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DIRECTIONS FOR SPRAYING. 
Thorough work in spraying must be done, or failure will result. 
To accomplish this, power sufficient to break up the liquid into a fine 
mist is essential. This makes it possible for the tree to be thoroughly 
and thinly wetted with the spray without waste, and the ideal appli- 
cation is to accomplish this without causing the liquid to collect in 
drops and fall from the tree. More of the spray is left on the leaves 
with a light spray than with a heavy application, which causes the 
globules to coalesce and a shower of drops to fall to the ground. To 
get a proper spray, it should be possible to produce a pressure of at 
least 75 pounds, or, with power outfits, of 125 to 150 pounds. 
Fruit trees of average size or, if apple, such as would produce 
10 or 15 bushels of fruit, will require from 3 to 7 gallons of spray 
to wet them thoroughly. For smaller trees, such as plum and cherry, 
1 gallon to the tree may be sufficient. In spraying orchard trees 
and other fruit trees it will often be found convenient, especially with 
a smaller apparatus, to spray on each side half of each tree in a 
row at a time, and finish on the return. 
A light rain will remove comparatively little of the poison, but 
a dashing rain may necessitate a renewal of the application. 
HYDROCYANIC-ACID GAS TREATMENT. 
The use of hydrocyanic-acid gas originated in southern California 
in work against citrus scale insects, and was perfected by a long 
period of experimentation by an agent of this Bureau, Mr. D. W. 
Coquillett. It is undoubtedly the most thorough method known of 
destroying scale insects and especially is it the best treatment for 
citrus trees, the abundance of foliage and nature of growth of 
which render thorough spraying difficult, but, on the other hand, 
enable the comparatively heavy tents employed in fumigation to be 
thrown or drawn over the trees rapidly without danger of breaking 
the limbs. One good gassing is usually the equivalent of two or 
three sprayings, the gas penetrating to every particle of the surface 
of the tree and often effecting an almost complete extermination, 
rendering another treatment unnecessary for two years or more. 
(See fig. 6.) 
The gas treatment is just as effective against scale insects on decidu- 
ous orchard fruit trees, as has been demonstrated by a good deal of 
work done in the East, notably in Maryland by Professor Johnson; 
but the difficulty and expense of the treatment as compared with the 
value of the crop protected makes it as a rule prohibitive in the case 
of deciduous fruits. This does not apply, however, to nursery stock, 
which may be brought together compactly and treated in mass in 
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