334 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1942 



miles of this barrier were installed to good effect during cricket out- 

 breaks, but this method has now been almost entirely superseded by 

 the much cheaper and less laborious poison-baiting method of control. 



Metal barriers of the type just described have been and are still 

 very extensively used in Argentina against the great hordes of grass- 

 hoppers prevalent in some parts of that country. 



Another expedient frequently used against Mormon crickets where 

 circumstances permit is the oil-on-water barrier. The crickets do 

 not hesitate to plunge in and swim across streams or irrigation 

 ditehes that they may encounter in their migrations. Advantage is 

 taken of this fact to turn strategically located watercourses into 

 barriers by coating the surface of the water with oil. Barrels of 

 cheap, light oil are placed at intervals of about y^ ^^il^ along the 

 streams and canals and their contents allowed to drip or dribble 

 slowly so as to form a thin film on the water. This coats the swimming 

 crickets and kills them. 



SPRAYS AND DUSTS 



Often the first question asked when an insect infestation is discovered 

 on any crop is "What can I spray it with?" Unfortunately, the acre 

 value of cereal crops is ordinarily too low to permit the direct use of 

 insecticides on them. As has already been mentioned, control measures 

 for the insects attacking these crops must be largely cultural or, if 

 insecticidal, must be of extremely low cost as in the case of poison 

 bait, which costs only about 20 to 50 cents per acre. It is true that 

 insecticides are recommended for the direct control of the European 

 corn borer in market sweet corn, but this is a specialized crop which 

 cannot properly be considered a cereal. Because of the rapid growth 

 of the corn and the fact that the egg-laying and hatching period of the 

 borers extends over a month or more, several applications of a spray 

 or dust are required for effective control. This method is too expensive 

 for practical use on field corn, or even on sweet corn grown for the 

 cannery. It is possible, however, that future developments may reduce 

 costs to tlie point where insecticides may be used profitably against 

 the borer, at least on canning corn. 



FUMIGATION 



Insects continue to take their toll of our cereal crops even after they 

 have been safely harvested and stored. As has already been noted, 

 they destroy annually some 300 million dollars worth or more of stored 

 grains and cereal products. With the wartime need of conserving our 

 supplies of these most essential foods, the prevention or reduction of 

 such enormous losses becomes extremely important, and at the same 

 time more diflicult owing to the interference with normal distribution 



