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Introduction
Plants have
evolved for over 400 million years and to defend themselves from insect
attack they have developed protection mechanisms such as repellents and even
insecticidal effects. The oldest known pest control method is human
sacrifice, but since it is not effective, or maybe because of the lack of
volunteers, dusts began being used and also plant extracts which are
mentioned even in the Bible. Massive use of these insecticides has had a
long and difficult road because the earliest data gathering done by
researchers among farmers and natives revealed a lot of practices based on
superstition, which, when tested by scientific methods. were not shown to
be effective. After the Second World War the few plant and plant extracts
that had shown promising effects, and were of widespread use, were replaced
by synthetic insecticides. When synthetic insecticides appeared in the
1940’s some people thought that botanical insecticides would disappear
forever but problems like environmental contamination, residues in food and
feed and pest resistance brought them back to the fore. There is no doubt
botanical insecticides are an interesting alternative to insect pest
control, and on the other hand only a few of the more than 250,000 plant
species on our planet have been properly evaluated for this purpose. This
means the potential for the future may be huge. In fact, plants like neem (Azadirachta
indica J., Meliaceae), have shown excellent results and there already
are commercial products in the market made from it. But one should not think
success is at hand and botanical insecticides will replace all synthetic
products. They are only alternatives that may be used in Integrated Pest
Management programs and they should be used together with other available
control measures.
Brief historical review
Use of plant
extracts and powdered plant parts as insecticides goes back at least as far
as the Roman Empire. For instance, there are reports that in 400 B.C. during
Persian king Xerxes’ reign, the delousing procedure for children was with a
powder obtained from the dry flowers of a plant known as pyrethrum (Tanacetum
cinerariaefolium, Compositae). The first botanical insecticide, used as
such, dates back to the XVII Century when it was shown that nicotine,
obtained from tobacco leaves, would kill plum beetles. Around 1850 a new
plant insecticide known as rotenone was introduced. It is obtained from the
roots of plants called timbó. Up to that time this plant was used for
fishing purposes only as natives had known for a long time that throwing
root pieces to the water caused fish to start floating a few minutes later,
making them very easy to catch. Later on plants with irritating properties
like incense and sabadilla were used: extracts from the latter plant were
used as decongestants. These plants did not kill insects directly but it was
said that they “scared them off.” More recently other plants used are quasia
(Quaisa amara, Simaroubaceae), neem or Margosa (A. indica)
mentioned above, which besides giving excellent results for insect control
are also a source of compounds used against cancer.
In México and
several Central American countries even today it is common practice to treat
pests with plants known for their insecticidal properties as far back as the
era of the Aztecs and Mayans. A case in point is the use of a mixture of
corn and beans with chili peppers (Capsicum frutescens; Solanaceae),
rue (Ruta graveolens; Rutaceae) or garlic (Allium cepa;
Alliaceae).
At the present
time there are a number of botanical insecticides being marketed which are
extracted from neem, grapefruit seeds and garlic, among other plants.
Besides, there are the synthetic copies of natural active ingredients like
neonicotinoids where imidacloprid stands out. Lastly, it is worth mentioning
that this is a field where new discoveries are made every day. One example
is the development of a new kind of insecticide obtained from the plant of
Chilean Andes origin known as Calceolaria andina (Scrophulariaceae).
Nature of the compounds
Plants are
like natural laboratories where a great number of chemicals are
biosynthesized and in fact they may be considered the most important source
of chemical compounds there are. Primary plant metabolism synthesizes
essential compounds which are present in all plant species. On the other
hand, the end products of secondary metabolism are neither essential nor
universally present in all plants. Common among these metabolites are
componds with protective action against insects, such as alkaloids, non-proteic
amino acids, steroids, phenols, flavonoids, glycosids, glucosinolates,
quinones, tanins and terpenoids. Some people support the idea that these
compounds do not have a clearly known role, and have even classified them as
“metabolic garbage.” However, other researchers point out that those
products are important chemical signals in the ecosystem. There is a great
deal of variation as far as the concentration of secondary compounds that
individuals of a population can express. Besides, there is neither a pattern
of maximum yield nor particular storage organs for secondary metabolites,
even though it is common that the highest concentrations of these kinds of
compounds are found in flowers and seeds.
