46 
exhibit the effects of the wash or gas for some time. Little can be 
judged, ordinarily, of the ultimate results before two weeks, and it is 
often necessary to wait one or even two months to get final conclu- 
sions. In the case of liquid washes the slow progressive death of the 
scales is apparently due to the gradual penetration of the insecticide, 
and also to the softening and loosening of the scale itself, enabling 
subsequent weather conditions of moisture and cold to be more fatal. 
With such biting insects as caterpillars and slug worms, after treat- 
ment with arsenicals or other poisons death rapidly follows, the time 
being somewhat in proportion to the size of the larve and their 
natural vigor. Soft-bodied larve, such as the slug worms and very 
young larve of moths and beetles or other insects, are killed in a day 
or two. Large and strong larve sometimes survive the effect of 
poison for eight or ten days, and leaf-feeding beetles will often fly 
away and perish from the poison in their places of concealment. 
Many larve or other forms of leaf-feeding insects, after taking one 
or two meals of poisoned foliage, will remain in a semitorpid and 
diseased condition on the plants for several days before they finally 
succumb. The protection to the plant, however, is just as great as 
though they had died immediately, but misapprehension may often 
arise and the poison may be deemed to have been of no service. 
The complete extermination of insects on plants is often a very 
difficult, if not an impossible undertaking. This is especially true of 
scale insects. In California even, where the work against. these 
enemies of fruits has been most thorough and successful, experience 
has shown that the best that can be done is a practical elimination of 
the scale for the time being, and it is often necessary to repeat the 
treatment every year or two. In exceptional cases once in three years 
suffices. With leaf-feeding insects it is often possible to effect com- 
plete extermination with the use of arsenical poisons. Such sucking 
insects as aphides may also be completely exterminated. But in gen- 
eral all applications or methods of treatment must be recognized, 
more or less, as a continuous charge on the crop, as much so as are 
the ordinary cultural operations. 
CONTROL OF INSECTS BY CULTURAL METHODS. 
It is much easier to ward off an attack of insects or to make condi- 
tions unfavorable for their multiplication than to destroy them after 
they are once in possession; and in controlling them, methods and 
systems of farm and orchard culture have long been recognized as of 
the greatest value, more so even than the employment of insecticides, 
which, in most cases, can only stop an injury already begun. Insects 
thrive on neglect, multiply best in land seldom or never cultivated, 
and winter over in rubbish, prunings, or the undisturbed soil about 
their food plants, and become, under these conditions, more numerous 
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