INSECTICIDES AXD FUNGICIDES 293
function in a normal way. Non-volatile insecticides in the
form of powdered solids may function by sticking to certain
body secretions, then being absorbed into the tissues. As
examples of this class, borax and sodium fluoride are frequently
used to exterminate cockroaches. The powder sticks to the body
of the insect and is partly absorbed but it also acts as a stomach
poison because some of it is usually licked off and swallowed
by the animal.
The vapor of white hellebore is insufficient to kill insects
but Shafer has noted that rose slugs which come into contact
with this insecticide gradually become numbed and fall from
the leaves. This occurred even in cases where none of the
insecticide had been eaten. It is concluded that the numbing
effect is due to slight dissolving of the powder and surface
absorption by the excretions, little if any of the insecticide passing
through the cuticular covering, and that the cause of the final
death of the insect is due more to drying and starving than to
any other reason.
Finally, the natural cells of some insects contain enzymes,
the normal functioning of which is of the greatest importance to
the well-being of the insect. The interference of the various
insecticides with the activity of these enzymatic bodies is known
to be serious and this may be the cause of the death of the insect
in some cases.
Preparation of Insecticides.—The internal poisons are usually
prepared in considerable quantities and their preparation should
be under chemical control. The contact and fungicide poisons
are freshly prepared by the sprayer and their efficiency depends
upon the composition and proportions of the ingredients. Ar-
senic has been so universally used as an active internal poison for
insects that the determination of this element is highly important.
Free arsenous acid in solution has a destructive action on foliage,
therefore it is necessary also to limit the per cent of arsenic in
this form. The maximum quantity which is safe for foliage
varies from 4 to 6 per cent.
Mixing of Sprays.—The question of combining insecticides
and fungicides for the control of orchard pests is important
from the standpoint of saving time and money as well as from
that of increasing the efficiency of the spray. Choosing the