1920 ENTOMOLOGICAL SOCIETY. IIJ 



Fumigation with hydrocyanic acid gas is a control method which served a 

 very useful purpose before the superheating process was perfected, but it must 

 now be relegated to the '' out-of-date " class, since it has the following disadvant- 

 ages when compared with the rival method: 1. It is dangerous to human life. 

 2. While the initial exj)ense is less than that of installing an efficient heating 

 system every subsequent operation is far more ex})ensive than that of turning on 

 the steam. 3. The gas fumes are less penetrating than the heat, and since a high 

 concentration is required for the destruction of eggs many of these, v/hich are 

 laid in protected places, ma^^ escape. 4. The mill must be idle for a longer period 

 at each operation. 



Freezing is a method much in vogue in the Prairie Provinces, where extremely 

 low temperatures can be relied upon at almost any time in the winter. When 

 there is so much of this " natural resource " annually going to waste it would 

 seem to be desirable that it be utilized to the greatest extent possible. We have 

 no records of experimental data as to what low temperature is necessary to destroy 

 the different stages of the various pests, and there is some doubt as to whether 

 the extreme cold experienced in this country will destroy all of the stages. Some 

 of the smaller mills do not run at all in the winter but they never appear to be 

 quite free from pests when they commence operations in the spring. Tliis may. 

 however, be due to an annual re-infestation. 



A mill, when it is opened up to freeze for a couple of days, is usually sub- 

 mitted just before or after the operation to a more vigorous cleaning than it 

 receives at any other time in the year. To what extent the evident benefit derived 

 can be ascribed to the cold or to the broom is a debatable point. Adults of the moth 

 and the beetle certainly perish without exception at 25° below zero, but we have 

 ]io definite data as yet upon the effect on immature stages. 



Freezing is, in most cases, acknowledged to be hard on the mill. Steam 

 pipes obviously must be completely drained, and this is not always easy. Some 

 lubricating oils stiffen up at low temperatures, and the mill should not be re-started 

 until it has warmed up to normal temperature. Metal work warms up more 

 slowly than the rest of the mill. This results in sweating, Avhich collects dust 

 and may even cause rust. These difficulties have been overcome in several mills, 

 among them some of the largest in the country, and freezing is practised by them 

 with evident success. The first cold snap of winter is, however, usually rather 

 anxiously awaited in such mills since, by the time it arrives, the moth is often 

 '■ getting pretty numerous again." This is the main disadvantage to freezing as 

 the sole method of controlling pests. It cannot be applied at any season of the 

 year, and is not available in the summer when the moths are most active. It is, 

 liowever, to be hoped that an opportunity will be offered this winter for us to 

 obtain some definite data upon the value that low temperatures have in the ex- 

 termination of mill pests. 



These, then, are the chief methods of reducing the pests in our mills, but 

 we are faced with one more problem in this connection, namely, that of re-infesta- 

 tion. This possibly is the main problem, and certainly, had it been solved in the 

 first ]»]ace. the problems of eradication would have been non-existent, for mill 

 pests are not indigenous to mills. Some of the newest mills in the countr\' have 

 been heavily infested almost as soon as they were ])ut into commission, while some 

 others have remained almost free after many years of running. 



Often this infestation, and re-infestation after eradication, is well-nigh 

 unavoidable. A citv mill with a local trade stands little chance of immunitv. 



