48 



NATURAL ENEMIES OF INSECTS 



l:)esan to grow cxcessivel}^ nuniorous, some 

 one of its enemies, stimulated by the abun- 

 dance of food, increased so rapidly that with 

 the next generation or the next season the 

 injurious species was well-nigh wiped out of 

 existence. 



In truth, there is a sort of natural balance 

 between the numbers of a given species of 

 insect and those of its enemies. If the insect 

 increases abnormally, the parasites are stimu- 

 lated to heavy increase and the numbers of 

 Fig. 51. — Predaceous the host are rapidly diminished. If, on the 

 beetle, Calosoma. Orig- ,^ ^ ^ .1 ■, . ^ 1 n j.i 



. J other hand, the host decreases abnormally, the 



parasites perish from lack of food, and thus, 

 freed temporarily from their attack, the host is enabled to increase 

 once more. 



Consideration of the above law hel]3s greath^ to explain the fact 

 that injurious species imported 

 from a foreign country are so 

 often intolerable pests. We 

 have brought over the host with- 

 out its enemies. Finding con- 

 ditions here congenial, it multi- 

 plies to excessive numbers, 

 escaping the attack of the 

 parasites that would have taken 

 it in hand in its native home. 



Occasionally, it has been found 

 possible to import artificially 

 the parasites of an introduced 

 pest, and to establish them 

 successfully in this country. 



But the venture is tedious and 



,. , ,.^ ,, ^ Fig. 52. — Adult tachina fly; enlarged 



exceedmgly difficult. Some con- and natural size. The larva, are para- 



dition of weather or tempera- sites. Original. 



