48 ENTOMOLOGY. 



himself when healthy and liviug under favorable conditions, 

 resist their attacks. Even if one or a few individuals were 

 inoculated, the disease might not spread. When, however, 

 insects are superabundant and crowded, and the conditions 

 favorable to any disease arise, the timely inoculation of even 

 a few individuals might result in the destruction of im- 

 mense numbers of insect-pests. Future experiments in 

 this direction may give a new phase to economic entomol- 

 ogy- 



Unusual Increase in the Number of Insects. — It is fre- 

 quently noticed that certain insects abound in profusion 

 which are ordinarily rare or not common. This is due, as 

 we shall see farther on, either to favorable weather or to 

 the absence of their parasites. Thus canker-worms, the 

 Hessian fly, the chinch-bug, the cotton-worm, as well as 

 the Rocky Mountain and other locusts, may in certain 

 years become vastly more numerous, and consequently 

 more destructive, than in others. If all the eggs laid by 

 insects came to maturity, the earth would be overwhelmed 

 with them, and every green thing would be devoured. In 

 what a ratio insects might increase, were it not for these 

 natural checks, may be seen by the following statements. 



Tom i cits typographus in 1874, in the Bohemian forests, 

 had three broods. Judeich assumes that in the middle of 

 April the female laid in its maternal gallery 90 eggs; and 

 he therefore reckons that early in June at least 30 in- 

 dividuals became capable of reproduction. Each of these 

 30 females again lavs in the maternal gallery 90 eggs, pro- 

 ducing also in all 2700 individuals; and by the beginning 

 of August of the third brood again, only a third part of 

 them being females, these would gnaw 900 maternal gal- 

 leries and lay in them 8100 eggs. Having reached this 

 number again, the next spring a third would be ready for 

 oviposition, so that there would be of the first brood in 

 April already 27,000 descendants of the single female which 

 flew about the preceding April, and which would be now 

 capable of laying 2,430,000 eggs. 



