336 ANNUAL REPORT SMITHSONIAN INSTITDTTON, 1942 



carbon disulphide mixed with carbon tetrachloride to make them 

 noninflammable, are most generally used on stored grains (pi. 18). 

 These are liquids at ordinary temperature and when sprayed on the 

 surface of the grain immediately evaporate to form heavy gases which 

 quickly sink down through it. Another material often used for grain 

 fumigation is calcium cyanide in granular or powder form (pi. 18). 

 This is run into the grain stream as it enters the bin where it quickly 

 reacts with atmospheric moisture to form deadly hydrocyanic acid gas. 



The fumigants most commonly used in mill and warehouse fumiga- 

 tion are liquid hydrocyanic acid, methyl bromide, chloropicrin, and 

 a mixture of ethylene oxide and carbon dioxide. These must be lib- 

 erated by suitable means and in sufficient quantity to obtain an insect- 

 killing concentration throughout the entire building. Chloropicrin, 

 by the way, is the tear gas introduced during the first World War. All 

 these gases are extremely poisonous to humans as well as insects and 

 should be handled only by experienced persons equipped with gas 

 masks adapted to the particular fumigant to be applied. 



The ready susceptibility of most stored-product insects to heat and 

 cold is often utilized instead of fumigation for their control. Some 

 cereal products are run through steam or electrical heating appliances 

 to kill any insects they may contain and fair control is sometimes ob- 

 tained by heating entire buildings to 125° F. or more in hot weather 

 when these temperatures can be produced without too great expense. 

 In the more northern latitudes advantage is often taken of zero and sub- 

 zero temperatures to eradicate insect infestations in mills and ware- 

 houses by turning off all heat and opening the buildings to the outside 

 air. The low winter temperatures and short cool summers of northern 

 climates ordinarily serve to hold insect infestations in grains or cereals 

 in unheated storage to such a low level as to make fumigation unneces- 

 sary. 



The preemption of carbon disulphide and fumigants containing 

 chlorine for use in various war industries has made them unavailable 

 at times for the treatment of stored grains, and has increased the need 

 of finding available substitutes. Efforts are now being made in this 

 direction. 



INSECT-RESISTANT VARIETIES GE CROPS 



Although it has been known for many years that some varieties of 

 the different cereal crops are less susceptible than others to their 

 respective insect enemies, deliberate efforts to find and breed this 

 character into improved varieties are comparatively recent develop- 

 ments. As in the case of other ventures into new fields, these efforts 

 were considered by some investigators to be very unlikely to yield 

 results of practical value. Nevertheless, they have been continued and 

 have become extremely promising. Wheats highly resistant to the 



