414 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



poisons. These may be considered under four heads: first, the 

 internal poisons, or those which take effect by being eaten along 

 with the ordinary food of the insect; second, the contact in- 

 secticides that kill the insects when applied directly to their 

 body; third, the gases that are used for fumigation, and, fourth, 

 various substances that act as repellents. The kind of insec- 

 ticide to be used depends in a large degree upon the structure 

 of the mouth-parts of the insects. For beetles, the larvae of 

 moths and butterflies, and others with biting mouth-parts 

 the internal poisons are used, while for the various insects 

 with sucking mouth-parts, as the plant-lice, scale-insects, etc., 

 the contact insecticides are used. The reason for this is very 

 plain. It would be useless to spray a plant with poison when 

 it was infested by an insect with sucking mouth-parts, for 

 such insects obtain their food from the juices of the plant 

 which are not reached at all by the poison. But if the plant 

 is infested by biting insects, such an application of poison to 

 all the leaves and tender shoots would be effective, for with 

 each bitten off and swallowed mouthful of leaf, the insect would 

 get a dose of the poison. 



Internal Poisons. Paris green was for a long time regarded 

 as the most efficient of the internal poisons and is still used to a 

 limited extent. 



The poison is applied as a spray, using one pound of Paris 

 green to 150 to 200 gallons of water for such trees as apples, 

 pears, etc. For peach and plum trees and other plants with 

 delicate foliage which is very susceptible to the poison the 

 mixture should not be stronger than one pouud of Paris green 

 to 250 or 300 gallons of water. A pailful or two of fresh lime 

 water should be added to every 200 gallons of the mixture to 

 prevent the poison from scalding the foliage. In preparing 

 the mixture the Paris green should first be mixed into a fine 

 paste with a small amount of water; it should then be strained 

 thoroughly and the bulk of water added. The lime may be 

 added to the paste before straining, in amount equal to the 

 amount of poison used, instead of being added to the mixture as 

 above recommended. The mixture must be kept well stirred 

 during the spraying. 



