130 Control of Parasites 



These conditions, to be sure, are not under control, and 

 the practical value of this knowledge applies mainly to the 

 forester, with whom economy is a first consideration and 

 who cannot afford preventive measures if their need is not 

 surely established. 



That the encouragement of the enemies in the bird world 

 can be successfully used for ridding a locality of insect 

 pests was proved with a \'engeance by the city of Brooklyn, 

 when the unusual pest of ''inch worms" in the early seventies 

 was subdued by the importation of the common English 

 sparrow. It is a pity that the common sparrow was 

 selected for this duty, as, in the presence of the lavish waste 

 of grain and other palatable food which characterizes the 

 American community he soon prefers this easier mode of 

 living. But there are, outside of this common sparrow, few 

 insectivorous birds that care to live in city streets, even if 

 the small boy would let them. In the parks and country 

 places, on the other hand, the cuckoos, thrushes, nuthatches, 

 chickadees, wrens, creepers, vireos, and warblers should be 

 encouraged by providing nesting places and insuring them 

 peace. In fact, as almost all birds are, at least occasionally, 

 insect eaters, especially when there is a large supply, they 

 are all helpful in combating insect pests. 



Unfortunately, birds may also take the useful insects 

 which prey on the depredators, although this happens rarely, 

 and the most prominent of these useful insects, the wasp- 

 like ichneumons, perhaps always escape. These wasp-like 

 parasites, as well as the Tachina flies, lay their eggs on or in 

 the caterpillars or grubs, their larvae developing within the 

 host, which they sooner or later kill, either before or after 

 its pupation. Since from six hundred to twelve hundred 

 parasites may develop from a single pupa or larva of the 

 hosts, the destruction of these would certainly be the oppo- 



