INSECT ENEMIES AND THEIR CONTROL 109 



may be purchased which are highly satisfactory. The 

 liquid extracts may also be diluted and applied as a spray. 

 This plan is not regarded as satisfactory for lettuce, be- 

 cause it is not desirable for any kind of a tobacco solution 

 to come into contact with the leaves. 



Sheets of paper, impregnated with concentrated 

 tobacco extracts, are popular with some gardeners, and 

 especially with frame vegetable growers. The papers are 

 easily ignited and convenient to use. 



Ordinary tobacco powder is often dusted on cucumber 

 plants and sometimes on lettuce, but it is only moder- 

 ately effective, because it serves mainly as a repellent. 

 It also acts as a preventive when placed on top of the 

 soil when the lettuce is planted. 



Hydrocyanic gas fumigation. This method of destroy- 

 ing insects which feed on greenhouse vegetable crops is 

 now employed by most of the large commercial growers. 

 It made slow progress for many years, mainly for two 

 reasons the danger to the fumigator and to others likely 

 to be about the establishment, and the possibility of 

 injuring the plants. 



Numerous experiments, however, made by scientists as 

 well as by practical growers, have demonstrated that 

 hydrocyanic gas, when properly used, is a cheap, safe and 

 effective fumigant. By its use insects may be destroyed 

 which are extremely difficult to exterminate by any other 

 method. Among such pests is the white fly, a most 

 serious enemy of greenhouse tomatoes and cucumbers. 

 This gas is a deadly poison also to thrips, plant lice and 

 mealy bugs, but it does not kill the red spider or scale 

 insects unless used sufficiently strong to kill the plants. 



The equipment needed for fumigation with hydro- 

 cyanic gas is stone or earthenware jars or crocks, which 

 should be of gallon size and fairly narrow. Wrapping 

 paper or preferably small paper bags will be needed for 

 the crystals of cyanide of potassium or cyanide of sodium, 



