6 CIRCULAR 3 64, U. S. DEPARTMENT OF AGRICULTURE 



much as one-fourth to one-half pound over the dosage required may- 

 run in before the valve can be closed. The concentration dropped to 

 0.4 mg per liter in an average of 56 minutes. 



FUMIGATION TESTS ON INSECTS AND MITES 

 APPARATUS AND METHODS 



The studies on the lethal effects of the fumigating gas were made 

 at Arlington Farm, Va., where equipment for such purposes was 

 available. The fumigation chamber used consists of a small metal- 

 lined room containing 254 cubic feet. It is well insulated, and con- 

 tains apparatus for temperature and humidity control. At tempera- 

 tures up to 120° F. a relative humidity of around 90 percent can 

 be maintained. A 16-inch electric fan gives rapid and uniform 

 distribution of gas. 



The insects used were the springtails Lepidocyrtus lanuginosus 

 (Gmel.) and Xenylla sp., adults and larvae; and all stages of the 

 mushroom fly Sciara sp. The mites were Tyroglyphus lintneri Osb. 

 and Histiostoma sp., of which all stages were present. The insects 

 and mites, upon finely divided mushroom spawn, were placed in metal 

 boxes having the tops and bottoms of fine wire screen. As exact 

 figures on the percentage of mortality were not desired, no more 

 than an approximate count of the insects used was made. In each 

 box there were 15 to 20 individuals of L. lanuginosus; 10 to 20 larvae, 

 several pupae, and 10 to 50 eggs of Sciara sp. ; 25 to 30 each of adults 

 and larvae of T . lintneri and Ristiostonna sp. ; and several thousand 

 hypopi of each of the last two genera. The boxes were placed in the 

 chamber upon a support about 3 feet from the floor in such a way that 

 the gas could circulate freely through them. The insects were re- 

 moved when the concentration had dropped below 0.4 mg per liter. 



In order to simulate as nearly as possible the actual conditions 

 existing in a mushroom house, the boxes containing the insects and 

 mites were placed in the chamber at 75° or 80° F., and the tempera- 

 ture slowly raised to 100° through a period of from 6 to 8 hours. It 

 was held at 100° for from 7 to 9 hours and the gas then introduced. 



A series of test fumigations was conducted in the fumigation' 

 chamber at 100° F. and 90 percent relative humidity, beginning with 

 a dosage that produced a peak concentration of 1 mg of hydrocyanic 

 acid gas per liter, and in succeeding tests increasing the dosage in small 

 steps until a point was reached at which no insects survived. The 

 exposure consisted of either a single dosage or two or more dosages 

 applied at intervals of 15 minutes, the total quantity being the same 

 whether applied at once or in smaller quantities at intervals. The 

 approximate lethal point having thus been found, a series of tests 

 was run at each of several concentrations immediately above and be- 

 low this point to fix it more closely. 



In any given series of duplicate tests, the concentrations (deter- 

 mined at intervals of 5 minutes) were averaged, and an average curve 

 for the series constructed from these figures. Mean concentrations 

 were determined from these curves by the use of a planimeter. The 

 fumigations and the curves drawn from them were designated as 

 " sublethal " in cases in which approximately 1 percent or fewer of 

 the insects or mites survived. Lethal fumigations were those in 



