6 BULLETIN 796, U. S. DEPARTMENT OF AGRICULTIJRE. 



veniently carried in an open pail and the dose measured in a gradu- 

 ate holding just the desired amount. A little preliminary practice 

 with the graduate and scales enables one to gauge the dosage very cor- 

 rectly. It is probably best to keep the material from actual contact 

 with the bark by perhaps half an inch, although the practical im- 

 portance of this precaution is not known. Finally, the material is 

 covered to a depth of 2 inches with two or three shovelfuls of soil, 

 and this slight mound compacted by a few sharp blows with the 

 back of the shovel. The soil is the container for the gas, and the 

 success of the fumigation depends upon the vapor being given off 

 faster than it is lost. At best there is considerable surface loss of 

 gas, and the final compacting of the soil is of considerable impor- 

 tance. No lumps or stones are left against the trunk above the 

 surface to furnish a harbor behind which newly-hatched larvae 

 may begin feeding out of reach of the vapor. No effect, of course, 

 can be produced upon larvae feeding in galleries above ground. 



Labvicidal Action. 



To determine the actual larvicidal value of the vapor of p-dichlo- 

 robenzene, a series of applications were made to trees in the 

 field at Springfield, W. Va., in 1916. The approximate limits of 

 effective dosage had been fairly well established by preliminary 

 experiments. Each dose was applied to 20 trees and these examined 

 in lots of 5 at approximately weekly intervals. 



Table I gives the summarized results of several such tests. It is 

 impossible to include all of the experiments made in this connection, 

 but the data given bring out the essential facts in regard to dosage 

 efficiency. The soil on which these experiments were made is classi- 

 fied as Frankstown silt loam.'' 



1 Determination of soil type furnished by the Soil Survey of the Bureau of Soils, United 

 states Department of Agriculture. 



