18 BULLETIN 796, TJ. S. DEPARTMENT OP AGRICULTURE. 



TREES SIX YEARS AND OVER. 



A single early fall application of a moderate dose of p-dicliloro- 

 benzene has caused no injury on trees six years and over. Doses have 

 been applied to trees of this age varying from 1 ounce to several 

 ounces per tree. Where moderate doses of three-fourths of an ounce 

 or 1 ounce per tree have been applied once in early fall no injury of 

 any importance has ever been observed. Hundred of trees have been 

 fumigated in this way, and in some cases blocks of trees have re- 

 ceived the same doses for two and three years. While there can be no 

 doubt that trees of any age could be killed if the fumigation was 

 prolonged sufficiently, there is every indication that the time re- 

 quired would be many times that necessary to kill the borer. Occa- 

 sionally in the vicinity of wounds a slight amount of tissue is killed 

 even in fumigation with ordinary doses, but on the older trees this 

 has been so slight and superficial in character that it may be disre- 

 garded. , 



Table IV gives the results of prolonged fumigation on eight-year-- 

 old trees at Springfield in 1916. As will be seen from the data there 

 presented very little injury of importance was found on trees treated 

 with doses giving a continuous fumigation from June 24 to the last 

 of October. Of the entire 126 trees treated with doses ranging from 

 one-half ounce to 4 ounces each, only two developed serious injurj^ 

 and in both cases the trees had been seriously injured by both borers 

 and crown gall. Apparently the gas has considerable effect upon the 

 spongy galls which accompany this disease. Doses of 2 ounces 

 and over gave off a very pronounced odor when the earth was opened, 

 and the fumigation had probably been maintained at a toxic concen- 

 tration throughout most of the period from June to October. While 

 from the standpoint of control a fumigation of so great duration is 

 not necessary or desirable, it is interesting that all these larger doses 

 gave almost complete immunity from the insect throughout the 

 season. 



The greatest amount of injury ever observed on older trees resulted 

 from an application made at Springfield on November 1, 1916. (See 

 Table VI.) 



