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University
of Rhode Island GreenShare Factsheets
Least
Toxic Pesticides
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 practice.
Pyrethrins 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.
At present, 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.
Properties: Unlike 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.
Disadvantages:
- 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.
- Bt-based 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.
Various oils 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 normal metabolism.
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.
Vegetable oils 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.
Soaps can 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.
Adapted from the Cornell
Cooperative Extension and the Colorado State University Extension,
1999
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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