48 BULLETIN 918, U. S. DEPARTMENT OF AGRICULTURE. 



cotton seeds are very poor conductors of heat. Machines (4, 6) 

 have been perfected for passing the seed on an endless belt through 

 air heated to 130° F. and houses have (?) been constructed for 

 heating a mass of seed sufficiently high to kill the larvae, but in 

 Mexico fuel is scarce and very expensive, so fumigation with poi- 

 sonous gases seemed the most economical and practical method 

 to use. 



A fumigation chamber of adobe bricks set in mud, the usual type 

 of building in the Laguna, was constructed. The walls of this 

 chamber were about 20 inches thick and were plastered on the in- 

 side and outside with cement. The floor was of brick set in mortar. 

 During the fumigation the wooden door was closed and several 

 thicknesses of wall paper plastered over the cracks. The size of the 

 room was 5 by 8 by 10 feet, or 400 cubic feet. Three tons of cotton 

 seed filled it about 5 feet deep. Experiments were made with seed 

 placed in the room in bulk and in sacks. Carbon disulphid and 

 hydrocyanic-acid gas were used. 



Carbon Disulphid. 



Live larva 3 , pupae, and adults were placed at different depths in 

 the seed, usually near the top, center, and bottom of the pile. The 

 use of adults was discontinued after the firstj few experiments on 

 account of the death of all the moths, while in the same experiments 

 the mortality of the larvae varied a great deal. Pupae did not seem 

 more difficult to kill than larvae. Larvae, and pupae when they could 

 be obtained, were placed in single, double, and triple pill boxes, at 

 least 60 larvae and sometimes more being used for each experiment. 

 Larvae in double and triple boxes were thought to be about as diffi- 

 cult to kill as larvae webbed up in seed would be. Sometimes the 

 larvae would spin cocoons in the boxes before they were placed in the 

 house, but such larvae were killed along with the others. Where 

 only a few larvae were not killed they were always in the triple boxes 

 on the bottom of the pile. 



As only one lot of 3 tons of seed was available, the seeds were 

 taken from the house and left exposed to the sun for several days 

 after each experiment. When the seeds were returned to the house 

 the boxes of larvae were tied in a muslin bag and placed in position, 

 the disulphid placed in shallow vessels on the top of the seed, and 

 the door sealed. 



Considerable difficulty was experienced at first in determining 

 whether larvae were dead or not. All of the larvae would appear 

 dead when removed, but they still retained their pink color and some 

 of them would revive and pupate after lying in a comatose state for 

 a week or more. Later all the larvae, were kept until we were abso- 

 lutely sure whether they were dead or alive. Table XXV is a de- 



