422 ELEMENTARY BIOLOGY 



450. Fighting insects. One of the first suggestions that 

 insects could be controlled by encouraging other insects was 

 made about a hundred years ago by two English entomologists, 

 who declared that an increase in the number of ladybirds in 

 greenhouses and fields would clean out the aphids, or plant 

 lice, and insure the hops against destruction (see Fig. 224). 



In this country various species of native ladybirds serve as effec- 

 tive checks upon plant lice of many kinds. It has been possible to 

 control the destructive Hessian fly by means of the parasite Polygnotus. 



FIG. 224. The calosoma beetle (Calosoma sycophanta) 



This beautiful green animal was used by a French scientist in a campaign against the 

 gypsy moth in 1840. In recent years this method of fighting undesirable insects by 

 encouraging the spread of an enemy insect has been rapidly extended, especially in the 

 United States, which leads the world in applied entomology, a, adult ; <, larva feeding 

 on pupa of gypsy moth ; c, adult feeding on larva (caterpillar) of gypsy moth 



Shipments of such parasitic insects from one part of the 

 country to another are frequently made to meet outbreaks of 

 injurious insects. A further step was taken when specialists 

 were sent abroad by the government to look for natural 

 enemies of injurious insects in the regions from which these 

 insects originally came. 



The United States probably suffers more from injurious insects 

 than any other country, because there have come here with the mi- 

 grations of peoples a large number of foreign insects, without their 

 natural enemies. This country has also done more than any other in 

 the scientific and practical study of insects and of methods of control. 



