The Sirex Wood-wasps and their Importance in Forestry

Abstract

(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. cyaneus, F., which has been studied at Tubney Wood, Oxford, during the past two years, is described, together with supplementary 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 Sirex are usually markedly unhealthy from one cause or another. At Tubney unsuitable soil conditions were the principal factors.(4) Sirex and Tetropium 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 Sirex problem in New Zealand. Emphasis is therefore laid upon the importance of studying the silvicultural conditions in relation to Sirex attack.