TOXIC GASES AS A CONTROL OF THE PEACH-TREE BORER. 21 
When examined, about one month later, it was found to be two-thirds 
girdled by the fumigation. While these observations are very lim- 
ited they indicate a wide difference in susceptibility of apple and 
peach. This difference apparently is due mostly to the difference in 
the thickness of the bark and its layers of protective tissue, which 
exists between the two. 
Discussion. 
The experience so far obtained indicates that there are pronounced 
possibilities in the application of poison gases to the control of the 
peach-tree borer. At present p-dichlorobenzene remains the only 
material of decided value. Despite the fact that for three seasons it 
has given uniformly good results, however, it is not the purpose 
of this paper to encourage its use except in an experimental way. 
The control obtained has been by no means 100 per cent efficient, 
and it is doubtful if such a degree of control could be secured safely 
by any artificial means. Certainly it is not obtained in practice by 
the ordinary " worming." Unfortunately the use of this gas is 
restricted to trees of somewhat advanced age, but this limitation 
might apply as well to the application of other fumigants for the 
control of this insect. 
The question of the volatility of p-dichlorobenzene was submitted 
to the Bureau of Chemistry. 1 It was found that the vapor pressure, 
while very low at ordinary temperature, is, roughly, about ten times 
as great at 100° F. as at 50° F. In the soil, however, the relation of 
vapor pressure and temperature to the rate of volatilization is greatly 
modified by such factors as barometric pressure, humidity, circula- 
tion of .air, surface exposure, etc. Although there is undoubtedly a 
considerable variation in the rate of volatilization within the sea- 
sonal range of soil temperatures, it has not seriously interfered with 
the effectiveness of the gas in the field tests so far made. Soil tem- 
perature records taken throughout the season of 1916 at Springfield, 
W. Va., showed a variation at a depth of 6 inches from about 50° 
to 55° F. in April and October to 75° and 80° F. in July and 
August. 
The vapor of p-dichlorobenzene has a very decided repellent effect 
upon the ovipositing moths. On several occasions females were 
followed through treated blocks of trees and observed to visit tree 
after tree, hovering about the base for a short time or alighting for 
an instant without depositing a single Qgg. 
The effective range of the gas is rather local, being confined to the 
area reached in practice by " worming." On the bole of the tree at 
a depth of from 8 to 12 inches and on lateral roots at less than that 
depth, but more than 6 to 8 inches from the trunk, larvae usually are 
not affected. 
1 Correspondence and notes of the Bureau of Entomology. 
