12 BULLETIN 796, U. S. DEPAKTMENT OF AGRICULTURE. 



cations of 1918 were made from two to three weeks later than was 

 originally planned, and, while the results of these experiments were 

 quite satisfactory so far as the larvicidal action was concerned, a con- 

 siderable amount of the material remained about the trees at the time 

 examinations were made. There is reason to believe that the dates 

 originally adopted are more satisfactory. These dates are as follows : 



Michigan, Ohio, Connecticut September 1 



New Jersey, West Virgina, Maryland September 10 



North Carolina and the Ozarks September 25 



Georgia and Texas October 30 



In the study of Table VI it should be remembered that the exam- 

 inations were made several weeks and in some cases several months 

 after application. Dead larvae, of course, were largely decomposed 

 after such a lapse of time. Hence no account was taken of anything 

 but living larvse. Dead skins and partly decomposed bodies were 

 always in evidence but no attempt was made to determine the actual 

 infestation from their numbers. The check plat by which the relative 

 infestation was judged in all cases was made up of consecutive trees 

 in adjoining parallel rows. 



DOSAGE. 



Table VI sets forth with sufficient clearness the essential fea- 

 tures of dosage efficiency. It will be seen that the doses applied as 

 a single fall treatment gave results which agree very closely with 

 their previously determined larvicidal value given in Table I. The 

 trees treated in these experiments varied in age from 6 to 15 years 

 and in size at the butts from one to several inches in diameter. 

 It might be argued that the collar girth of the trees would be a bet- 

 ter index to the dosage required than their age. There are certain 

 facts, however, which make this undesirable. First, as will be shown 

 later, injury is more dependent upon the age of the tree and the 

 development of protective tissue than upon its size. Second, there 

 has been a wide latitude in the size of trees treated successfully by 

 a given dose. Under field conditions there is at all times a heavy 

 surface and lateral loss of gas. It seems probable for that reason 

 that there is a certain area of gas diffusion necessary before the 

 vapor in the center of this area attains toxic concentration. There 

 are, of course, limits in both directions in the application of this 

 statement, very large trees undoubtedly requiring larger doses than 

 small ones. In practice, however, the latitude in collar girth of 

 trees which a given dose will treat successfully has been enough to 

 cover the usual variation in trees between the ages of 6 and 15 years. 



It will be seen in Table VI that the one-fourth-ounce dose gave 

 only partial control. Although only one experiment included the 

 application of this amount, the results agreed fairly well with its 



