44 INSECT PESTS OF FARM, GARDEN AND ORCHARD 



as directed the amount of arsenic which would be deposited on 

 the plant would not be sufficient to cause any injury, and Professor 

 C. P. Gillette has shown that twenty-eight cabbages dusted in the 

 ordinary way would have to be eaten at one meal in order to pro- 

 duce poisonous effects. Occasionally growers dust cabbage with 

 an unreasonable amount of poison, and very rarely instances of 

 poisoning are recorded, but there is no value in applying any more 

 poison than is necessary to make a thin film over the surface, and 

 more than that is wasted. Because a certain amount of poison 

 will kill an insect does. not indicate that a larger amount can kill 

 it any " deader." Experiments have also shown that tobacco 

 sprayed as recommended cannot possibly bear enough arsenic to 

 be injurious, and that cattle or horses may be pastured under 

 trees sprayed with arsenicals with impunity.* 



2. Contact Insecticides 



Contact insecticides are used against insects with sucking 

 mouth-parts and soft-bodied biting insects which may be more 

 readily destroyed by this means than by arsenicals. These sub- 

 stances are fatal to the insect either by clogging the spiracles 

 or trachea, and thus causing suffocation, or by corroding the 

 skin. It should be remembered that the chitinous skin of most 

 insects is not easily corroded, and that in most cases a material 

 strong enough to penetrate the skin will also injure foliage, so that 

 only soft-bodied insects can be combated with corrosive sub- 

 stances upon foliage. 



In the application of contact insecticides it is absolutely essen- 

 tial that the spray come into contact with the insect, as a mere spray- 

 ing of the foliage is of no value whatever. 



1. Kerosene emulsion is one of the oldest remedies for plant- 

 lice and other sucking and soft-bodied insects, and is often 

 resorted to because it is readily made and the materials are 

 always at hand. 



Dissolve J pound of hard or whale-oil soap (or 1 quart soft 

 soap) in 1 gallon of boiling water. Add 2 gallons of kerosene and 



* This is not true of grass beneath trees which have been sprayed with 

 a straight-jet fire-hose, as is commonly done in Massachusetts in the extensive 

 operations against the gypsy moth, but refers to spraying which has been 

 done with an ordinary spray nozzle, which applies the material as a fine spray. 



