276 INSECTS 



keep a long time if tightly covered. To prepare for use 

 dilute each of the stock mixtures with ten gallons of 

 water, combine them in a barrel of sufficient size, and 

 then add water to make up the full amount. Never 

 combine the concentrated stock mixtures as it pro- 

 duces a heavy coarse precipitate that is much less 

 effective. The above mixture is the full strength that 

 is reasonably safe for general use. The tendency is to 

 use only three pounds of copper instead of four on 

 orchard trees, and on peach it should be even weaker, 

 if used at all. 



Against plant lice, scale insects and other sucking 

 insects our battery is much larger, much less satisfac- 

 tory, and applications must be much more carefully 

 made. We can spray a tree with arsenate of lead to 

 reach an insect which we expect will make its appear- 

 ance to-morrow; but a contact insecticide must be 

 applied when the insects are actually present and must 

 be brought into actual touch with the specimen before 

 it can be effective. 



Contact poisons kill either by direct corrosive action 

 on the skin of the insect, as where dry hydrate of lime 

 is dusted on a soft slug, or by clogging or entering the 

 body through the spiracles or breathing pores. Soap 

 mixtures clog by forming a film over the openings and 

 matting up the hairy guards that as a rule protect 

 them; a soap that makes a rather thick slimy suds is 

 therefore preferable to one that forms a clean, thin 

 suds. Mineral oils are very penetrating and kill by 

 getting into the body cavity through all openings. 

 Decoctions like those of tobacco and hellebore get into 

 the spiracles and perhaps also through the mouth into 

 the stomach and set up a convulsive affection that re- 

 sults in death. Dry powders like hellebore and tobacco 

 are effective in proportion as they are finely ground, and 



