ECONOMIC ZOOLOGY 413 



number of allies among the insects themselves, the help- 

 ful forms who destroy those who are noxious or mis- 

 chievous. 



325. Economic entomology. The enormous number of 

 insects which feed on useful plants gives this branch of 

 science a great practical importance. Most insects feed on 

 plants, and those cultivated by man seem to be especially 

 chosen. This is due to the great masses of the same spe- 

 cies brought together in cultivation. An apple orchard of 

 300 acres in New York gives opportunity for the breeding 

 of enemies of the apple. A 3,300-acre vineyard in Cali- 

 fornia breeds in numbers the insect enemies of the vine. 

 The great orchards and gardens may be compared to 

 bounteous feasts, and the insect guests come in families and 

 remain until the feast is over. 



Probably half the insect species inhabiting the United 

 States (upward of 60,000 in all) are injurious to vegetation. 

 There is scarcely a plant, wild or cultivated, that does not 

 harbor insect pests, there being on an average six insect 

 enemies to each species. In Europe 500 species are said to 

 attack the oaks, and 400 species the willows. In America 

 250 species feed in one way or another on the apple. 



Of each species the number of individuals is enormous, 

 for most of them are excessively prolific. Of aphides or 

 plant lice there are 12 generations in a year ; 12,000,000 

 aphides have been found on a single cherry-tree. The 

 grape- destroying insect phylloxera was first discovered in 

 New York in 1854. It was carried to France in 1863, and 

 in 1879 the valuable vines on 3,000,000 acres in France had 

 been destroyed by its root-attacking larvae. 



326. Orchard pests, In 1878 the " cottony-cushion scale " 

 was brought to California from Australia on an orange-tree. 

 In less than ten years almost every orange-tree in Califor- 

 nia was attacked by it, and the industry seemed doomed to 

 destruction. The pest was checked by the introduction of 

 its natural enemy, an Australian lady-bird beetle, called 



