246 A COUNTRY READER. 



Rotation of Crops thins out Insects. 



So if after roots you grow a straw crop, the 

 insects that the year before lived on the root 

 crop will not find the food of the straw crop 

 suitable for them, and they will therefore decrease 

 considerably in numbers. 



You will therefore understand how wise is the 

 system of the rotation of crops, even as a protec- 

 tion against injurious insects. 



It is found that the destruction amongst woods 

 and forests from insects where rotation cannot 

 be carried out is very considerably greater than 

 amongst field crops, where a wise rotation is 

 followed. 



The attack of caterpillars and other insect pests 

 on forests always lasts for many years. 



In a forest, one generation of insects, having 

 increased in numbers, leaves descendants who find 

 all round them in abundance, just the very food 

 that they require, and as a consequence these again 

 increase in numbers till the whole forest swarms 

 with them. 



One Insect Checks Another. 



Sometimes two different species of insect attack 

 the same plant, often with very satisfactory 

 results in getting rid of an insect plague. 



In a certain district of Holland the oaks are 



