132 



the period of exposure. In this way the men are kept continuously 

 busy, the time for the removal of the first tent arriving by the time that 

 the last tree is covered. 



' Absence of Sunlight necessary. — The originators of the fumiga- 

 tion process observed that the gas was most efficacious, and chat Jess 

 injury resulted to the foliage when the operations were performed at 

 night than when they were carried on in sunlight. It is said that chem- 

 ical changes are produced in the gas by the action of sunlight, and 

 that the resulting gases are more injurious to the plant life and less to 

 animal than hydrocyanic acid gas. Whether or not these theories are 

 correct is of small practical importance, tor the foliage of a tree will 

 suffer serious injury if the tree is left covered with an air-tight oiled 

 tent for naif an hour in sunlight, without the gas being present. 

 Having ascertained this fact by experience, the foreman in charge 

 of the Board's outfit refrained from covering trees until the sun 

 had sunk from sight on any but cool dull days. The great majority of 

 the trees treated have been fumigated after sunset. The ideal night for 

 fumigating is quiet, cool, and moonlight without dew.' 



It is evident, from the above, that the period available for this 

 process is somewhat limited. However, when only a few trees have to 

 be treated, the hour immediately preceding nightfall will be ample for 

 the purpose. I have personally found no ill effects following the opera- 

 tion when performed on dull, cloudy days, when the sun is entirely 

 obscured. 



Although hydrocyanic acid gas will certainly kill every insect — 

 and even their eggs if used in sufficient strength and for a sufficient 

 length of time, both the necessary strength and time will be found to 

 vary with different species of insects, and must be made the subject of 

 careful experiment. I find that Orthezia insignis is a very difficult 

 insect to kill, and requires a double strength of gas, continued for fuUy 

 three-quarters of an hour. 



Mr. D. W. Coquillet, one of the first to employ this process, gives 

 the following directions for making an air-tight tent : * 1 The material 

 commonly used in the construction of the tent is what is known as blue 

 or brown drilling. A few persons have used ducking instead of drill- 

 ing, but this is much inferior to the latter ; in the ducking the threads 

 extend only lengthwise and crosswise, whereas in the drilling they also 

 extend diagonally — this belonging to the class of goods to which our 

 merchants apply the term " twilled " — and for this reason the drilling 

 is both stronger and closer in texture than the ducking. 



1 After the tent is sewed up it is given a coat of black paint, as it 

 has been ascertained that tents treated in this manner last longer than 

 those which have been simply oiled with linseed oil. Some persons mix 

 a small quantity of soap-suds with the paint in order to render the, latter 

 more pliable when dry, and therefore less liable to crack. Instead of 

 thus painting the tent some persons simply give it a coating of size. 

 Sometimes a small quantity of whiting or chalk is added to this sizeing 

 with or without the addition of lampblack. A few made use of the 



* Bulletin, No. 23, U. S. Department of Agriculture (Division of Ento- 

 mology. 



