286 GARDEN FARMING 



have a proboscis, or bill, which they force into the tissue of the 

 plant to suck its juices. When present in sufficient numbers these 

 insects make such a draft upon the juices of the plant that the 

 infested leaves lose color, curl up, and soon die. 



Treatment. Because of the manner in which the insect obtains 

 its food it is evident that it cannot be controlled by the use of 

 poisons. The use of poisons on lettuce would also be precluded 

 because of the fact that, although the plant grows from the center 

 out like cabbage, yet its leaves are usually greatly convoluted and 

 are capable of holding the poison on their surface. Moreover, both 

 the outer and the inner leaves are used for human food without 

 cooking, and even if they were cooked it would be dangerous to 

 human life to use arsenical poisons upon them. The habits of the 

 insect and the character and use of the plant make necessary an 

 insecticide which is not poisonous but which kills by contact or by 

 inhalation. This insecticide has been found to be suffocating gases, 

 one of the best of which is the fumes of tobacco in some one of its 

 forms. The burning of tobacco stems, and the destructive fumes 

 of liquid tobacco extract, either evaporated over a low flame or placed 

 in metal vessels into which hot irons are plunged, are effective. 

 Other more convenient preparations, in the form of burlap or paper 

 impregnated with a concentrated tobacco extract, are now on the 

 market. These when burned in the atmosphere of a cold frame or 

 greenhouse produce fumes destructive to insect life. Such insecti- 

 cides, to be most effective, should be used in the following manner. 

 Give three light treatments on consecutive nights and then wait 

 until the presence of insects gives evidence of the need of another 

 treatment. A good rule is to use the " smudge " for three nights, as 

 above suggested, and then withhold treatment for ten days. The 

 reason for the three treatments is that a few insects will escape 

 destruction the first time and even the second, while the third treat- 

 ment should kill all except those protected in some manner. The 

 three light treatments are more effective in destroying the insects, 

 and at the same time less harmful to the plants, than a single 

 strong treatment. 



While there are other insects which sometimes prove annoying 

 to lettuce growers, there are none so persistent as the aphis and 

 none which demand special consideration or treatment. 



