294 
JOURNAL OF ECONOMIC ENTOMOLOGY 
[Vol. 17 
to the methods used by eastern workers. The soil was leveled about 
the trees, and one ounce of the crystals was placed in a circle about two 
inches from the base of the tree. Soil was then mounded over the ma¬ 
terial and firmly packed. The dates of application and results of the 
treatments are indicated in table II. 
Time Factor. To be effective, it is necessary that sufficient time shall 
elapse after the application to permit of the volatilization of the crystals 
and diffusion of the gas about the base of the tree. It was found that 
when the results were noted in the fall, the number of living larvae 
varied inversely to the length of time that had elapsed since the applica¬ 
tion of the treatment. In no case were any living larvae found where 
the application was left undisturbed until the following spring. 
Position of Larvae. Larvae situated on the trunk of the tree above 
the level of the mounted soil are not affected by the paradichloroben- 
zene treatment. In some orchards of western Oregon this constitutes a 
distinct disadvantage in the paradichlorobenzene treatment. Young 
larvae in superficial tunnels below the soil level are the first to succumb 
to the fumes. Large larvae in deep tunnels are more resistant, but 
larvae situated in deep tunnels as far as six inches below the level of 
the application were eventually killed. Larvae above the level of the 
application but below the level of the mounded soil were also killed. 
Influence of Soil Type. It was found that the type of soil had con¬ 
siderable influence upon the action of the material. Most of the or¬ 
chards of western Oregon are on heavy soils, varying from silt loam to 
heavy “gumbo.” It was found that on the heavier soils, volatilization 
and consequently the lethal effects of the paradichlorobenzene were 
much less rapid than on the lighter soils. In the Skyline Orchard this 
was especially well illustrated. Where the material was applied in 
clay loam, approximately one-half of the crystals remained at the end 
of twelve days; while in a lighter silt loam in the same orchard the 
crystals had almost entirely disappeared at the end of the twelve day 
interval. So far as we have observed, none of the material has remained 
in crystalline form throughout the winter. However, in examining 
trees which had received a fall application, a faint trace of the charac¬ 
teristic odor could sometimes be detected in the spring. 
Injury to Prune Tree. In no case was injury observed where one 
ounce of paradichlorobenzene was properly applied to healthy, mature 
prune trees, even though the material was allowed to remain undisturbed 
throughout the winter. 
