Forest Preservation 69 



which occur over a wide area is by biological rather than 

 mechanical methods. The control of the gipsy and brown- 

 tail moths in the eastern States, by importing their insect 

 enemies, affords the best known illustration of this method. 



Another good example of this is seen in the treatment of 

 the larch sawfly. One of the chief means by which this 

 is controlled is by the attack of insect parasites. The 

 sawfly did not seem to have any deadly parasite among 

 natural Canadian insects and thus was able to run its course 

 unchecked. A study of conditions in England by Dr. 

 Hewitt revealed the insect which prevented the sawfly 

 becoming a serious pest there. This insect enemy was 

 introduced into Canada and distributed, not only in 

 Ontario and Quebec, but also in several places in Manitoba. 

 This parasite has now become well established through 

 the whole range of the sawfly, and the destruction of the 

 larch is decreasing. Several other parasites which prey 

 upon the sawfly have been introduced and are bringing 

 it under control. Some good is also being done by some 

 native species, and we may look for a re-establishment 

 of the balance that has been disturbed. 



In British Columbia bark-beetles have done great harm. 

 These multiply very rapidly in fallen timber and from 

 there are likely to attack the green trees, especially the 

 weaker ones. But nature as a rule does not have great 

 quantities of fallen timber in one place, and under natural 

 conditions these insects only occasionally did much damage 

 to standing trees. With the advent of the lumbermen this 

 was changed. Large quantities of refuse were left on the 



