26 CIRCULAR 6 3 5, U. S. DEPARTMENT OF AGRICULTURE 



100 pounds on the hydrocyanic acid in the cylinder forces it through 

 the piping system and sprays it through nozzles into the warehouse. 

 When less than one cylinder of hydrocyanic acid is required, the amount 

 needed should be weighed from the cylinder on portable platform scales. 

 When the required dosage has been admitted, the valves on the 

 cylinder should be closed and the piping system blown clear of hy- 

 drocyanic acid gas. The inlet pipe entering the warehouse should 

 be capped after the pipes and nozzles are blown clear of gas. 



The most satisfactory dosage for closed warehouses is 10 ounces of 

 hydrocyanic acid per 1,000 cubic feet with an exposure of 72 hours, 

 repeated at intervals of 20 to 45 days, depending upon the degree of 

 infestation. It is recommended that in most warehouses a minimum 

 of two and a maximum of four fumigations be planned for each year 

 in order to keep the populations of insects under control. When trap 

 records indicate that a heavy infestation is present, the dosage should 

 be increased to 16 ounces per 1,000 cubic feet. Owing to the leakage 

 of gas, even from well-sealed warehouses, it is rarely economical to 

 apply a dosage greater than 16 ounces in these periodic fumigations. 



FACTORS AFFECTING CONTROL 

 DOSAGE, TEMPERATURE, AND EXPOSURE 



If satisfactory results are to be obtained in the periodic fumigation 

 of tobacco warehouses, careful attention must be paid to a number of 

 details. The dosage of hydrocyanic acid, the temperature, and the 

 period of exposure are all of importance. It is concluded after several 

 years of work on the problem that dosages higher than about 16 ounces 

 of hydrocyanic acid per 1,000 cubic feet are rarely economical owing 

 to the loss by leakage. In many instances a dosage of 10 ounces of 

 hydrocyanic acid per 1,000 cubic feet was the most efficient and eco- 

 nomical, provided the application was repeated at proper intervals. 

 The temperature of the tobacco should be 70° F. or above, which re- 

 quires that the control program be carried out during the period from 

 about June 1 to October 31. 



Whenever possible the period of exposure should be 72 hours. The 

 concentration of hydrocyanic acid gas will vary with the different 

 types of warehouses treated, and often much of the gas has leaked 

 out after the first 6' to 10 hours have elapsed. Nevertheless the results 

 are uniformly more satisfactory with a 72-hour exposure than with 

 an exj:>osure of 24 or 48 hours. 



TYPES OF BUILDINGS 



The type of construction of tobacco warehouses affects materially 

 the results obtained from periodic fumigations. Those having floors 

 of coal cinders on a foundation of earth and crushed rock gave less 

 effective results than storages with floors of concrete or wood. The 

 thickness of walls and the general tightness of the warehouse at such 

 locations as eaves, ventilators, skylights, windows, elevator shafts, and 

 doors all affect the results obtained. In warehouses of good construc- 

 tion and where all openings are sealed properly, two fumigations for 

 the season are usually satisfactory, whereas in warehouses of poor" 

 construction or improper sealing four and sometimes five fumigations 

 are required to obtain control. 



