INSECTS ON GREENHOUSE PLANTS 225 



strength or density of the gas is necessary. This is also true for 

 plants. Fortunately, the insects in most cases are killed more 

 easily, i.e. with a smaller amount of the gas, than are the plants. 

 At the present time, the main gap in our knowledge is a lack of 

 knowledge as to how strong the gas must be in order to kill all 

 the individuals of each kind of insect concerned. A strength 

 sufficient to destroy all the plant-lice in a house would probably 

 fail to kill the thrips ; thus we must learn how strong the gas 

 should be to accomplish the result desired, for each different 

 kind of insect." 



267. The use of contact poisons. When but few plants are 

 attacked by mealy bugs or scale insects, contact poisons are 

 frequently used to exterminate the pests. These poisons are 

 various oil solutions, such as kerosene emulsion, whale oil soap 

 or similar preparations. The solutions are applied with a 

 brush, and the resulting chemical action on the bodies of the 

 insects destroys the skin and tissue beneath. 



