TOXIC GASES AS A COISTTKOL OF THE PEACH-TREE BOREK. 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 diflPerence 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.^ 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 egg. 



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. 



-^ Correspondence and notes of the Bureau of Entomology. 