By definition,
an insecticide is a substance that performs a biocidal action on insects due
to the nature of its chemical structure. For example, if we kill an insect
for our insect museum using a cyanide killing jar we can say that cyanide
has an insecticidal effect. However, we cannot say the same about water if
raindrops kill aphids, since that mortality cannot be attributed to water’s
chemical structure or properties.
Most plant
species used for plant protection exhibit an insect deterrent rather than
insecticidal effect. This means that in some way those compounds inhibit
normal development in insects. This can be done in different ways that will
be briefly described below.
Insect Growth Regulators
The growth
regulator effect may be seen in several ways. First are those molecules
inhibiting metamorphosis, in other words, compounds preventing metamorphosis
from taking place at the right time. Other compounds force the insect to go
through an early metamorphosis, so that development takes place at a time
not favorable for the insect. Lastly, it has been observed that certain
molecules may alter hormones related to this function so that insects suffer
malformations, are sterile, or die. In fact, one of the most common
entomological anecdotes tells that an experiment was being carried out
simultaneously in the United States and Hungary. In the latter country the
insects went through an additional immature stage before molting to adults.
All methods were reviewed but no difference was found until the paper towels
used to provide water for the insects were analyzed and then it was found
that in the two countries the paper towels were made from different trees.
In Europe they used Abies balsamea, a coniferous tree, very common in
Hungary, which contains a plant hormone which induced the additional molt. A
more practical example is the case of basil plant (Ocimum basilicum)
from which the compound juvocineme II is extracted and which later on served
as the model for synthesis of piriproxifen and fenoxicarb.
Feeding deterrents
Feeding
deterrence is perhaps the most studied mode of action for plant derivatives
used for insect pest management. Strictly speaking, a feeding deterrent is a
compound that once probed by the insect, causes it to stop feeding and
starve to death. Many compounds showing this activity are terpenes and most
have been isolated from medicinal plants native to Africa and India.
Repellents
The use of
plants as repellents is very old but has not received the necessary
attention for proper development. Use has been achieved with compounds
having bad odor or irritant effects as is the case for garlic and hot
peppers. A clear example can be seen in uses by Guatemalan and Costa Rican
natives who used to "paint" containers or sprinkle them with garlic powder
before using them to store corn or beans to prevent the grain from being
damaged by weevils (weevil punctures) and also to shoo away rodents. Lastly,
it is not uncommon to hear of home recipes mentioning the use of fennel (Foniculum
vulgare), rue (Ruta graveolens) and eucalyptus (Eucaliptus
globolus) among other aromatic plants to repel clothes moths.
Confusants
Chemical
compounds in a given plant are unequivocal signs for insects to find their
food source. In fact, there are cases like the monarch butterfly, which
feeds on plants that are highly poisonous to other organisms, but that are
found by the butterfly because of the poisonous substance. One way to use
this characteristic in IPM has been setting up traps and either spraying
them with infusions of certain plants that are more attractive to the insect
or of the same plant but from other areas so that the insect will have so
many sources of stimuli that they will not be able to find the plant we wish
to protect. Another option is to set up traps containing watery extracts of
the plant so that insects will “land” in the traps and not on the crop.
So, thinking
about the above mentioned information we should consider all those compounds
which we know have insect deterrents and have preventive rather than
curative effects.
Advantages
and Disadvantages of Botanical Insecticides
Advantages
1. Plants
producing the above mentioned compounds are known by the farmer because most
of the time they grow in the same general area.
2. Often these
plants also have other uses like household insect repellents or are plants
with medicinal applications.
3. The rapid
degradation of the active product may be convenient as it reduces the risk
of residues on food.
4. Some of
these products may be used shortly before harvesting.
5. Many of
these products act very quickly inhibiting insect feeding even though long
term they do (will?) not cause insect death.
6. Since most
of these products have a stomach action and are rapidly decomposed they may
be more selective to insect pests and less aggressive with natural enemies.
7. Most of
these compounds are not phytotoxic.
8. Resistance
to these compounds is not developed as quickly as with synthetic
insecticides.
