SECTIONAL TRANSACTEONS.— K. 623 



Mr. R. Neil Chrystal. — Biological Studies on two Parasites of the Sirex 

 Woodwasps (Hymenoptera-Siricidae). 



The Sirex wood wasps are well known in Europe and America as borers in the 

 wood of coniferous trees (pine, spruce, larch, silver fir, &c.), and they have recently- 

 appeared in the young coniferous plantations of New Zealand, where their activities 

 have occasioned some alarm. An attempt is now being made by the Imperial Bureau 

 of Entomology to collect and export to New Zealand the parasites of these insects, 

 and the author has collaborated with the Bureau in this work during the past j^ear. 

 Preliminary biological studies of the principal species of wood wasps and their parasites 

 in Britain have been made, and this paper deals with some of the results obtained. 

 There are two common species of wood wasp in Britain, Sirex gigas L., the large black 

 and yellow species, and Sirex cyaneus Fabr., one of the steel-blue species, of which 

 another species, Sirex juvencus L., is said to be the commonest form occurring in 

 New Zealand. There are two important parasites of Sirex in Britain, Rhyssa 

 persuasoria L., one of the largest of the Ichneumonid parasites, and Ibalia leucospoides 

 Hochenwarth, one of the parasitic species belonging to the Cynipidae, a family in 

 which most of the species are phytophagous in habit. 



The biology of Rhyssa persuasoria L. has already been studied in some detail by 

 previous European workers, and the studies of Riley in America on an alUed species 

 Thalessa Lunator Fabr., also a Siricid parasite, should be mentioned. The biological 

 studies dealt with in this paper concern the oviposition habits, upon which some 

 interesting new data have been collected. 



Ibalia leucospoides Hochenw., the Cynipid parasite, although long known to be 

 associated with Sirex, has always been considered a very rare species both in Britain 

 and on the Continent, and its biology has remained obscure. This has now been very 

 largely unravelled during the present work, and the details afford an interesting 

 counterpart to those of the larger Ichneumonid. The main features of the life cycle 

 can be briefly stated as follows : the oviposition takes place in the egg tunnel of the 

 Sirex, and the eggs are laid within the larva either before it has emerged from the 

 egg shell or just immediately after. This stands in sharp contrast to the habit of 

 Rhyssa, whose eggs are laid on the body of the Sirex larva, on which it feeds externally 

 throughout its life ; whereas the Ibalia spends most of its life as an internal parasite. 



The larval stages in both parasites show marked ' Hypermetamorphosis,' but with 

 considerable divergence in form between the two, as was to be expected. A com- 

 parison between the two types throws an interesting light upon the question of 

 adaptation of the morphological structure to environment. 



The larval life cycle differs in length in the two forms, and in Ibalia the specialised 

 method by which oviposition is accomplished renders its period of activity somewhat 

 more restricted. At present it is thought that this parasite may confine its attentions 

 to S. cyaneus alone in this country. 



In conclusion, the biological studies on the two parasites have shown that the 

 technique required for their successful transmission to New Zealand and their 

 subsequent treatment there may have to be somewhat different in the two species, 

 and that in the case of Ibalia an accurate knowledge of the season and duration of 

 the Sirex oviposition period in New Zealand will play a large part in the success 

 attending its introduction. 



Afternoon. 



Excursion to view Loch Katrine Afforestation Scheme. 



Tuesday, September 11. 



Discussion on The Interpretation of Growth Curves. (Mr. G. E. Briggs, 

 Dr. F. G. Gregory ; Dr. F. F. Blackman, F.R.S. ; Prof. V. H. 

 Blackman, F.R.S. ; Dr. R. A. Fisher ; Dr. W. H. Peaesall.) 



Prof. H. H. Dixon, F.R.S. — On the Transport of Organic Substances in 

 Plants. 



