PROCEEDINGS OP SECTION D. 329 



the tuber or isolated from it, succumbs after 15 to 16 hours' ex- 

 posure to an atmosphere containing this compound in the propor- 

 tion of 1 to 2 lbs. per 1,000 cubic feet of air. It requires, however, 

 at least 48 hours' fumigation at either of the above rates to destroy 

 the pupa, and a single application under these conditions does not 

 invariably suffice to insure the destruction of this life form. The 

 egg, it was found in the majority of the experiments, succumbs 

 to a single fumigation of 48 hours' duration at the above-men- 

 tioned rates, but in order to destroy any eggs which may in some 

 cases survive, a second fumigation must be applied six to eight days 

 after the first. 



The choice of this interval between the application of the first 

 and second fumigation rests on the fact, derived from a study of 

 the life history of the insect, that the egg hatches in from four to 

 • six days after being laid. It follows, therefore, that when the 

 second fumigation is applied, any eggs which may have survived 

 the first fumigation are either in or approaching the larval con- 

 dition, and are then, consequently, easily destroyed. 



In regard to the effect of fumigation on the vitality of the 

 tuber, it may be stated that experimental investigation has shown 

 that commercially sound unsprouted tubers may be subjected to 

 intermittent fumigation with carbon 'bisulphide (1 to 2 lbs. per 

 1,000 cubic feet of space) two, three, or even four times without 

 producing serious damage to the tuber buds, if each fumigation is 

 limited to a period of 48 hours' duration. Tubers fumigated inter- 

 mittently under the above-mentioned conditiozis, and subsequently 

 placed under dry, airy storage conditions for periods of from two 

 to three months, have, on planting, yielded quite as good crop 

 results as similar, but unfumigated, (control) tubers. In other 

 experiments in which tubers v/ere fumigated for a single period 

 of 72, 96, or 120 hours at the above rates, and then stored for 

 two to three months prior to planting, the crops were satisfactory, 

 but, owing to the risk of injury to the tuber buds, intermittent 

 fumigation is the preferable method. 



The comparative insensitivity of the life forms of the potato 

 moth to the various poisons in solution, which have been examined, 

 render very doubtful the use of steeping methods as a means 

 of combating the pest in stored tubers. The lengthy duration of 

 the period of effective steeping required, even in the case of the 

 larva — the least sensitive of the life forms examined — involves ex- 

 tensive injury to the tuber buds. In short, destruction of the pest 

 involves destruction of the vitality of the tuber. Carbon-bisulphide 

 fumigation, if properly applied, even to badly infested material, 

 affords an effective, comparatively cheap, and much superior method 

 of destroying the various life forms of the pest, without at the 

 same time seriously impairing the vitality of the tuber. 



