8 BULLETIN" 841, U. S. DEPARTMENT OF AGRICULTURE. 



Roughly speaking, so far as is now known, the sawfly inhabits an 

 area bounded on the north by a line far into Canada; on the east by 

 the Mississippi River, or probably a little east of that; on the south 

 by latitude 36°; and by the Pacific Ocean on the west. 



From the foregoing brief summary of its history it will be seen that 

 Cephus ductus is distributed over an immense territory and that it 

 constitutes a potential menace to the small grains throughout this 

 vast area. As the acreage of native grasses is decreased from year 

 to year by the bringing of wild lands under the plow, pests such as 

 the sawfly will be forced to depend in an increasingly large measure 

 upon the small grains and other products of the farms. On this 

 account the injury caused by these formerly harmless insects bids 

 fair to increase steadily. In the past the numbers of grass-feeding 

 insects such as the one considered in this paper have been governed 

 mainly by the supply of food plants. A dry summer that retarded 

 the growth of long-stemmed grasses would automatically reduce the 

 numbers of the insects that lived within these grass stems and perhaps 

 bring certain species to the point of extinction. It is easy to see 

 how seasonal fluctuations in vegetation would, to a large extent, 

 either multiply or diminish the numbers of these insects. 



Then again, the farmer, by introducing fields of grain into a region 

 previously uncultivated, brings in conditions unknown before and 

 invites the attack of these and other formerly harmless insects, 

 making it possible for them to become a menace to his future. Such 

 a study of life history as has been attempted in this paper is urgently 

 necessary in order that control measures may be undertaken suc- 

 cessfully when such insects become pests. 



FOOD PLANTS 



The various species of Agropyron and Elymus, genera both of 

 which are well represented in the West, appear to have been the 

 original hosts of the larvae. Since their feeding habits have been 

 modified by changing agricultural conditions, the list of their present 

 host plants, so far as known, stands as follows: 



Elymus canadensis 



Agropyron occidentale 



Calamagrostis spp. 



Elymus condensatus 



Agropyron caninum 



Festuca sp. 



Agropyron tenerum 



Hordeum jubatum 



Wheat 



Agropyron richardsoni 



Bromus inermis 



Durum 



Agropyron smithii 



Phleum pratense 



Spelt 



Agropyron repens 



Deschapsia sp. 



Rye 



Barley probably should be added to this list. 



Since the larva is wholly unable to move from one stem to another, 

 it is very obvious that the host stem must be large enough to afford 

 both shelter and food during its entire growing period. Hence only 

 the larger-stemmed grasses can be mined successfully by the Cephus 



