48 NATURAL ENEMIES OF INSECTS 
Fig. 51. — Predaceous 
beetle, Calosoma. Orig- 
inal. 
began to grow excessively numerous, 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 
the host are rapidly diminished. If, on the 
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 helps greatly 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- 
numbers, 
the 
parasites that would have taken 
plies to excessive 
escaping the attack of 
it in hand in its native home. 
Occasionally, it has been found 
possible to import artificially 
the parasites of an introduced 
pest, and to 
successfully in 
establish them 
this 
But the venture is tedious and 
exceedingly difficult. 
country. 
Some con- 
dition of weather or tempera- 
Fic. 52.— Adult tachina fly ; enlarged 
and natural size. The larve are para- 
sites. Original. 
