CHAPTER IX 



INSECTICIDES, FUNGICIDES, AND SPRAYING 



INSECTICIDES 



Insecticides are of two general types, those that kill by- 

 being eaten, or stomach poisons ; and those that kill by actu- 

 ally touching the insects, or contact poisons. The distinc- 

 tion is an important one. Insects that eat up the foliage can 

 be killed by applying to the leaves some mixture containing 

 arsenic. Insects that suck the juices of a plant are safe 

 from all poisons because no poison can be got into their 

 food. They make a minute puncture in the leaf -tissue and 

 then pump the sap, and get none of the poison. Such 

 insects must be killed by a contact poison which must cover 

 them and corrode their tissues, or choke them by clogging 

 their breathing pores. 



Stomach Poisons. — Against those insects that feed upon 

 plant tissue there is nothing better than arsenic in some 

 form. Paris green, which contains about 50 per cent, of 

 arsenic, has been frequently employed; but the safest arsen- 

 ical poison for spraying trees is arsenate of lead. Its use 

 has become quite common during the last few years. Paris 

 green is an effective insecticide, but is liable to injure the 

 foliage. Arsenate of lead is effective against insects and is 

 harmless to foliage when used at any reasonable strength. 

 It remains in suspension longer than Paris green, and adheres 

 to the foliage a longer time than any other stomach poison. 



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