Horticulture Principles and Practices

8.11.1 CLASSIFICATION BASED ON KILLING ACTION
Chemicals used to control insect pests vary in the way they kill, which thus provides a
basis for classifying insecticides. This method of classification is outmoded. Since insects
differ in morphology and feeding habits, for example, it is critical that the insecticide
attack the pest where it is most vulnerable. The various action modes under which insecticides
may be classified are as follows:
1. Contact action. Insecticides that kill by contact action are also called contact
poisons. They are effective when sprayed directly onto the pests or when the
pests come into contact with poisons as they move on plant parts that have been
sprayed. Once in contact with the pest, contact poisons attack the respiratory
and nervous systems, with lethal consequences. Most insects succumb to contact
poisons (e.g., malathion). Insects that hide on the undersides of leaves are hard to
hit directly by contact poisons.
2. Stomach action. Stomach poisons must be ingested by the pest to be effective.
As such, chewing insects (e.g., grasshoppers, beetles, and caterpillars) are
effectively controlled by this class of poisons. Once ingested, the poison
(e.g., rotenone) is absorbed through the digestive tract.
3. Systemic action. Systemic insecticides permeate the entire plant so that any
insects that suck or chew are exposed to the poisons. They may be applied as
foliar sprays or directly to the soil to be absorbed by roots. Insects cannot hide
from this chemical since once they feed (whether by sucking or chewing) they
ingest the toxin. The caution to observe with systemic poisons is that when
applied to food crops, the produce must not be eaten until the toxin (e.g., orthene)
has broken down to a safe level.
4. Fumigation. Fumigants are volatile chemicals that enter the target pest through its
respiratory system. They are effective when used in closed systems such as
storage houses and greenhouses. The soil can also be fumigated to control
soilborne diseases such as root-knot nematodes. Though gaseous, fumigants have
contact action. The fine particles settle on the body of the insect before entering
through the pores.
5. Repellent action. Most insecticides are designed to kill pests. However, some
chemicals, called repellants (e.g., Bordeaux mixture), repel insects (e.g., leaf
hopper and potato flea beetle) from plants without any killing action.
6. Attractant action. Females of many insect species secrete certain chemicals
called pheromones that attract male partners. Scientists have successfully synthesized
these chemicals for use in luring male insects to traps, where they are
caught and destroyed. The Japanese beetle moth and gypsy moth are easily baited
by the use of pheromones. By baiting males and destroying them, most females
are left unfertilized, thus reducing the population of the insects.
7. Suffocation. Scale insects are widely controlled by spraying oils that plug the
breathing holes in their bodies and suffocate them.
Active Ingredient (A.I.)
The amount of actual
pesticide in a formulation
that is toxic or inhibiting to
the pest.
8.11.2 CLASSIFICATION BASED ON CHEMISTRY
OF ACTIVE INGREDIENT
Modern classification of insecticides is based on chemical composition, since many
modern insecticides have both contact and stomach actions. The two broad classes of
insecticides are based on the chemistry of the active ingredient (the compound responsible
for the killing action).
Inorganic Compounds (Inorganics)
Insecticides made up of inorganic compounds or minerals are becoming increasingly less
common. They are usually designed to kill by stomach action and include compounds
such as arsenic (lead arsenate or calcium arsenate), sulfur, and fluorine.
262 Chapter 8 Principles and Methods of Disease and Pest Control