2 BULLETIN 796, V. 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 id^a 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 

 larvse 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- 



