136 Journal of Agricultural Research voi.xv, no. j 



As shown in experiments 2 and 3, satisfactory results are not obtained 

 where the gas is held in the presence of a partial vacuum throughout 

 the entire exposure. It has also been proved that a 30-minute exposure 

 of gas in the presence of a partial vacuum and a 30-minute exposure 

 under normal atmospheric conditions are not effective/ The addition 

 of 10 pounds' pressure for 30 minutes in lieu of normal atmospheric 

 pressure yielded practically the same results. It is obvious from Table 

 I that the most satisfactory results were invariably secured where a 

 15-minute preliminary vacuum was followed by an exposure of one or 

 more hours under normal atmospheric conditions. Especially was this 

 true in the case of dry and moist soil used in experiments i, 6, 7, and 8, 

 where 100 per cent of the larvae of Cotinus nitida, Agriotes mancus, 

 Popillia japonica, and Lachnosterna sp. were killed. In the soaked-soil 

 tests, however, the results were unsatisfactory. 



While there was no notable difference in the resistance to the gas by 

 the various larvae used, it was apparent that Popillia japonica was the 

 most difficult to kill, whereas Cotinus nitida was the most susceptible 

 to fumigation. 



SUMMARY 



(i) The effectiveness of the hydrocyanic-acid gas under the vacuum 

 process is influenced by the water content of the soil. 



(2) The death of 100 per cent was not obtained with larvae in soaked 

 soil at dosages ranging from >2 ounce to 3 ounces per 100 cubic feet of 

 space. 



(3) Eliminating the soaked-soil tests, by far the best results were 

 secured where a preliminary 15-inch vacuum preceded an exposure of 

 one and a half hours under normal atmospheric conditions. 



(4) Hydrocyanic-acid gas in the presence of a 26-inch vacuum through- 

 out the entire exposure gave negative results with a dosage of one ounce 

 of sodium cyanid per 100 cubic feet and an exposure of i hour. An 

 exposure of the gas for one-half hour under 10 pounds' pressure, follow- 

 ing a half-hour exposure to a 27-inch vacuum, yielded very indifferent 

 results. 



(5) With our present knowledge of vacuum fumigation with hydro- 

 cyanic-acid gas, a dosage exceeding i ounce of sodium cyanid per 100 

 cubic feet of space with an exposure oi ij4 hours is not recommended 

 for plants in foliage. Inasmuch as all larvae in soaked soil were not 

 killed with dosages varying from ^2 ounce to 3 ounces per 100 cubic 

 feet of space, fumigation at the port of entry with a dosage which will 

 not injure the plants can not prevent the introduction and establish- 

 ment of all subterranean pests. 



