2 BULLETIN 796, JJ. S. DEPARTMENT OF AGRICULTURE. 
infinite variation of such factors under field conditions it has been 
found impossible to correlate these data with the results of the 
experiments except in a very general way. For the most . part, 
therefore, conclusions have been based on the results of fairly ex- 
tensive field tests with only a general analysis of the conditions in- 
volved. 
A number of substances have been tested and many data accumu- 
lated which it is impossible to include in the present paper. In so far 
as promise of providing a safe and effective control measure for the 
insect is concerned, all of the gases tested have given negative results 
except one, p-dichlorobenzene. The results obtained from the use of 
this material are so promising that it has seemed best to report upon 
the present experimental status of, the work for the information of 
others interested in the problem. Also it is hoped that as far 
as possible useless duplication of work with some of the other less 
promising gases may be prevented. 
EXPERIMENTAL PROCEDURE. 
As already stated, the essential idea of the experiments involves 
application of volatile toxic compounds to the soil at the base of the 
trees. In one experiment small tents were used to confine the gas, but 
this method, on account of the mechanical difficulties involved, was 
never used to any great extent. The soil under all ordinary condi- 
tions proved a fairly satisfactory container for the gas. Whether the 
material was applied in water solution or in its original form de- 
pended upon its chemical properties. Whenever possible both 
methods of application were adopted. When applied either in solu- 
tion or suspended in water, a trench was dug about the collar of the 
tree as for " worming " and the required dose, in from 1 to 2 gallons 
of water, puddled about the trunk, after which the soil was replaced 
and compacted. When applied in its original state the material, if 
a liquid, was poured into the soil about the collar, and if a solid, was 
buried in the surface soil about the trunk. 
Two methods of ascertaining results were followed. In the first the 
treated trees were examined within a short time after application, 
usually within a few days, the results being judged by the mortality 
of the larvae. The interval from application to examination was 
usually varied sufficiently to determine the toxicity of the fumigation. 
In the second method the treatments were made either in late fall or 
early spring and examination made several weeks or several months 
later, before the infestation of the next season. By this time dead 
larvae usually had largely decomposed, and it was necessary to 
judge the effectiveness of the fumigation by a comparison of the 
number of active larvae escaping on the treated trees with the num- 
