20 



requires repeated use at short intervals. Moreover, it may cause 

 serious injury to some plants. On the other hand, tobacco may prove 

 injurious to the foliage .and flowers of certain plants such as violets, 

 by bringing on epidemics of "spot." Tobacco is also useless against 

 scale insects in general and mealy bugs. 



Hydrocyanic-acid gas treatment. — At present the most satisfactory 

 manner of dealing with this and similar pests is by means of hydro- 

 cyanic-acid gas which has been developed as a medium for the fumiga- 

 tion of insect-infested greenhouse plants by the Division of Vegetable 

 Ph} 7 siology and Patholog} r of this Department, having been used 

 under the direction of Mr. A. F. Woods in 1894, and later by Mr. 

 Woods and by Mr. P. H. Dorsett. Detailed directions for its use 

 were given by these two gentlemen in Circular No. 37, 2d Ser., of 

 this Division, from which publication the directions which follow on 

 the hydrocyanic-acid-gas method haA^e been largely transcribed. 



This remedy has proved particularly effective against the present 

 species on violets and against aphides on similar greenhouse flowers, 

 and has the advantage of being useful against all other insects 

 except the so-called red spider, which it does not entirely destroy, but 

 it can not } T et be safely used for the fumigation of certain other plants 

 owing to the danger of bleaching and otherwise injuring them; hence 

 it will be wise, before undertaking fumigation on a large scale on 

 other plants which will be mentioned, to first try this remed}' experi- 

 mentally on a few plants and in a small way. 



THE HYDROCYANIC-ACID-GAS METHOD OF FUMIGATING GREENHOUSES AND COLD FRAMES. 



Hydrocyanic-acid gas, since its introduction by the Division of Entomology in 

 1886 as a remedy against scale insects of the orange, has proved of great value as an 

 insecticide. Previous to our experiments early in 1895, though it had been occasion- 

 ally tried in greenhouses, hydrocyanic acid was not recommended on account of its 

 injurious effects upon plants. As a result of a series of careful experiments we found 

 that, as a rule, plants were less injured by a short exposure to a relatively large 

 amount of gas than they were by a long exposure to a relatively small amount. On 

 the other hand, a strong dose for a short time was the most effective in killing 

 insects. Different species and varieties of plants, however, were found to vary 

 remarkably in their power of withstanding the poison. This in many cases appeared 

 to depend upon the open or closed condition of the breathing pores, as well as upon 

 peculiarities of the cell contents. Fumigation an hour or two after sundown, with 

 the temperature as low as practicable, was found to give the best results. In each 

 case the proper amount of gas to use and the length of exposure must be determined 

 by experiment. It is impossible at present to give a general rule applicable to all 

 plants in all stages of development. ' When the conditions are once determined they 

 must be strictly followed to insure the greatest success. Methods of experimenting 

 will be described in the latter part of this circular.' The quantity of gas in each case 

 is always given in terms of the potassium cyanide from which it is made, and on a 

 basis of a cubic foot of space, as will be explained more in detail further on. The 

 cyanide, as well as the gas made from it, is exceedingly poisonous, and both must 

 be used with the greatest care. 



