1 82 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



parasitic species than one. The ravages of the caterpillars of 

 the Large Larch Sawfly on larch have caused this insect to be 

 studied from the standpoint of its parasites, and the observa- 

 tions of Gordon Hewitt, Mangan, Wardle and others have 

 shown that in Britain at least eight different Ichneumonidae and 

 one Braconid may be reared from Large Larch Sawfly material. 

 To this list has to be added another Braconid and four Chalcids 

 in America. Some of these insects are few in number, but two 

 at least are in numbers sufficient to have an economic importance. 



The Ichneumonid Mesoleius teuthredinidis has in some districts 

 greatly reduced the number of the Large Larch Sawfly, and as a 

 means of fighting this sawfly great numbers of this parasite have 

 been set free in other districts in Britain under the supervision 

 of the Board of Agriculture and Fisheries, while Gordon Hewitt 

 has imported the parasite into Canada. 



Nowhere has the study of parasitism as a factor in the 

 destruction of injurious insects been studied with so much 

 thoroughness as in the United States., Two European insects, 

 very destructive to trees — the Gipsy Moth and the Brown Tail 

 Moth — have been accidentally introduced to America and have 

 done enormous damage. This led to a study of the European 

 parasites of the caterpillars of these two moths. The result has 

 been not only a mass of important and interesting scientific 

 information, but the importation and colonisation of a number of 

 useful parasites. In India also there are successful records 

 following on the importation of a parasite. 



The problem is often a complex one. One must not hastily 

 assume that all the Ichneumonids and Braconids and Chalcids 

 associated with an injurious species of insect are parasitic on that 

 species. Some are, and others are likely to be, parasitic on the 

 parasite. If an injurious insect be parasitised by more than 

 one species, then these two parasitic species compete with one 

 another. Again, if the parasite be parasitised in turn the 

 numbers of the useful species will be reduced. 



Annihilative measures adopted against an injurious species 

 of insect are often practised without consideration of the 

 effect of these destructive measures on the parasites. The 

 destruction of the caterpillars of the second brood of White 

 Cabbage Butterflies — a brood generally much parasitised — will 

 carry with it the destruction of the useful parasites. It would 

 be better if the collected caterpillars could be so confined that 



