ROTATION OF CROPS 197 



holding, and let it revert to prairie con- 

 ditions, or to introduce crops other than 

 cereals, so that the maintenance of cattle 

 may become possible. 



To the credit of a rational rotation must 

 also be put comparative freedom from the 

 attack of injurious insects. Most of our 

 most destructive insects are very fastidious 

 as regards the diet that they require, and 

 are incapable of living upon any plant but 

 the particular species, or at most genus, that 

 they affect. Thus, for instance, the carrot 

 fly is met with on no other crop ; the turnip 

 flea beetle is dependent for its existence 

 upon the presence of cruciferous plants ; 

 the mangold fly can only exist in the leaves 

 of beets and mangolds ; while the Hessian 

 fly confines its devastations entirely to the 

 Graminese, and, one might almost say, 

 entirely to cereals. A few insects are less 

 specialized in their food requirements, as, 

 for instance, the cockchafer grub, the larva 

 of the daddy-long-legs, and the wire-worm. 

 But, speaking generally, the first class 

 contains the majority of our thoroughly 

 destructive insects. In the case, then, of 

 an insect, highly specialized as regards its 

 nutriment, the farmer can do much to hold 

 it in check by depriving it in alternate years, 



