622 INSECT PESTS OF FARM, GARDEN AND ORCHARD 



The method consists, essentially, of the enclosure of the trees 

 to be treated, in individual tents made of material as nearly im- 

 previous to the gas as possible and of generating in the tents gas 

 according to a variable table, based on the cubic content, the 

 amount of leakage, which latter is less in proportion for large trees 

 than for small, and upon other factors. Very accurate dosage 

 tables have been worked out and are available wherever fumigation 

 is a common practice. Time of exposure to the action of the gas 

 is from 45 minutes to one hour, the same as for deciduous trees 

 being fumigated in houses for San Jose Scale. 



Special machinery for the generation of the gas is used by the 

 large fumigating concerns. With these the gas is generated out- 

 side the tents and piped into them and the necessity for separate 

 generating outfits for each tent is done away with. 



Fumigation has been tried rather extensively in Florida, but 

 not so well adapted for Florida conditions as for those in California 

 and so is not recommended for use in Florida. 



As mentioned before in connection with fumigation, it should 

 not be undertaken except under the direction and advice of experts 

 as otherwise there is danger to the trees and to the operator. 



The Citrus Thrips.* 



The Citrus Thrips is one of the smallest members of a group in 

 which the largest species are considered as minute insects. It has 

 been known to be injurious to citrus fruits for a quarter of a century 

 but the injury was not generally recognized until about ten years 

 ago, on account of the small size of the insect. So far as is now 

 known it is confined almost entirely to California and is not found 

 in the gulf states. In Florida a different species, the Florida 

 Flower Thrips f is similarly injurious. 



The thrips sucks the juices from foliage and fruit and kills the 

 cells surrounding its puncture. These dry up and form character- 

 istic scabs and much of the injury is due to their thus scabbing the 

 fruit, especially around the stem end where they cause a condition 

 described as "stem-end ring." They also cause curling of the 

 leaves and a grayish discoloration and interfere in this way with 



* Scirtothrips citri Moulton. See J. R. Horton, Bulletin 616, U. S. Dept, 

 of Agr. 



t Frankliniella bispinosus Watson. 



