324 SCHOOL ENTOMOLOGY 



Against insects affecting orchard and garden crops which 

 have a higher value per acre, the use of insecticides and 

 mechanical devices will also prove profitable, but to deter- 

 mine which will be effective we must know somewhat of the 

 insect's anatomy and habits, for it is entirely useless to apply 

 an arsenical poison for insects whose mouth-parts are so 

 constructed as to make it impossible for them to eat it. 



163. Methods of Farm Practice for Insect Control.* 

 (a) Crop Rotation. Many insects feed on only one crop. 

 Evidently, therefore, if a field be planted in a different crop, 

 they will have to migrate from it and a very considerable 

 mortality will result, while if it were left in the same crop 

 they would have ideal conditions, under which to multiply. 

 Thus the western corn root-worm may be practically con- 

 trolled by not growing corn on the same land for two suc- 

 cessive years, for it feeds only on corn and is not injurious 

 where rotation is practiced. Injury by the Hessian fly to 

 wheat and by the chinch bug to corn is also very materially 

 reduced by frequent rotation. Care should be taken to 

 arrange a rotation in which plants nearly related botanically 

 do not follow each other, for they are usually attacked by 

 the same insects. Thus, white grubs, wire-worms and cut- 

 worms live in sod land, where they often become exceedingly 

 abundant. If the land be put in corn these pests will con- 

 centrate on the fewer plants and do serious injury, whereas 

 if it had been planted in some small grain, buckwheat, "cow- 

 peas, potatoes or some other crop which they do not affect, 

 the land could then be safely planted in corn the next sea- 

 son. The same principle applies to various garden crops. 



* See F. M. Webster, Farm Practice in the Control of Field Crop 

 Insects, Yearbook, U. S. Department of Agriculture, 1905, and Some 

 Things that the Grower of Cereal and Forage Crops Should Know 

 About Insects, Yearbook, U. S. Department of Agriculture, 1908, 

 page 367. 



