BULLETIN 616, U. S. DEPARTMENT OF AGRICULTURE. 
HISTORY AND DISTRIBUTION. 
Although the citrus thrips has been recognized so recently as the 
cause of the scabbing and scarring of citrus fruits in California, there 
is unmistakable evidence that it is native to the orange-growing sec- 
tions of that State and Arizona. Its present known distribution lies 
within the arid portion of the Lower Austral life zone in central 
and southern California and south-central Arizona. It is most abun- 
dant and injurious in those sections of this 
area where the minimum rainfall occurs 
and where the average summer tempera- 
tures are highest, such, for example, as in 
the San Joaquin Valley, where a climate 
favorable to the development of the pest 
and extensive recent plantings of citrus 
offer an ideal environment. In southern 
California it becomes seriously injurious 
only in certain seasons and only in the 
local sections about Riverside, Red- 
lands, Highland, and Rialto, in- 
jury decreasing as the coast is 
approached. The natural hab- 
itat of the citrus thrips, there- 
fore, appears to be the 
arid Lower Sonoran life 
r \ zone of North Amer- 
ica. This belief is 
substantiated fur- 
ther by its oc- 
currence on 
plants na- 
tive to this 
region and 
distant from 
citrus trees. 
The citrus 
Fig. 1. — Map showing present known distribution of the citrus thrips thrips IS 
(Scirtothrips citri) in California. (Original.) known t-O OC- 
cur in eight counties in California (see fig. 1), viz., Sacramento, 
Fresno, Tulare, Kern, Los Angeles, San Bernardino, Riverside, 
and Orange. The infestation is most serious in Tulare, Fresno, 
Riverside, and San Bernardino Counties. In Kern County the 
insect is increasing rapidly and with increased plantings of citrus 
doubtless will become quite as injurious as in Tulare County. 
