring in Britain is given in this paper.
(2) The biology of S. cyancus, F., which has been studied at Tubney W ood’
Oxford, during the past two years, is described, together with supplementarv notes on S. gigas, L.
(3) A study of the forest relations of S. cyaneus at Tubney has shown that this
species cannot be considered a primary enemy of healthy green trees. Trees which
are favoured by Sir ex are usually markedly unhealthy from one cause or another. At
Tubney unsuitable soil conditions were the principal factors.
(4) Sir ex and Tetr opium gabrieli, Weise, the larch longicorn beetle, may occur
almost simultaneously as indicators of pathological conditions in larch woods.
(5) In North Devon Fomes annosus, a root fungus, was the predisposing factor
in the case of silver fir attacked by Sirex.
(6) Both S. cyaneus and S. gigas may occur in the same tree. This was found
to be the case at South Molton, North Devon, in silver fir. S. gigas appears to prefer
larger trees, and it is not present in the larch at Tubney Wood, which is in the pole
(7) The primary object of the work was to acquire a knowledge of the parasites
of Sirex, Rhyssa persuasoria, L., and Ibalia leucospoides, Hochenw. Both parasites
were studied at Tubney, and have already been dealt with (Bull. Ent. Res. xix p. 67
(8) It is considered probable that the results obtained in the above study will
throw some light on the Sir ex problem in New Zealand. Emphasis is therefore laic
upon the importance of studying the silvicultural conditions in relation to Sim
STUDIES OP THE SIREX PARASITES | Part I. The Biology and Post-embryonic Development of Ibalia leuoospoides Hochenw. (Hymenopjsera-Cynipidae)
(1) This paper contains an account of studies made during
the past three years, on the biology and post-embryonic
development of l'balla leucospoid.es Hochenw. (Eymenoptera
Gynipoidea) a parasite of the Sirex wood-wasp, S.cyaneus
Pabr. (Hymenoptera Sirioidae.)
(2) The oviposition-habit, which is quite specialised, is one
of the features of the life-history, as the parasite cam
only reach the host while it is still in the ovipositiontunnel.
(3) The larval stages, the morphology of which has been studied
in detail, are distinctly hypermetamorphic. The primary
stage is "polypodeiforrn" , and thus differs from that of
any other known Cynipoid.
(4) The duration of the larval stage is estimated at three years,
and for almost two-thirds of this time the parasite remains
within the body of the host-larva. Parasitised Sirex
larvae hardly ever burrow deeper than the sapwood, and the
length of tunnel which they make averages two to three
inches, but sometimes attains a length of six to seven inches.
(5) Dissections of large numbers of Sirex larvae from different
localities has supplied data from which the percentage of
parasitism could be estimated. These dissections have
also thrown much light on the course of the larval
(6) The interrelations of Ibalia and Rhyssa persuasoria L y
the Ichneumonid parasite of the wood-wasp have also been
studied. It has been found that Sirex parasitised by
Ibalia are also liable to be parasitised by Rhyssa, and
for this reason it is believed that in regions where
Rhyssa is abundant, Ibalia will occur sparingly.
Statistics bearing on this question are given.