390 BOARD OF AGRICULTURE. [Pub. Doc. 



as possible. As the punk and fumigating papers are stronger, they 

 give better results than the stems and leaves, though more expensive 

 than these last. 



Certain tobacco extracts are also on the market under various trade 

 names, and, as they contain fixed strengths of nicotine, are more 

 reliable in their actions than ordinary tobacco stems, in which the 

 amount of this narcotic varies considerably. They are usually diluted 

 with more or less water and then heated, producing a vapor which 

 spreads through the house. The objection to these substances is that 

 it is often inconvenient or impossible to vaporize them rapidly enough 

 to obtain a density of vapor sufficient to produce the desired effect. 



The insects most successfully controlled by fumigation with tobacco 

 are plant lice, and, when vaporization is successful, some of the thrips. 

 It does not seem to kill all the plant lice, however, for others soon 

 appear, evidently derived from some which had escaped the treatment. 

 The white fly adults are temporarily affected by it and fall to the 

 ground, but most of them soon recover. Red spiders, mealy bugs 

 and scale insects are but little affected. 



Fumigation with sulfur will undoubtedly destroy insects when the 

 fumes are strong, but unfortunately it also seriously affects the 

 plants. For that reason, then, this method of treatment cannot be 

 considered of value in greenhouses. 



Fumigation with carbon disulfid has been but little tested in green- 

 houses, the weight of the gas causing it to settle rapidly. A few tests 

 of it for thrips on tomato plants indicate that, used at the rate of one 

 cubic centimeter to each cubic foot of space, the insects would all be de- 

 stroyed without injury to the plants; but this treatment would prob- 

 ably be successful only on very low plants, placed on the floor of the 

 place to be fumigated, while if enough of the disulfid should be used 

 to reach higher plants the gas would be likely to become so dense 

 lower down as to cause injury. Its use, therefore, can hardly be 

 advised without more knowledge of its capabilities. 



Fumigation with hxjdrocyanic acid gas is certainly the most effective 

 treatment thus far discovered; but the gas is a most deadly one, and 

 for this reason many are afraid to use it, and a hearty respect for its 

 powers is a feeling which should be encouraged. If proper care be 

 taken, however, it can be used with safety; and the only other draw- 

 back it has is our ignorance as to how much can be used without injury 

 to the plants exposed to its fumes. This subject is now being in- 

 vestigated at the Massachusetts Agricultural Experiment Station, 

 and the maximum strength of fumigation without injury to the plants 

 has been determined for the more common greenhouse varieties of 

 tomato and cucumber at all ages and under different conditions of 

 light, temperature, moisture, etc., and the experiments will be ex- 

 tended to other plants as rapidly as possible. 



To fumigate with hydrocyanic acid gas, potassium cyanide, commer- 



