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. 



