TOXIC GASES AS A CONTROL OF THE PEACH-TREE BORER. 15 



satisfactory on this type of soil, and the mechanical difficulty in- 

 volved in its application would probably be no greater than in 

 " worming." 



For the types of soil upon which experiments have been made no 

 consistent variation in control has so far been observed. The com- 

 parative mildness of the fumigation and the long period over which 

 it acts apparently tend to neutralize the influence of soil type. 



Injury to Teees. 



The margin of safety in the use of p-dichlorobenzene against the 

 peach-tree borer apparently depends entirely upon the relative extent 

 to which the insect and the tissues of its host are exposed to the 

 action of the gas. In the main this relation varies in proportion to 

 the age of the tree and the development of its protective tissues. It 

 has been found impossible to fumigate young trees with safety. On 

 the other hand trees beyond a certain age have shown so far no ill 

 effects from a fumigation of several times the duration necessary 

 to kill the insect. 



In the course of three years' work many hundreds of observations 

 have been made on the collars of trees treated with a gTeat variety 

 of doses on a number of different soils. In some cases blocks of 

 trees have been treated for two successive seasons and in one case for 

 three years. In part, the observations have been made in connection 

 with " worming " data, and in part they are the results of separate 

 dosage experiments. 



NUBSERY TEEES. 



In 1916, 57 nursery trees were treated in a nursery at Hagerstown, 

 Md. The trees were Belle of Georgia buds of the previous fall. The 

 soil was a stiff clay loam. The dosage was distributed over the 57 

 trees as follows: 10 received one-fourth ounce each; 10 one-half 

 ounce; 10 three-fourths of an ounce; 10, 1 ounce; 5, 2 ounces; 5, 3 

 ounces; 2, 4 ounces; 2, 5 ounces; and 3 individual trees received 6, 

 8, and 10 ounces, respectively. The application was made on Sep- 

 tember 6. On October 21 the trees were dug and examined. In this 

 interval of 45 days all of the trees had been injured severely by the 

 gas. No distinction could be made between the various doses in the 

 extent of the injury, which was as severe on trees receiving one- 

 fourth ounce as on those receiving more. The injury was localized 

 on the collar and larger roots lying within 6 to 8 inches of the ma- 

 terial. The greatest amount of tissue killed was in the vicinity of 

 wounds, but even on uninjured stems where the epidermis was not 

 ruptured the surface was peppered with tiny lesions extending into 

 the cambium. 



