Horticulture Principles and Practices

Organic Compounds (Organics)
Organic insecticides may be natural or synthetic.
Natural (Botanicals) Botanicals are products from plants that have insecticidal effects.
Many plant species produce organic compounds that are toxic to insect pests that
feed on them. Plant organic substances are usually safe and nontoxic to humans. An
example of a botanical is pyrethrum, which is obtained from chrysanthemum. Another
organic substance is nicotine, which is obtained from tobacco plants and is an addictive
substance found in cigarettes. Other botanicals are rotenone, ryania, and sabadilla. Many
organic compounds act as stomach or contact poisons.
Synthetic Organic Compounds Synthetic organic chemicals are artificially compounded
and are effective against a wide variety of insects and pests. On the basis of the
active ingredients in chemicals, several classes of synthetic organic insecticides are
identified:
1. Organochlorines (chlorinated hydrocarbons). Organochlorines are most readily
associated with dicarbo-, dihydrotetrachloride (DDT), which is one of the earliest
and most successful to be developed. It has wide application, being effective
against horticultural field pests, mosquitoes, fleas, and flies. It has a long residual
effect, working long after initial application. Organochlorines are not readily
biodegradable, which contributes to their rapid buildup in the environment in the
soil and water, as well as in the tissues of plants and animals and their products.
They are thus not only toxic to humans through their action on direct contact but
also through the ingestion of food that has been contaminated, such as dairy and
meat products (i.e., through a cow eating contaminated feed) and fish from
contaminated waters. Consequently, DDT (as well as its close relatives) has been
banned in many parts of the world. Other organochlorines are chlordane, lindane,
methoxychlor, heptachlor, and aldin.
2. Organophosphates. Unlike chlorinated hydrocarbons, organophosphates (or
organic phosphates) have shorter residual action (breakdown within thirty days)
and are more readily biodegradable. As such, they do not build up in the environment.
However, certain types (e.g., parathion) are extremely toxic to humans.
Organophosphates are generally effective insecticides. Malathion is less toxic and
widely used as a horticultural spray. Other organophosphates include diazinon,
phorate, dameton, and chlorpyrifos.
3. Carbamates. Carbamates are relatively safer to use than those previously
described. They have somewhat low mammalian toxicity and short residual
action (breakdown within seven days). They are effective against sucking and
chewing insects. One of the earliest and most successful was carbaryl (trade
named Sevin). Others are carbofuran, aldicarb, and propoxur.
4. Pyrethroids. Pyrethroids are synthetic equivalents of natural pyrethrins found in
species such as chrysanthemum. They are less toxic to humans and effective against a
broad spectrum of insects.
Fumigants Fumigants act in the gaseous state and are best used in closed environments
(e.g., as storage pesticides) or injected into the soil. One of the most common types
is methyl bromide, an odorless and colorless gas that is highly toxic to humans but is
used widely to fumigate stored vegetables, seeds, fruits, and grains. It is also used to
chemically sterilize soil mixes for use in greenhouses. Malathion is one of the most
widely used fumigants of stored grain.
Spray Oils Spray oils are obtained by specially distilling and refining crude oils. They
are used to combat scale insects and mites in orchard plants and ornamentals. A common
form is called dormant spray.
8.11 Insecticides and Their Use 263