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 great 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. 
NURSERY TREES. 
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. 
