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 Cephus ductus 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 larvae. Indeed, it and one other parasitic 



Fig. 16. — Larvse of Pleurotropis utah- 

 ensis, a parasite of the western 

 grass-stem sawfly, in situ. 



