20 



farmers' bulletin 896. 



1. The virulence is not great enough to kill a sufficiently high per- 

 centage of rats that eat food containing the microorganisms. 



2. The virulence decreases with the age of the cultures. They de- 

 teriorate in warm weather and in bright sunlight. 



3. The diseases resulting from the microorganisms are not con- 

 tagious and do not spread by contact of diseased with healthy 

 animals. 



4. The comparative cost of the cultures is too great for general use. 

 Since they have no advantages over the common poisons, except that 

 they are usually harmless to man and other animals, they should be 

 equally cheap ; but their actual cost is much greater. Moreover, con- 

 sidering the skill and care necessary in their preparation, it is doubt- 

 ful if the cost can be greatly reduced. 



The Department of Agriculture, therefore, does not prepare, use, 

 or recommend the use of rat viruses. 



NATURAL ENEMIES OF RATS AND MICE. 



Among the natural enemies of rats and mice are the larger hawks 

 and owls, skunks, foxes, coyotes, weasels, minks, dogs, cats, and 

 ferrets. 



Probably the greatest factor in the increase of rats, mice, and other 

 destructive rodents in the United States has been the persistent kill- 

 ing off of the birds and mammals that prey upon them. Animals that 

 on the whole are decidedly beneficial, since they subsist upon harmful 

 insects and rodents, are habitually destroyed by some farmers and 

 sportsmen because they occasionally kill a chicken or a game bird. 



The value of carnivorous mammals and the larger birds of prey in 

 destroying rats and mice should be more fully recognized, especially 

 by the farmer and the game preserver. Rats actually destroy more 

 poultry and game, both eggs and young chicks, than all the birds and 

 wild mammals combined ; yet some of their enemies among our most 

 useful birds of prey and carnivorous mammals are persecuted almost 

 to the point of extinction. An enlightened public sentiment should 

 cause the repeal of all bounties on these animals and afford protec- 

 tion to the majority of them. 



ORGANIZED EFFORTS TO DESTROY RATS. 



The necessity of cooperation and organization in the work of rat 

 destruction is of the utmost importance. To destroy all the animals 

 on the premises of a single farmer in a community has little perma- 

 nent value, since they are soon replaced from near-by farms. If, 

 however, the farmers of an entire township or county unite in efforts 

 to get rid of rats, much more lasting results may be attained. If con- 

 tinued from year to year, such organized efforts are very effective. 



