50 FUMIGATION FOR THE CITRUS WHITE FLY. 



in the tables. It is evident from the results summarized in the fore- 

 going table that prolongation of the period of exposure beyond 40 

 minutes produces no noticeable increase in effectiveness. It is also 

 evident that the dosage recommended for use with sheet tents of 

 a good quality of 8-ounce duck is ample for bell tents of a good quality 

 of 6J-ounce drill. The smaller amount of leakage surface with bell 

 tents as compared with sheet tents may be entirely responsible for the 

 apparently wide margin between the recommended dosage and the 

 dosage actually required for efficiency, but it seems safe to conclude 

 that the 6J-ounce drill used in the bell tent held the gas approxi- 

 mately as well as the 8-ounce duck, the difference in leakage surface 

 considered. 



MISCELLANEOUS EXPERIMENTS AND OBSERVATIONS. 



APPEARANCE OF LARV^ AND PUP^ OF THE WHITE FLY WHEN DE- 

 STROYED BY FUMIGATION. 



The opportunities for studying the efficiency of the gas against 

 citrus pests are far superior with the white fly as compared with 

 the true scale insects. Wliile it requires considerable skill in the 

 examinations, the vital conditions of the larvae and pupse, both 

 before and after treatment, can be recognized with practical cer- 

 tainty without removing the specimens from the leaves. When in a 

 normal condition the insects in the stages mentioned appear green, 

 owing to their translucence, and paired yellowish spots, due to inter- 

 nal organs, are sometimes visible in the abdominal region. As the 

 pupa reaches maturity the reddish eyes of the adult become conspicuous 

 and the location of the developing adult wings is indicated by whitish 

 patches on either side of the body. When destroyed by fumigation 

 with hydrocyanic-acid gas the larvas and pupae usually turn more or 

 less brownish in the course of a few days. This brownish discolora- 

 tion is most pronounced along the middle of the body. Frequently, 

 however, two or three weeks may elapse before they can be positively 

 determined as dead. In the first examinations made by the author, 

 pupae on fumigated trees were classed as alive, doubtful, and dead. 

 It was afterwards determined that in practically every case those 

 classed as doubtfid were in reality dead. Examinations under a 

 compound microscope were found to be of some assistance at times, 

 but on the whole unsatisfactoiy. In such cases movements of the 

 internal organs furnish positive proof that the insect is alive, but 

 when these movements can not be detected there may still be doubt 

 concerning the condition of the specimen unless granulation or dis- 

 coloration of the body contents is evident. The most satisfactory 

 method of observing the results of fumigation is to examine the 

 insects with a hand lens of 1 or 1| inch focal distance without dis- 



