28 



ate the scale in orchards with a liquid insecticide required that the 

 fluid should be so distributed as to reach every scale on every tree 

 in quantity sufficient to kill the insect. This wa.s obviously an 

 impossible task, especially if we take into account the frequency 

 with which these minute scales are secreted under bark, behind 

 buds, etc.; and this conclusion was confirmed by two years of pre- 

 vious experience with the use of whale-oil soap, during' which 

 orchards and town lots at twenty-one localities in Illinois had been 

 carefull}^ and thorouj^hly treated by two of my assistants. Prof. H. 

 E. Summers, now State Kntomolog-ist of Iowa, and Mr. R. W. 

 Braucher, a graduate in horticulture from the University of Illi- 

 nois. Subsequent inspections of the premises treated disclosed the 

 fact that the extermination of the scale was accomplished in no 

 case except where every visibly' infested tree and shrub was 

 destroyed, together with all adjacent vegetation to which the scale 

 might possibly have spread. 



The fumigation method had this theoretical advantage over 

 that with the liquid spray, that an insecticide vapor set free under 

 a close tent enveloping the infested tree would be carried by spon- 

 taneous diffusion to all parts of the inclosed space, and would thus 

 presumabl}^ reach every particle of the infested surface and kill 

 every scale, provided the operation were intelligently and carefully 

 conducted. There seemed, in short, a reasonable possibility that 

 expert fumigation would exterminate locally where the conditions 

 were not unfavorable to thorough work, while such a result seemed 

 clearly impossible with the liquid spray. I consequently decided 

 to make a trial of orchard fumigation with hydrocyanic acid gas, 

 applied after the method which had been in use for several 3^ears 

 in the citrus orchards of the Pacific coast. 



The opinion current among entomologists with respect to 

 fumigation as an orchard method was well shown by statements 

 of experimental results appearing at about this time, and especial- 

 ly by those published in August, 1898, in Bulletin 57 of the Mary- 

 land Experiment Station, and in the fall of that year in Bulletin 

 No. 17 of the Entomological Division of the U. S. Department of 

 Agriculture. In the first of these bulletins, entitled "A Report on 

 the San Jose Scale in Maryland and Remedies for its Suppression 

 and Control," Prof. W. G. Johnson, State Entomologist of Mary- 

 land, gives an account of various experiments with this insecticide 

 g-as, made in the fall of 1897 and the spring of 1898 on bearing 

 orchard trees in Maryland. As a conclusion from experiments on 

 fifty-three dwarf Bartlett pear-trees conducted from September 27 

 to October 1 by liberating gas under inclosing tents, he says (p. 86) 