Disadvantages
1. Most of
these products are not truly insecticides since many are merely insect
deterrents and their effect is slow.
2. They are
rapidly degraded by UV light so that their residual action is short.
3. Not all
plant insecticides are less toxic to other animals than the synthetic ones.
4. They are
not necessarily available season long.
5. Most of
them have no established residue tolerances.
6. There are
no legal registrations establishing their use.
7. Not all
recommendations followed by growers have been scientifically verified.
Which plants to use?
There are a
lot of publications with lists of plants with insecticidal properties. For
example, in 1950, Heal et al. reported approximately 2,500
plants in 247 families with some sort of toxic property against insects. But
to use them it is not enough that the plant be considered as promising or
even with proven insecticidal properties. It is also necessary to conduct an
analysis of the risks to the environment and to human health. Another
example is that it may not be appropriate to recommend the use of plants in
danger of extinction, that are difficult to find, or if their use results in
important alterations to their population density in natural conditions. For
example, if tomorrow it is discovered that baoba wood has a compound which
may be an insecticide, that does not mean we are going to cut the baoba
trees. For these reasons and with the purpose of obtaining maximum benefit
from a plant with insecticidal properties, but without degrading the
ecosystem, the properties that an ideal insecticidal plant should have are
listed below (Silva, 2001):
1. It should
be a perennial.
2. It should
have a wide distribution and be present in large numbers in nature.
Otherwise it should be possible to be grown by agricultural procedures.
3. The plant
parts to be used should be removable: leaves, flowers or fruit.
4. Harvesting
should not mean destruction of the plant (avoid the use of roots or bark).
5. The plants should
require small space, reduced management and little water and fertilization.
6. The plant should have
additional uses (for example medicinal use).
7. The plant should not
otherwise have a high economic value.
8. The active
ingredient should be effective at low rates.
If it is natural, is it not
poisonous?
It would be a
big mistake to consider products of plant origin, and this includes
botanical insecticides, harmless merely because they are natural. There are
large numbers of plant products which are highly toxic; remember that
ancient history mentions that Socrates was sentenced to death by drinking a
diluted extract of hemlock (Cicuta spp.). Schmutz and Breazeale
(1986), in their book “Plants that Poison” point out around 120 species of
plants containing substances that are toxic to humans, including plants as
common as almond trees, some bean species, garlic, strawberries and apples,
among others. So we must not forget that the toxic potential of a molecule
is the nature of its chemical structure and not its origin. As Paracelsus
wrote in 1564 “the difference between something that kills and something
that cures is the rate.”
Laboratory Copies
Plants have
been used not only directly as insecticides but also as a source for a
series of synthetic insecticides developed in the laboratory. From the pest
control point of view. one problem is that botanical insecticides disappear
soon after application because they are decomposed by the action of light
and temperature so that they remain on the plant only briefly (typically no
more than 24 hours). This has prompted industry to modify molecules found in
plants so that they are more toxic and more persistent.. These improvements
offer the advantage that the crop does not have to be sprayed virtually
every day and that there will be less pressure to harvest the insecticidal
plants into extinction. A clear example of this may be found in two
insecticide families of widespread use in agriculture, medicine and urban
environments: pyrethroids and carbamates which are synthetic derivatives of
molecules isolated from plants like pyrethrum (T. cinerariaefolium)
and Calabar beans (Physostigma venenosum), respectively.
Description of Some Compounds
Rotenone
Rotenone is a
flavonoid extracted from the roots of two plants: Derris spp. (Fabaceae)
and Lonchocarpus spp. (Fabaceae). The first one gives up to 13% of
rotenone while the second only about 5%. Derris spp. is a native to
Eastern tropics, while Lonchocarpus spp. is native to western
hemisphere. Rotenone is a contact and ingestion compound, which acts as a
repellent too. Its mode of action involves the inhibition of the electron
transport at the mitochondrial level, thus blocking phosphorylation of ADP
to ATPthereby inhibiting insect metabolism. Insects poisoned with Rotenone
show the following symptoms of intoxication: a drop in oxygen consumption,
respiratory depression and ataxia leading to convulsions and finally to
paralysis and death by respiratory arrest.
Figure 1
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