PREFACE. 
An investigation into the methods of fumigating citrus trees with 
hydrocyanic- acid gas was commenced by the Bureau of Entomology, 
United States Department of Agriculture, during the summer of 
1907, and for a period of three years has been carried on in California 
by the writer under the direction of Mr. C. L. Marlatt, assistant chief 
of the bureau. This work was undertaken in response to urgent 
requests from the horticultural commissions of the principal citrus- 
fruit-producing counties of southern California and of many active 
fruit growers. Prominent in this movement was Mr. J. W. Jeffrey, 
former secretary of the Los Angeles County horticultural commission, 
and now State commissioner of horticulture — a man entirely familiar 
with the unsettled condition of fumigation practice at that time and 
with the need of placing it on a more scientific basis. At the com- 
mencement, the writer spent from three to four months in a thorough 
field investigation to acquaint himself with the conditions of citrus 
culture throughout southern California, the distribution of the dif- 
ferent citrus pests and the damage caused by them, the existing 
methods for their control, and the methods of fumigation practiced 
in the various citrus districts. 
During the early part of November, 1907, active experimental 
field work was commenced at Orange, Cal., using an outfit belonging 
to this bureau, consisting of four tents and the other paraphernalia 
necessary for practical fumigation. Field work of this character 
has been continued throughout, it being the writer's effort to conduct 
the investigation on as nearly a commercial basis as possible so that 
the conditions and results would be those normal to the ordinary 
care of citrus groves. During the work there have arisen many 
problems of a laboratory nature, the solution of which would have 
been most interesting, but these problems for the most part have 
been set aside except in those cases where they had a direct economic 
bearing on practical work in the field. 
The results of this investigation have very little of the nature of 
original discoveries, although there has been acquired a vast amount 
of exact information never before thoroughly understood. The 
advance is largely the result of correcting, correlating, systematizing, 
and placing upon a more scientific as well as a more practical basis 
methods which had been practiced in California or elsewhere for 
many years. 
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