189 



Blakeslee (E. B.). Use of Toxic Gases as a possible Means of 

 Control of the Peach-Tree Borer. — U.S. Dej}t. Agric, Washington, 

 D.C., Bull. 79G, 21st October 1919, 23 pp., 1 plate. [Received 

 3rd March 1920.] 



Investigations were begun in 1915 with the object of developing a 

 possible method of controlUng the peach tree borer, Aegeria {Sanni- 

 noidea) exitiosa, Say, by the use of local applications of volatile toxic 

 compounds to the soil at the base of the tree. 



The substances experimented with include carbon bisulphide, 

 carbon tetrachloride, hydrocyanic acid gas, naphthahne and para- 

 dichlorobenzene. With the exception of the last-named all the 

 tests gave negative results. 



Some account of para-dichlorobenzene is given and its previous uses 

 are reviewed. The action of the gas generated is quite local and an 

 €ven distribution about the tree is desirable ; in the present experi- 

 ments it was found most convenieW to pulverise the material to 

 about the fineness of coarse salt. The soil around the tree was broken 

 up to a depth of 1 or 2 inches but not scraped away from the collar 

 of the tree unless there was a decided mound. Para-dichlorobenzene 

 was then sprinkled in a band 1 or 2 inches wide round the collar of 

 the tree as nearly as possible at the level of the uppermost galleries. 

 It is probably best to keep the material from actual contact with the 

 bark to the extent of about half an inch, but the practical importance 

 of this precaution is not known. The material is finally covered to 

 a depth of about 2 inches with soil and the mound shghtly compressed. 

 No lumps or stones should be left against the trunk above the surface 

 as they furnish shelters for the newly hatched larvae out of reach 

 of the vapour. 



A total of 126 eight-year-old trees were treated with various doses 

 ranging from ^ oz. to 10 oz. per tree. It was found that less than 

 I oz. would probably not give consistent control, but there is no 

 advantage in giving a larger dose than 1 oz. The best time for 

 apphcation is 3 wrecks before the end of the hatching period, when 

 all larvae will be killed except a few that have already entered the 

 tree. The seasonal fluctuations in the period of oviposition prevent 

 the determination of the time of application to any very great degree 

 of exactness, but during these observations applications made between 

 the end of August and end of September proved most satisfactory, 

 the larvae being killed in about a fortnight. There is apparently 

 no advantage in making a second application, which only increases 

 the risk of injury to the tree. Although it has been found that, with 

 all gases, trees recover more readily from injury in the spring at the 

 beginning of the growing season, apphcation s made at that time are 

 not efficacious, as the majority of the larvae have penetrated the tree 

 and their galleries provide a chance of escape from the gas. 



Trees of six years standing or older do not appear to show any 

 ill effects from fumigation, but the injury to younger trees was too 

 great to warrant the use of this substance. It has so far been used 

 with almost uniform results on a variety of soils. 



This method of fumigation has also been tried on apple trees against 

 Saperda Candida, but the trees were severely injured, there being 

 apparently a wide difference in susceptibiUty between apple and 



