UNITED STATES DEPARTMENT OF AGRICULTURE 
A BULLETIN No. 616 
Contribution from the Bureau of Entomology 
L. O. HOWARD, Chief 
^kr^Su 
Washington, D. C. 
PROFESSIONAL PAPER 
February 14, 1918 
THE CITRUS THRIPS. 
By J. R. Horton, 
Tropical and Subtropical Fruit Insect Investigations. 1 
CONTENTS. 
Page. 
Introduction 1 
History and distribution 2 
Nature and extent of injury 3 
Dissemination 7 
Food plants , 8 
Life history and habits 10 
Page. 
Seasonal history 22 
Natural checks 24 
Natural enemies 25 
Control experiments 29 
Bibliography 41 
INTRODUCTION. 
The citrus thrips [Scirtothrips 2 (Euthrips) citri Moul.] has caused 
more or less injury to oranges, grapefruit, and lemons in California 
for the last 20 or 25 years. Owing to its minute size the insect re- 
mained undiscovered for many years, and its injury to the fruit was 
attributed to other causes, such as wind whipping, freezing, etc. 
About the year 1908 the injury became so severe in Tulare County 
that the old theories began to be doubted and the need of expert in- 
vestigation was realized. The preliminary examination in behalf of 
the Bureau of Entomolog}^ was made by Dudley Moulton (2) 3 and 
resulted in a description of the insect and its injury. Subsequently 
the bureau undertook a thorough investigation of the life history and 
control of the insect, this project being started at Lindsay, Tulare 
County, Cal., in 1909. A report of progress by Jones and Horton 
(5) and a Farmers 5 Bulletin on the control of the thrips by the 
writer (15) have appeared since. 
1 Transferred to Cereal and Forage Insect Investigations, October 1, 1917. 
2 Hood (14) places this insect in the genus Scirtothrips on the apparently adequate 
grounds that the thorax is finely, transversely striate and the abdomen clothed with 
minute hairlike processes. 
3 See Bibliography, p. 41. 
13138°— 18— Bull. 616 1 
