REFACE 
Fumigation under tents with hydrocyanic-acid gas has been the 
principal moans of controlling scale-insects on citrus fruit trees in 
California for many years. Most of the commercial orchards in the 
State are fumigated at intervals of one or two years, at a cost rang- 
ing from 25 cents to $1.50 a tree, or a probable total annual expendi- 
ture of about $1,500,000, on the basis of fumigation of 50 per cent 
of the trees each year. It becomes, therefore, a matter of very great 
importance to conduct the operation of fumigation in the most effect- 
ive and economical manner. The work being done on the subject in 
California by this Bureau is aimed to thoroughly standardize the proc- 
ess. It was undertaken in response to urgent demands from the hor- 
ticultural commissioners of the principal citrus-fruit-producing coun- 
ties of California, and of many prominent growers. The need of this 
investigation was most urgently championed by Mr. J. W. Jeffrey, for- 
mer secretary of the Los Angeles County horticultural commission and 
now State commissioner of horticulture of California. Recognizing 
the general usefulness of the process of fumigation, Mr. Jeffrey called 
attention strongly to the unevenness of results in the work of differ- 
ent manipulators and against different scale pests, and that the whole 
practice had grown up experimentally without ever having been given 
thorough scientific examination. He urged that such an examina- 
tion necessitated carefully conducted and recorded field work, sup- 
plemented by chemical tests of ingredients and the determination of 
reactions, and expert study of the physiological effect of the gas on 
the trees and fruit; in other words, to remove the process from the 
mere guesswork of the field man and to place it on an exact scientific 
basis. 
This investigation has been under the direct charge of the writer, 
who made a personal study of the situation in southern California in 
September and October, 1907, and planned the work to cover the fol- 
lowing subjects: 
(1) Dosage, or the amount of gas and duration of exposure neces- 
sary for different purposes. The strength of gas necessary to effect- 
ively control the three prominent scale pests of citrus trees in Cali- 
fornia, namely, the red scale, the purple scale, and the black scale, 
under different climatic conditions, as in the drier foothills regions and 
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