TOXIC GASES AS A COlsTTROL OF THE PEACH-TREE BORER. 3 



ber active on untreated trees in adjoining rows. In all cases the effect 

 of the fumigation on the tissues of the tree was carefully noted, 

 the margin of safety being considered as fully as the larvicidal 

 action of the gas. Injury to the trees has been classified under 

 the following terms: Trace, slight, moderate, severe, and maxi- 

 mum. Maximum injury applies to cases where the tree has been 

 girdled completely by the fumigation, and no doubt remains of 

 its immediate death. Severe injury indicates that the tree has been 

 girdled jpartially, sufficient to destroy permanently its jproductive 

 value, but not to cause its immediate death. The other terms, moder- 

 ate, slight, and trace, represent a gradually diminishing degree of 

 injury, the last term indicating that the effect of the gas was only 

 sufficient for identification. The first two terms, trace and slight, 

 represent injury which is of little or no importance as far as the 

 effect upon the tree is concerned. 



EXPERIMENTAL RESULTS. 

 MISCELLANEOUS GASES. 



The principal materials that have been tested are carbon disul- 

 phid, carbon tetrachlorid, hydrocyanic-acid gas, naphthalene, and 

 para-dichlorobenzene. In the discussion of the relative merits of 

 these materials the results are given in detail only for the last, p-di- 

 chlorobenzene. 



Carbon Disulphid. 



In 1902 Woodworth^ recommended the use of carbon disulphid 

 against the western species of peach borer. Under the proper con- 

 dition of soil moisture and temperature it is a most efficient larvicide. 

 In practice, however, it was found impossible to standardize its use 

 to an extent which would make it both safe and effective. The main 

 difficulty with this material arises from its great volatility at or- 

 dinary temperatures, which makes it very sensitive to variations in 

 soil porosity. In very dry or j)orous soils, large doses of several 

 ounces may be applied with no effect upon the larvse. On the other 

 hand, in very moist, tight soils, large vigorous 12-year old trees 

 have been entirely girdled by an application of as small a dose as 

 one- fourth of an ounce. Its application in water emulsion was 

 found to standardize soil conditions greatly, and successful treat- 

 ments by this method with greatly-reduced doses have been made on 

 hundreds of trees without injury. A very slight mis judgment in 

 dosage for a given set of conditions, however, gave results of an 

 opposite character, and after two and one-half years' experience with 

 this gas its use has been entirely abandoned. 



1 WooDWOETH, C. W- The Califorxia peach-tree borer. Cal. Agr. Exp. Sta. Bui. 

 143. 1902. 



