60 INSECTS AND HUMAN WELFARE 



Thus, in spite of their simplicity of action, arsenical sprays 

 can be prescribed with confidence only after a careful and 

 painstaking diagnosis has been made. 



The arsenical insecticides are usually referred to as stom- 

 ach poisons as their toxic action is accomplished through the 

 medium of the alimentary tract. As we have seen, they are 

 readily ingested by chewing insects when applied to the 

 plants upon which such insects are feeding. Many of our 

 most destructive insects do not feed by chewing however, 

 but subsist upon juices which they extract from their food 

 plant by means of a piercing, sucking beak which is thrust 

 into the tissues of the plant. Such haustellate insects mani- 

 festly will not take into the mouth any arsenical poison pres- 

 ent upon the surface of the plant, and no successful means 

 have been devised for introducing poisonous materials into 

 the sap with which they might be taken into the body of 

 the insect. 



For the destruction of such insects, other materials, known 

 as contact insecticides are in general use. These consist of va- 

 rious substances, such as nicotine, certain sulphides and oil 

 emulsions, etc., which kill through contact, by actual poison- 

 ing, suffocation or otherwise. 



Tliese are often used in combination with the arsenicals 

 and with a fungicide such as the copper compound known as 

 Bordeaux mixture. Thus when several insects of different 

 types and fungous diseases are to be dealt with upon the 

 same plants, specific combinations may be prescribed. Some 

 of the methods of treatment available for application to va- 

 rious fruits are shown in the accompanying diagram (Fig. 29) . 



After foods are harvested they are commonly stored for 

 considerable periods before they are finally utilized, espe- 

 cially in the case of staple crops such as cereals, beans, etc. 

 During this time they are by no means free from insect in- 

 jury, in fact, their deterioration is undoubtedly hastened 

 more by insect activity than by any other agency. The in- 

 sects concerned are a most cosmopolitan lot, gathered from 

 all quarters of the globe, for their world-wide dissemination 



