18 STATE BOAKD OF AGRICULTURE. 



CHAPTER IV. 



INSECTICIDES AND THEIR RANGE. 



Nothing so forcibly demonstrates the importance of checking or 

 reducing insect injury, as the number of patented or proprietary 

 insecticides that have been of late years put upon the market, and 

 the great variety of machinery for its application that has been 

 devised and is being constantly manufactured. 



Insecticides are of two general types — those that kill by being 

 eaten or stomach poisons, and those that kill by corroding the tissue 

 or clogging the spiracles or breathing pores, i. e., contact poisons. 

 The distinction is an important one, for while contact poisons may 

 kill insects of any kind, stomach poisons kill only such as eat the 

 tissue upon which the poison is spread. Thus the apple tree cater- 

 pillar may be readily killed by an arsenical spray which poisons its 

 food, while the plant lice clustering on the same shoot will be entirely 

 unaffected. 



Arsenic in one or the other of its combinations is the chief stomach 

 poison, and paris green is the form in which it is most commonly 

 used. Paris green is, strictly speaking, an aceto arsenite of copper, 

 manufactured as a coloring material and not primarily for insecticide 

 purposes. When pure and well made it contains from 50 % to 60 % 

 of arsenious acid combined in such a way as to be practically insol- 

 uble in cold water. Arsenious acid soluble in water is as destructive 

 to plant as to animal tissue and will "burn" foliage at a strength 

 necessary to kill the insects feeding upon it. Combined with copper 

 or lime it is insoluble in water and therefore harmless to plant life, 

 though yet soluble in the digestive juices of insects and therefore 

 fatal to them. It is slightly soluble in warm water, hence, when 

 sprayed liberally on a plant in the hot sunshine, the water becoming 

 heated before evaporation dissolves some of the acid and causes a 

 burning effect. Applications are best made, therefore, in the morning 

 or evening, or on a cool day. 



When applied dry it may be either dusted very finely on the plant 

 surface to be protected, or it may be mixed with lime, plaster, flour 

 or other material and applied more freely, yet not in such intimate 

 contact with the plant. Good paris green, well made, may be safely 

 applied pure or unmixed on a dry foliage or one moist with cold 

 water, provided it is spread in an even, thin layer. Indeed, this is 

 really the most effective way it can be employed on low plants. 



