32 CIRCULAR 4 2 2, TJ. S. DEPARTMENT OF AGRICULTURE 



from the results of this work that dosages of hydrocyanic acid higher 

 than 16 ounces were not economical to apply owing to the loss by 

 leakage. In many instances it was found that a dosage of 10 ounces 

 to 1,000 cubic feet was the most economical, when used for periodic 

 fumigations during the warm months of the year. For best results 

 in the fumigation of warehouses for control of the tobacco moth, 

 it was found that the temperature of the stored tobacco should be 

 70° F. or above. This requires that the control program in Virginia 

 and North Carolina must ordinarily be limited to the period from 

 June 1 to October 31. 



The period of exposure which gave the most effective results in 

 warehouse fumigation was 72 hours. It is recognized that some in- 

 vestigators, notably Page and Lubatti (i^, IS), have found that, 

 owing to factors of leakage and absorption, the concentration of hydro- 

 cyanic acid gas was greatly reduced after 6 or 8 hours. Neverthless, 

 the results obtained in periodic fumigations of tobacco warehouses 

 were more satisfactory with a 72-hour exposure than when exposures 

 of 24 and 48 hours were used. The concentration of hydrocyanic 

 acid gas in the air of tobacco warehouses under fumigation will vary 

 with the different types of closed buildings, and this subject will also 

 receive further investigation. The writers, unfortunately, were not 

 equipped to determine the gas concentrations following the periodic 

 fumigations mentioned in this circular. The control data given were 

 obtained in experiments where only the factors of dosage, temperature 

 of tobacco, and duration of exposure were considered. 



CONSTRUCTION OF BUILDINGS 



The type of construction in closed tobacco warehouses affected the 

 results in fumigation. In warehouses having dirt floors covered with 

 coal cinders, less satisfactory results were obtained from fumigations 

 than in warehouses in which the floors were constructed of concrete 

 or wood. The thickness of walls and the tightness of eaves, venti- 

 lators, skylights, windows, and doors all affect the results obtained 

 from periodic fumigations. It was observed that in well-constructed 

 warehouses two fumigations for the season were satisfactory, while 

 in warehouses of poor construction three, and sometimes four, fumiga- 

 tions were necessary for satisfactory control. 



MOVEMENT OF TOBACCO' 



Another important factor in the number of periodic fumigations 

 required annually for controlling the tobacco moth is the normal 

 movement of tobacco in storage. In warehouses, especially in large 

 storage centers, there is a rather constant movement of aged tobaccos 

 from the warehouses throughout the year, either for export or domes- 

 tic manufacture, and of new-crop tobaccos into the warehouses from 

 redrying plants (i, p. 46). In many instances fermented tobaccos 

 which have been stored for 1 year or more in isolated localities are 

 transferred to the large storage centers. A record was made of all 

 hogsheads of cigarette-type tobaccos, which had been stored for 1 

 year or more in other localities, that were moved into the 3,318,000 

 cubic feet of warehouse covered by the data shown in figure 22. The 

 receipts of the hogsheads of tobacco were summarized for the period 

 May 1 to October 31, 1932, and these data are shown in table 8. 



