. Bulletin of the Department of Agriculture. Agriculture. 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 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
. Bulletin of the Department of Agriculture. Agriculture. 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 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 Pleurotroins utah- 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 measm'e from mm. to mm. in length. They are somewhat active and travel slowly about the cell when matm-e. They are often found crowded together in one end of the cell, but when distm-bed 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. I
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