7 
For this class of insects the application of poisons, which penetrate 
little, if at all, into the plant cells, is of trifling value, and it is neces- 
sary to use substances which will act externally on the bodies of these 
insects as a caustic, or will smother or stifle them by closing their 
breathing pores, or will fill the air about them with poisonous fumes. 
Of value also as repellents are various deterrent or obnoxious sub- 
stances. 
Where it is not desirable to use poisons for biting insects some of 
the means just enumerated may often be employed. 
GROUPS SUBJECT TO SPECIAL TREATMENT. 
The two general groups outlined above comprise the species which 
live and feed upon the exterior of plants for some portion or all of 
their lives, and include the great majority of the injurious species. 
Certain insects, however, owing to peculiarities of habit, inaccessi- 
bility, or other causes, require special methods of treatment. Of 
these, two groups properly come within the scope of this bulletin: 
(1) Those working beneath the soil, or subterranean insects, such as 
the white grubs, root maggots, root aphides, etc.; and (2) insects 
affecting stored products, as various grain and flour pests. 
Three other groups, which include species requiring very diverse 
methods of treatment, and which are not considered in this bulletin, 
are (1) such internal feeders as wood, bark, and stem borers, leaf 
miners, and gall insects, and species living within fruits; (2) house- 
hold pests; and (3) animal parasites. 
The classification of insects outlined above, based on mode of nour- 
ishment, and indicating groups amenable to similar remedial treat- 
ment, simply stated, is as follows: 
I, External feeders: III. Subterranean insects. 
(a) Biting insects. IV. Insects affecting stored products. 
(6) Sucking insects. VY. Household pests. 
II. Interna! feeders. VI. Animal parasites, 
INSECTICIDES FOR EXTERNAL BITING INSECTS (FOOD POISONS). 
THE ARSENICALS. 
The arsenical compounds have supplanted practically all other 
substances for insects falling under this heading. Of these, Paris 
green is the best known and most generally employed, and probably 
from 2,000 to 3,000 tons of it are used for horticultural purposes 
every year. Arsenate of lead is a new arsenical coming into very 
general use, and arsenite of copper, a near ally of Paris green, is also 
increasingly employed. Arsenite of lime is usually a home prepara- 
tion, and London purple, the least uniform in composition of all the 
mixtures, israther going out of use. The powdered white arsenic or 
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