62 THE EEPOET OF THE No. 36 



SOME OBSERVATION'S ON THE PRACTICAL IMPORTANCE OF THE 

 STUDY OP PARASITIC INSECTS. 



C. GoEDON Hewitt^ D.Sc.^ Dominion Entomologist, Ottawa. 



This subject of the practical significance of a study of the parasitic species of 

 insects is becoming increasingly important each year, and my reasons for bringing 

 the subject forward are chiefly two, first to reiterate the significance of this study; 

 and secondly, to give an account of a little of the work that we have begun at 

 Ottawa, some of which is a continuation of the work I was carrying on in England. 

 We are all acquainted with the importance of parasitic insects in the natural control 

 of serious outbreaks of injurious insects; so much so, in fact, that the sudden dis- 

 appearance of an injurious insect which has attained very -large proportions is not 

 infrequently attributed to the work of parasites, upon the slightest, and often in the 

 entire, absence of evidence in support of the supposition. Nevertheless we have 

 a very large number of instances on record when such disappearances are due to the 

 enormous increase of parasites. These sudden disappearances of serious outbreakf 

 usually occur when man would be perfectly powerless and on that account, therefore, 

 it is of great importance that we should study, so far as is possible, the course of such 

 outbreaks, with the idea of learning the methods by which control is obtained and 

 a balance regained in a state of nature. This is becoming increasingly necessary. 

 Our own present methods of combatting insect attacks are comparatively inadequate. 

 They are practicable only when the area attacked is not large, where the vegetation 

 makes the expense entailed justifiable and where the insects have certain definite 

 types of life-histories. But take an insect devastating a large area, as the Larch 

 Sawfly has done in Canada in past years, no means could be adopted to control it, 

 the area under attack was too large. Our cut-leaved birches are being killed by the 

 Bronze Birch Borer, Agrilus anxius, nothing can be done at present on account of the 

 peculiar life-history and habits of the insect. Many other instances may be quoted 

 of cases similar to these where we are powerless at present to adopt practicable 

 remedial measures. The reason for this is that we know so little. Our knowledge 

 of the natural means of control, the parasitic insects and also the parasitic fungi 

 which under natural conditions frequently obtain control, is still very meagre. 

 One reason for this is, I think, that economic entomologists have been too much 

 occupied in devising sprays and other mechanical means of attacking injurious 

 insects, and the study of parasitic means of control, the natural means of control 

 has been unduly, though on account of instances well-known to all, one cannot say 

 entirely neglected. 



The impossibility of using ordinary methods of insect control in the case of 

 the Gipsy and Brown-tail Moths in Massachusetts has resulted in a great stimulus 

 being given to the study of parasitic means of control and I firmly believe that the 

 work now carried on under the Bureau of Entomology of the United States De- 

 partment of Agriculture under the direction of Dr. Howard, who was responsible 

 for its initiation, is the real beginning of the study and the practical applications of 

 that study of parasitic and other natural means of control. This type of control 

 will increase in importance as the importance of the conservation of our forests, 

 for example, increases, as it is doing year by year. 



During the last few years an interesting instance of the value of this kind of 

 work occurred in connection with my investigations upon the Larch Sawfly, Nematus 

 erichsonii, in England. I had the pleasure of bringing to your notice last year some 



