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 vengeance 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 Tachine flies, lay their eggs on or in 
the caterpillars or grubs, their larve 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- 
