UNITED STATES DEPARTMENT OF AGRICULTURE 



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BULLETIN No. 841 



Contribution from the Bureau of Entomo'ogy 

 L. O. HOWARD, Chief 



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Washington, D. C. 



PROFESSIONAL PAPER 



May 7, 1920 



THE WESTERN GRASS-STEM SAWFLY 



By C. N. Ainslie, 1 Entomological Assistant, Cereal and Forage Insect Investigations 



CONTENTS 



Introduction . 1 



History 2 



Food plants 8 



The egg 9 



Development of the egg 10 



The larva 11 



The pupa 16 



The adult 17 



Oviposition 19 



Key to North American species of Cephus. . . 22 



Natural control 23 



Artificial control 24 



Cephus pygmaeus (L. ) 26 



Description , 27 



INTRODUCTION 



The western grass-stem sawny (CepTius cinctus Norton) (fig. 1) is 

 in many ways one of the most interesting and important insects that 

 has attracted the especial attention of economic entomologists in 

 recent years. It is a species native to the L'nited States and has been 

 gradually coming into prominence since the beginning of the present 

 century by reason of the change which the feeding habits of the larvae 

 have been undergoing subsequent to its discovery. Originally a grass 

 feeder, it is becoming a serious menace to the grain growers of the 

 Northwestern States because of its acquired appetite for small grains, 

 within the stems of which it now subsists. 



Such changes of diet are probably occurring everywhere with 

 greater frequency than formerly was deemed possible, especially 

 among the phytophagous insects of the Middle West. When given a 



1 The writer wishes to express his appreciation of the assistance afforded by Messrs. J. C. Crawford, A. B. 

 Gahan, and S. A. Kohwer, of the Bureau of Entomology, in the preparation of this paper, the two 

 former in determining parasitic material reared during the progress of the studies of the sawny, the latter 

 in making a critical examination of a large series of sawny individuals reared or collected from various 

 parts of North America, and for furnishing detailed descriptions of Cephus cinctus besides a key to the 

 North American species of the genus Cephus. Helpful criticisms from these men have added to the value 

 and accuracy of the paper. 



The writer desires also to mention the valuable assistance and cordial cooperation of Mr. Norman Criddle, 

 entomological field officer for Manitoba, Canada; of Mr. A. P. Heuderson, of Bottineau, N. Dak., and of the 

 several county agents in the infested areas, who have aided in various ways in the accumulation of infor- 

 mation and material. 



150056°— 20— Bull. 841 1 



