THE WESTERN GRASS-STEM SAWFLY. 
23 
NATURAL CONTROL 
In the usual scheme of things an undue increase of insect pests is 
controlled naturally by parasites that take a heavy toll of their hosts 
and prevent their multiplication. Under normal conditions, when 
the Cephas cinctus existed wholly in grass stems, the larvae were at- 
tacked with varying success by two or more species of parasites 
that destroyed numbers of them and kept them within reasonable 
bounds. Since the fly has begun to 
change its habits and to subsist on wheat 
and other small grains to a certain extent, 
these parasites apparently have not yet 
learned of the change and are confining 
their attacks, as heretofore, almost en- 
tirely to those larvae that they find in grass 
stems. A very few parasites have been 
taken from infested wheat stubble, and 
there is little question but that in course of 
time the busy little parasites will hunt 
their prey in the grain stems and do their 
share in helping to control this pest. 
The most common parasite found every- 
where in the grasses is Pleurotropis utdh- 
ensis Cwfd., a beautiful little bronze-green 
chalcid that was reared by the writer from 
numerous larvae taken near Salt Lake City, 
Utah, from Cephus hibernation cells. This 
species appears to kill the larva only after 
it has formed its hibernation cell. It is 
gregarious and seldom or never attacks its 
host singly. As many as 12 of its larvae 
have been taken from a single cell, but 5 or 
6 is a more common number. These larvae 
are white and measure from 2.5 mm. to 
3.5 mm. in length. They are somewhat 
active and travel slowly about the cell 
when mature. They are often found 
crowded together in one end of the cell, but when disturbed will 
scatter about the chamber (fig. 16). 
Although this species is widely distributed and propagates in 
numbers it appears to destroy but a small percentage, possibly 10 
per cent of the Cephus larvae in the native grasses of Utah. In 
Bottineau County, N. Dak., it attacks the sawfly very freely in 
Bromus and timothy, and in some localities has killed more than 50 
per cent of the Cephus larvas. Indeed, it and one other parasitic 
Fig. 16.— Larvse of Pleurotropis Utah- 
ensis, a parasite of the western 
grass-stem sawfly, in situ. 
