626 AGRICULTURAL, ANALYSIS 



hold pests, and animal parasites. The insects which principally 

 injure plants however, and for which insecticides are most often 

 applied are external feeders, which include "biting" and "suck- 

 ing" insects. 95 



Biting insects are those which actually eat some part of the 

 solid substance of the plant as the leaf, bark, flower, etc. For 

 these some poisonous substance is used, which can be sprayed 

 on the parts of the tree attacked and then be eaten by the in- 

 sect in its food. 



Sucking insects, are those which live by sucking the plant 

 juices from leaf, bark, fruit, etc. For these insecticides must 

 be used which kill the insect either by their causticity, by smoth- 

 ering, through closing the breathing pores, or by filling the air 

 around the insect with poisonous fumes. 



Classification of Insecticides. From the above it will be seen 

 that insecticides may be classified according to the group of 

 insects upon which they act, in the following manner: 



Insecticides used against 



1. External feeders: 



(a) Biting insects. 



(b) Sucking insects. 



2. Internal feeders. 



3. Subterranean insects. 



4. Insects affecting stored products. 



5. Household pests. 



6. Animal parasites. 



Since the methods used in combating, as well as the insecticides 

 used against, internal feeders and household pests are extremely 

 varied, it does not seem best to consider these classes of insecti- 

 cides in this section. 



Following are descriptions of the composition, adulteration 

 and methods of analysis of the principal commercial insecticides 

 under the various groups enumerated above, such as the chemist 

 is usually called upon to examine. Those insecticides which 

 are home-made and do not require analysis by the chemist are 

 not included. 



95 Marlatt, Department of Agriculture, Farmers' Bulletin 127, 1901 : 7. 



