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No. 37, SECOND SERIES. 
uted States Department of Agriculture, 
‘DIVISION OF ENTOMOLOGY. 
THE USE OF HYDROCYANIC ACID GAS FOR FUMIGATING GREEN- 
HOUSES AND COLD FRAMES. 
INTRODUCTION. 
The most widely used insecticide for greenhouse fumigation is 
tobaceo in its various forms. In many instances it is without doubt 
the cheapest and safest insecticide to use against plant-lice and a 
few other greenhouse insects. Its effectiveness, however, under the 
best conditions is not great, as it requires repeated use at short 
intervals. Moreover, it may cause serious injury to the plants. In 
the case of violets it has been found to be of only slight value against 
plant-lice working in the bud, and while it may destroy the so-called 
‘“oreen aphis” when exposed, it is not effective in killing the ‘‘ black 
On the other hand, tobacco may prove injurious to the 
“spot.” Tobacco is 
5 
aphis.’ 
foliage and flowers, bringing on epidemics of 
also useless against scale insects in general and mealy bugs. The 
desirability of some insecticide as easy to use as tobacco but more 
penetrating and effective and less likely to injure plants was appar- 
ent, and became absolutely necessary in our study of the diseases of 
certain crops. 
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 experi- - 
ments early in 1895, though it had been occasionally tried in green- 
houses, 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. 
EXPLANATORY NOTE.—The fact that a circular of the Division of Entomology 
should be written by expert employees of the Division of Vegetable Physiology 
and Pathology needs a word of explanation: Messrs. Woods and Dorsett in the 
course of their investigations of the diseases of greenhouse plants, found it neces- 
sary to destroy the insects living upon the plants which they had under observa- 
tion. For this purpose they used hydrocyanic acid gas and thus became by this 
practical experience so well fitted to discuss this particular phase of the use of 
this important gas that I have requested them to prepare this circular.—L. O 
Howarp, Entomologist. 
