36 HYDROCYAXIC-ACID GAS FUMIGATION IX CALIFORNIA. 



at the right and left margins are to be found blank spaces, and in the 

 horizontal lines corresponding to these the numbers at the top of the 

 chart are repeated in that part of the chart contammg dosage figures. 

 These numbers, repeated in this manner, make it easier for the eye 

 to locate with certainty the dosage figures sought. In the chart used 

 bv the writer the figures representing distances around and over are 

 printed in red. The lines bounding these columns of figures are also 

 red. Ah the rest of the lines and figures are black. 



The writer does not maintain that this table is accurate to the 

 minutest part of an ounce for every dosage, but that such variations 

 as do exist are so small that in practical work in the held the results 

 in killing scale insects will be found uniformly satisfactory through- 

 out. Two years of experience with the outfits belonging to this 

 investigation, as weU as with many practical outfits, in which work 

 thousands of acres have been fumigated, have proved that this 

 behef is weU founded. 



It is a common practice with tent-pullers in covering small trees 

 to kick in the edge of the tents httle if any so as to prevent as much 

 as possible the generator as well as the escaping gas from coming in 

 contact with the tent. This common practice leaves much more 

 space under the tent and incidentally makes more leakage surface in 

 small trees than was provided for in the original chart. (Bulletin 

 Xo. 79, fig. 28, p. 65.) To correct this feature of the fumigation 

 practice, the original chart has been revised by increasing the smaller 

 dosages to the extent which, in field practice has been found necessary. 

 In this revised chart the half ounces are not used as in the original. 

 Field experience has taught that it is desirable to have schedules as 

 simplified as possible. The writer would now advise all fumigators 

 to discard all old schedules and to use the revised one entirely. The 

 dosage strength on which this schedule was based was calculated for 

 use against the purple scale. However, this does not imply that it may 

 not be used against other insects: in fact, the greatest advantage 

 of it is that it can be manipulated so as to meet the requirements for 

 use against any insect. The schedule in its original form is not recom- 

 mended for use against all insects under ah conditions, as many have 

 believed. Some of the scale pests frequenting citrus trees require 

 a heavier dosage for their destruction than others. The first point 

 to be determined is the strength of gas required for a particular pest 

 under its special conditions. When this is known, then schedule 

 Xo. 1 can be manipulated to meet the requirements, provided it is 

 not already of the proper strength. This change is secured by 

 increasing or decreasing all the dosages throughout schedule Xo. 1 at 

 the same rate, i. e., ^, J, \, etc. The resultant schedule will be one 

 of uniformity even as is the original. So in treating an orchard the 

 first point to determine is what species of insect has to be combated. 



