UNITED STATES DEPARTMENT OF AGRICULTURE 
BULLETIN No. 841 «_ 
SU&*4&L 
Contribution from the Bureau of Entomology 
L. O. HOWARD, Chief 
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 
Page 
Introduction 1 
History 2 
Food plants 8 
The egg 9 
Development of the egg 10 
The larva 11 
The pupa , 16 
The adult 17 
viposition 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 sawfly (Ce/phus ductus 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 United 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. Rohwer, 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 sawfly, the latter 
in making a critical examination of a large series of sawfly individuals reared or collected from various 
parts of North America, and for furnishing detailed descriptions of Cephus ductus 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 officerfor Manitoba, Canada; of Mr. A. P. Henderson, of Bottineau, N. Dak., and of the 
several county argents in the infested areas, who have aided in various ways in the accumulation of infor- 
mation and material. 
150056°— 20— Bull. 841 1 
