38 FIFTH REPORT OF THE ENTOMOLOGICAL COMMISSION. 



8uch good results that the value of this comparatively new method of 

 combatiug out-of-door insects is now well established. It is not to be 

 supposed that work of this kind can be carried on in the case of forest 

 trees, except on a very limited scale, to protect cherished trees in lawns 

 or parks. This treatment is also efi'ective against Aphides and leaf- 

 mites — and indeed is calculated to destroy any insects whatever. 



Hydrocyanic acid Gas. — Of the several gases experimented with by 

 Mr. Coquillett, of which full accounts are given in my annual reports as 

 Entomologist for 1887 and l."^88, the one named has given much the 

 best results. 



A number of methods of generating this gas have been devised, of 

 which the most satisfactory is now known as the " dry-gas process." 



The necessity of drying the gas was very evident from the first, for 

 it was found that the injury to foliage was very serious when the gases 

 were charged with any considerable amount of aqueous vapor. In the 

 dry-gas process the cyanide is dissolved by boiling in water for a few 

 minutes, using I gallon of water for each 5 pounds of cyanide. To 

 generate the gas, sulphuric acid is caused to flow upon the cyanide 

 solution in a tine stream, causing the gas to be rapidly given oft' in the 

 form of a whitish fog. The moisture is taken up by passing the gas 

 through sulphuric acid, which by reason of the water taken up becomes 

 diluted, but may still be employed to generate fresh quantities of gas. 



The gas is confined to the trees under treatment by means of a suit- 

 able canvas tent or fumigator, of which a number of styles have been 

 patented. They are constructed so as to be lowered over the tree from 

 above or to inclose it from the sides. Full details for the construction 

 of these tents, together with figures, are given in the reports cited 

 above, to which the reader is referred, also for a detailed account of 

 the use of various gases. 



INSECTICIDE APPARATUS.— The application of insecticides to fruit 

 or forest trees may be successfully accomplished by the use of the same 

 devices employed in the case of low-growing plants, except that more 

 force will be required as a rule, and hence larger and stronger machinery. 

 The treatment of young trees or application to the lower part of the 

 trunk or to the base or roots of larger ones may easily be effected by 

 hand, but in the case of the branches and foliage of large trees other 

 means must be employed. 



As has been already indicated, the principal insecticides are now 

 used in the liquid form, and particularly in the case of work against 

 the insect enemies of forest trees will this method prove the only prac- 

 ticable one. The use of insecticides in the form of powders will occasion- 

 ally be desirable, however, and hence the treatment of the second part 

 of the subject may be discussed under (1) devices for applying pow- 

 ders and (2) devices for applying liquids. 



Devices for Applying Powders. — Poicder Blotcers. — The appli 

 cation of powders to trees may be successfully accomplished by the 

 use of long-discharge-tube power-bellows. 



