8 BULLETIN 796, U. S. DEPARTMENT OF AGPJCULTUEE. 



It was found necessary in this work to differentiate between what 

 Avere apparently normally active larvae and those which, although 

 slightly active, were visibly affected by the gas. The larvae there- 

 fore were classified into dead, stupefied, and active. Whether 

 stupefied larvae eventually recover or die seemed to be largely a ques- 

 tion of the duration and toxicity of the fumigation, and the degree to 

 which the larvae were affected. Stupefied larvae when placed in the 

 open air sometimes recovered and sometimes died, but the determina- 

 tion of the exact condition under which either took place was impos- 

 oible. It was determined, however, that larvae apparently much af- 

 fected may be revived. On the other hand, when the dosage was 

 sufficient to provide for a continuation of the fumigation much 

 beyond the time at which the examination was made it may be as- 

 sumed with safety that the stupefied larvae eventually would have 

 been killed. 



In Table I it will be seen that a 1-ounce dose, applied July 19, 

 showed 9 d'ays after application a small number of larvae stupe- 

 fied. By the time of the next examination, however, 14 days after 

 application and in the two succeeding examinations, the larvae were 

 all dead. 



Very little can be added to these data by an extended discussion. 

 The principal facts in regard to dosage are brought out in Table I. 

 In the study of the table, however, it should be emphasized that the 

 relative numbers of dead and living larvae in the last two examina- 

 tions for each dose are somewhat misleading, due to the early de- 

 composition of many of the dead, especially the smaller ones. The 

 1-ounce and one-half-ounce doses were more quickly and fully ef- 

 fective in the July applications than in the August applications. 

 The one-fourth-ounce dose applied August 17 was partially effective 

 but its action was not sustained, and by the time of the last examina- 

 tion, 39 days after application, the dose had been dissipated so com- 

 pletely that young larvae which were hatching at the time were able 

 to enter the trees. The one-half -ovmce and the 1-ounce doses both 

 gave very complete immunity from this current infestation of newl}- 

 hatched larvae. The importance of this point will be emphasized later 

 in connection with the time of application and the final control ob- 

 tained from the use of this gas. 



It was observed that the larvae of the fungus-gnat, Mycetobla 

 sp., which feed in the gum, apparently succumb to the action of the 

 gas in about the same proportion as do the larvae of exifwsa. 



From the results of the experiments with p-dichlorobenzene given 

 in Table I it will be seen that at the soil temperatures prevailing 

 in late July and August a dose of 1 ounce per tree has a very effective 

 larvicidal action. The vapor apparently acts rather slowly, how- 



