SUPPRESSION OF RODENTS 191 



field should first be gone over with poison, and 

 the bulk of the animals thus destroyed. 



Flooding the burrows. Where available, 

 water is one of the best means of combatting 

 pocket-gophers. Flooding the land in winter 

 is especially effective, as it wets the animals 

 and drives them to the surface, where they soon 

 succumb to the cold. In warm weather the 

 method can be made effective if men and dogs 

 are on hand to kill the animals as they seek 

 refuge on the embankments. S. E. Piper, of 

 the Biological Survey, reports that about the 

 middle of April, 1909, at Modesto, Cal., he saw 

 some boys killing pocket-gophers that had been 

 driven from an alfalfa patch by flooding. A 

 hundred gophers, more than half of them young 

 of the year, were killed from a three-acre tract. 



Difficulties of extermination. Much hope has 

 been entertained that a bacterial disease fatal 

 to rodents, and particularly to rats, might be 

 found, but thus far has been disappointed. 

 It was thought that a bacillus (B. typliimur- 

 ium) given to the field-mice which over-ran 

 Thessaly in 1892-3 had put an end to the 

 plague, but it is now thought it did little to 



