20 



requires repeated use at short intervals. Moreover, it uiav 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, 

 b}' bringing on epidemics of "spot." Tobacco is also useless against 

 scale insects in general and mealy bugs. 



T{ydr<. yanic-aeid gas treatment. — The most satisfactory manner of 

 dealing with the insect enemies of violets and some other greenhouse 

 plants is by means of hydrocA'anic-acid gas which has been developed 

 as a medium for the fumigation of greenhouses by Messrs. A. F. 

 Woods and P. H. Dorsett, of the Division of Vegetable Physiology 

 and Pathology of this Department. Instructions for its use were 

 given by these gentlemen in Circular No. 37, 2d Ser., of this Division, 

 from which publication the directions which follow on this method 

 have ))een largely transcribed. 



This remedy has proved particularly effective against aphides, and 

 has the advantage of being more or less useful against many other 

 insects except the so-called red spider, which it does not entirely' 

 destroy, but it can not yet 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 plants which will l)e mentioned, to first use this remedy 

 experimentally on a few plants and in a small way.^ 



THE HYDROCYANIC-ACIB-GA.S METHOD OP FUMIUATIN(i (iREENHOUSES 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 vahie 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 temper'ature 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 impossiVjle at present to give a general rule applicable to all 

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



^ In Galloway's Commercial Violet Culture (p. 215), where similar advice for the 

 treatment of this insect is given, the statement is made that "Where this gas is used 

 for other pests, the insect in question is easily kept in check." The writer is of the 

 opinion that this is true, but it is to be regretted, in the face of experiments that have 

 been conducted by Mr. M. V. Slingerland (Bui. 190, Cornell Univ. Agr. Exp. Sta., 

 May, 1901, p. 16.3), that he (the present writer) was not more guarded in his expres- 

 sions concerning the efficacy of this remedy in the first edition of this bulletin. Mr. 

 Slingerland tested this gas against this leaf-tyer, and his experiments show that the 

 gas does not kill moths, pupfe, and grown larvpe when used at the ordinary strength, 

 but only small larva?. These results agree with some experiments that have Iteen 

 made by the writer on other caterpillars; still it is the experience of INIessrs. Gal- 

 loway, Woods, and Dorsett that where this gas is frequently used the leaf-tyers will 

 be greatly reduced in numbers if not held under practical control. — F. H. C. 



