The Sirex wood-wasps and their importance in forestry ; Studies of the Sirex parasites. Part I, The biology and post-embryonic development of Ibalia leucospoides Hochenw. (Hymenoptera-Cynipidae)

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 stage.(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 1928).(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 attack.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 development.(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