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- 

 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 

 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 

 thrips is 

 known to 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. 



Fig. 1. — Map showing present known distribution of the citrus thrips 

 (Scirtothrips citri) in California. (Original.) 



