12 



FUMIGATION FOR THE CITRUS WHITE FLY. 



Wind. — The effect of wind upon the results is so marked that 

 fumigation should not be attempted with anything stronger than 

 a slight breeze, particularly if the tents have not been rendered 

 gas-tight or nearly so by the use of a "filler." It has been found, 

 with an untreated tent, that with a dosage sufficient to destroy 100 

 per cent of wliite fly pupae, a brisk breeze renders the results so 

 uncertain that the effectiveness may be as low as 30 per cent in 

 some sections of the tree, while in others the destruction of the 

 insect may be complete. 



Atinosplieric humidity and dews. — The presence of moisture in the 

 form of dew does not seem to have any deleterious effect upon the 

 foliage, although in California it is generally considered necessary to 

 materially increase the dosage in such cases to insure the effective- 

 ness of the work against scale insects. Prof. H. A. Gossard ^ con- 

 cluded that "moisture did not seem to interfere with the efficiency 

 of the work, unless the leaves were almost dripping, when it became 

 a factor of jnuch disturbance, though not as great as we had thought 

 probable." 



The experiments conducted by the writer and assistants during 

 January and February, 1907, show that moisture on the foliage 

 during the period of exposure has no marked effect on the foUage 

 or upon the efficiency of the gas against the white fly. In the six 

 instances where the leaves were wet with dew, examination showed 

 that 100 per cent of the insects were destroyed in all cases but one, 

 and in this only a single specimen out of 102 under observation, 

 before and after fumigation, survived the treatment. 



The results of the tests concerning the effect of atmospheric 

 moisture on the efficiency of the fumigation treatment are given in 

 Table I. " 



Table I. — Effect of atniosphcric moisture on efficiency of fumigation. 



a Bui. 67, Fla. Agr. Exp. Sta., pp. 047-048. 



!> The number preceding the decimal point indicates the length of exposure. 



