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, 
