CHAPTER XIV
INSECTICIDES AND FUNGICIDES
The large number of insect and fungus pests with which the
economic entomologist and the horticulturist have had to
contend in recent years has caused a renewed search for methods
for more efficient control. The insecticides used for this purpose
belong to one of two classes, depending upon whether they are
for external or internal action. Paris green and London purple
are examples of those of internal application, while lime-sulphur
mixture and kerosene emulsion are examples of those designed
to kill by contact. Bordeaux mixture is a well known remedy
for fungus pests.
Character of Insecticide as Related to Insect Anatomy.—There
is a close relation between the general character of the insecticide
sprays to be applied and that of the mouth parts of insects.
Generally speaking, insects secure their food either by biting out
and swallowing plant particles or by sucking juices from interior
portions of the plant. Those of the biting kind have jaws and
also certain accessory parts which enable the insect to cut and
pass on the small parts of food to the digestive organs. Most
sucking insects have mouth parts of long bristle-like structure.
These are inclosed in a tube and the bristles and beak together
constitute a sucking apparatus for the extraction of the plant
juices. It is possible to kill both sucking and biting insects by
poisoning the air with hydrocyanic acid or other poisonous gases,
as well as by poisons that are to be eaten by the insect.
Action of Contact Insecticides.—Considerable attention has
been given to the method by which contact insecticides kill.
Shafer1 found that in the case of certain volatile insecticides,
such as gasoline, carbon disulphide or chloroform, the fatty
membranes absorb some of the vapor, which renders them
less permeable to oxygen. The cells thus gradually cease to
1 Mich. Exp. Sta. Tech. Bull, 21 (1915).