It is important
to remember that the following "least toxic" pesticides are pesticides
and should be treated as such. Many botanically-derived insecticides
have gained favor in recent years, due in part to the perception
that, because they originate from plant material, they are more
safe or "natural." These pesticides are often used for growing crops
organically, according to guidelines set forth by certification
programs. However, it is important to be aware that they are pesticides,
and that they fall under the same state and federal regulations
as synthetic pesticides. Pesticides may only be used as indicated
on the label. It is important to realize that produce grown using
these less toxic alternatives is not "pesticide free." These pesticides
also leave residues, can be disruptive to natural enemies and may
be toxic to humans. The same precautions should be taken in the
use of "less toxic pesticides" as with synthetic pesticides.
Relying heavily on the use of any pesticide is an un-ecological
are botanical insecticides made from the dried flower heads of a
relative of the common chrysanthemum. Pyrethrins affect the insect
on contact, creating disturbances in the nervous system which eventually
result in convulsions and death. Low doses, however, often cause
temporary paralysis from which the insect may recover. Pyrethrins
are only slightly toxic to man and not hazardous to birds and other
wildlife. They have almost no residual activity, breaking down rapidly
from exposure to sunlight, air or moisture. Pyrethrins are effective
as broad-spectrum insecticides to control pests such as aphids,
whiteflies, stinkbugs and mites.
Neem is a botanical
insecticide derived from a tree native to the Middle East, where
it has been used for centuries to control insects. One of the most
desirable properties of Neem is its low degree of toxicity‹it is
considered almost nontoxic to humans and animals, and is completely
biodegradable. It is used as an ingredient in toothpaste, cosmetics,
pharmaceuticals and other products. Neem products can be used to
manage pests on vegetables, fruit, ornamentals, and lawns and can
be found at many home garden centers. Neem has been advertised as
effective or moderately effective for more than 200 pest insect
species and some of the plant diseases, including certain mildews
and rusts. Effectiveness, however, is variable and test results
have been inconclusive in many cases. Because the products are relatively
new, it is not yet clear how effectively the products control each
of these pests. Generally, chewing insects are affected more than
sucking insects. Insects that undergo complete metamorphosis are
also generally affected more than those which do not undergo metamorphosis.
Neem often works more slowly than other pesticides, and effectiveness
is reduced in cooler climates. Neem does not persist in the environment
and should be reapplied after rain. Neem has little effect when
applied directly on insects, except in the oil formulations; most
insects are affected only after consuming foliage that has been
treated. Neem is most effective as a foliar spray applied periodically
to new flushes of growth. On some species of plants Neem also works
as a systemic pesticide, absorbed into the plant and carried throughout
the tissues, ingested by insects when they feed on the plant. This
may make it effective against certain foliage-feeders that cannot
be reached with spray applications, such as leafminers and thrips.
Bt is the only "microbial insecticide" in widespread use. A naturally
occurring bacterial disease of insects, Bt is an insecticide with
unusual properties which make it useful for pest control in certain
situations. Bt insecticides are most commonly used against some
leaf- and needle-feeding caterpillars. Strains have recently been
produced which affect certain fly larvae, such as mosquitoes, as
well as the larvae of leaf beetles. Insecticidal activity is specific
to each Bt strain. Bt is considered safe to humans and wildlife.
Some formulations can be used on food crops.
typical nerve-poison insecticides, Bt acts by producing proteins
(delta-endotoxin, the "toxic crystal") which react with the cells
of the gut lining of susceptible insects. These Bt proteins paralyze
the digestive system, and the infected insect stops feeding within
hours. Bt-affected insects generally die from starvation, which
can take several days.
Bt is susceptible to degradation by sunlight; most formulations
persist on foliage less than a week following application. Some
of the newer strains developed for leaf beetle control become ineffective
within 24 hours.
- The highly
specific activity of Bt insecticides might limit their use on crops
where problems with several pests occur, including nonsusceptible
insects (aphids, grasshoppers, etc.).
- As strictly
a stomach poison insecticide, Bt must be eaten to be effective,
and application coverage must be thorough. This further limits its
usefulness against pests that are susceptible to Bt but which rarely
have an opportunity to eat it in field use, such as codling moth
or corn earworm which tunnel into plants.
products tend to have a shorter shelf life than other insecticides.
Manufacturers generally indicate reduced effectiveness after two
to three years of storage. Liquid formulations are more perishable
than dry formulations. Shelf life is greatest when storage conditions
are cool, dry and out of direct sunlight.
have been used for centuries to control insect and mite pests. Oils
remain an important tool to manage certain pest problems (e.g.,
scales, aphids, mites) on fruit trees, shade trees and woody ornamental
plants. Several recently developed oils extend this usefulness to
flowers, vegetables and other herbaceous plants. Oils also can control
some plant diseases, such as powdery mildew. Horticultural oils
block the air holes (spiracles) through which insects breathe, causing
death by asphyxiation. In some cases, oils also may act as poisons,
interacting with the fatty acids of the insect and interfering with
Oils pose few
risks to people or to most desirable species, including beneficial
natural enemies of insect pests. However, a mask or respirator should
be worn when applying horticultural oils. Oils quickly dissipate
through evaporation, leaving little residue. Oils also are easy
to apply with existing spray equipment and can be mixed with many
other pesticides to extend their performance.
The main limitation
of spray oils is their potential to cause plant injury (phytotoxicity)
in some situations. Oils also can stain some surfaces, particularly
dark-colored house paints. Essentially all commercially available
horticultural oils are refined petroleum products. Impurities in
the oil that are associated with plant injury, such as aromatic
compounds and compounds containing sulfur, nitrogen or oxygen are
removed. Filtration, distillation and dewaxing complete the production
of the finished base oil.
also can be used as insecticides, although the type of oil can greatly
affect its activity. Cottonseed oil is generally considered the
most insecticidal of the vegetable oils. Soybean oil, the most commonly
available vegetable oil used in cooking, has also provided fair
to good control.
be used to control a wide range of plant pests. Small, soft-bodied
arthropods such as aphids, mealybugs, psyllids and spider mites
are most susceptible to soaps. In most cases, control results from
disruption of the cell membranes of the insect. Soaps and detergents
may also remove the protective waxes that cover the insect, causing
death through excess loss of water. Insecticidal soaps act strictly
as contact insecticides, with no residual effect. To be effective,
sprays must be applied directly to, and thoroughly cover, the insect.
Insecticidal soaps are considered selective insecticides because
of their minimal adverse effects on other organisms. Lady beetles,
green lacewings, pollinating bees and most other beneficial insects
are not susceptible to soap sprays, for example. Predatory mites,
however, which are often important in the control of spider mites,
are vulnerable to insecticidal soaps.
One of the
most serious potential drawbacks to the use of soap-detergent sprays
is their phytotoxicity. Certain plants are sensitive to these sprays
and may be seriously injured. The risk of plant damage is greater
with homemade preparations of household soaps or detergents. When
in doubt, test soap-detergent sprays for phytotoxicity problems
on a small area a day or two before an extensive area is treated.
Plant injury can be reduced by using sprays that are diluted more
than the 2 to 3 percent suggested on label instructions. To reduce
leaf injury, wash plants within a couple of hours after the application.
Limiting the number of soap applications can also be important,
as leaf damage can accumulate with repeated exposure.
from the Cornell Cooperative Extension and the Colorado State University