Azadirachta indica: One Tree's Arsenal Against Pests
Kirk Howatt
Colorado State University
Fort Collins, Colorado 80523
ABSTRACT
The tree Azadirachta indica is native to parts of South Asia
where it has been used for many things. Of primary interest to
research scientists is its activity as an insecticide. Many of
the tree's secondary metabolites have biological activity, but
azadirachtin is considered to be of the most ecological
importance. Studies have shown a wide spectrum of activity and
species affected. Research has increased in the past few years
as the desire for safe pest control methods increases and it
becomes apparent that this tree will be able to play a role in
integrated pest management systems.
INTRODUCTION
Azadirachta indica has been used for centuries as the country
store of developing nations. Earliest reference to it is in
Sanskrit writings that are over 4,000 years old (Larson, 1990).
Parts of this tree have been used for medicine, shade, building
materials, fuel, lubrication, and most of all as pesticides. It
is the use of this tree as an insecticide that now draws interest
from industrialized countries. It is seen as an environmentally
safe alternative to synthetic pesticides. To date over 195
species of insects are affected by this trees extracts at
concentrations ranging from 0.1 to 1,000 ppm, and insects that
have become resistant to synthetic pesticides are controllable
with these extracts (Lindquist et al., 1990; Menn, 1990).
THE NEEM TREE
Azadirachta indica, commonly referred to in many countries
as the neem tree, is a member of the Meliaceae family. This
broad-leaved evergreen can reach heights of 30 meters with a
trunk girth of 2.5 meters and live for over two centuries. Its
deep root system is well adapted to retrieving water and
nutrients from the soil profile, but this deep root system is
very sensitive to waterlogging. The neem tree thrives in hot,
dry climates where shade temperatures often reach 50 degrees
celsius and annual rainfall ranges from 400 to 1,200 millimeters.
The tree can withstand many environmental adversities including
drought and infertile, stony, shallow, or acidic soils. The neem
produces ellipsoidal drupes, that are about two centimeters in
length, borne on axillary clusters. These fruits contain kernels
that have high concentrations of secondary metabolites (National
Research Council, 1992). There is evidence, but no scientific
correlation, that trees grown in climates with lower rainfall
produce kernels with higher content of metabolites (Schmutterer,
1990a).
The neem tree is believed to have originated in Assam and
Burma of South Asia, but other reports suggest various areas of
Pakistan, Sri Lanka, Thailand, Malaysia, and Indonesia (National
Research Council, 1992). The tree also grows well in other
tropical and subtropical areas around the world (Verkerk et al.,
1993). This is very important to commercial neem extract
production so that a broad raw material base for industrial
refinement can be established. Neem trees have successfully been
established in Australia, Haiti, West Africa, the Dominican
Republic, Ecuador, Puerto Rico, the Virgin Islands, and in the
continental United States in Florida, California, Oklahoma, and
Arizona (Jacobson, 1990; Schmutterer, 1990a; Verkerk et al.,
1993). The trees growing in Arizona are part of a breeding and
selection program aimed at developing a variety that will be
frost tolerant to temperatures as low as 18 degrees below zero
celsius. Such a development would allow this tree to be
established in many more regions. The seed for this project was
obtained from natural tree populations growing in northern India
where the climate is cooler than most areas where neem grows
(Jacobson, 1990).
Cultivation of the neem tree is also an important
consideration as the tree is established in new regions. Very
little problems arise in vegetative propagation. Transplanting
seedlings, saplings, or root suckers achieves a high success rate
(National Research Council, 1992). Seeds are more desirable to
use when transporting a long distance for ease of packing,
however, minor problems have been observed when growing these
trees from seeds. It was found that dry or unripe seeds would
rot in soil. Large scale establishment of neem trees required
germination in sand, transplanting to clay pots after a month,
and then planting in the field when the seedlings reached 30 to
45 centimeters in height (Jacobson, 1990).
NEEM CHEMISTRY
The chemicals that have pesticidal activity can most
efficiently be extracted from neem seed kernels. Neem trees
begin their reproductive stage at about three to five years of
age but don't become fully reproductive until they are ten years
old. From this time on, the tree will yield an average of about
20.5 kilograms of fruit per year, with maximum production
reaching 50 kilograms per year (National Research Council, 1992).
Of the fruit yield, only about ten per cent is attributed to seed
kernels, and desired biologically active compounds comprise only
ten grams per kilogram of kernel weight. This means that an
adult neem tree will only produce about 20 grams of pesticidal
compounds in a season (Schmutterer, 1990b).
Many biologically active compounds can be extracted from
neem, including triterpenoids, phenolic compounds, carotenoids,
steroids, and ketones. The tetranortriterpenoid azadirachtin has
received the most attention as a pesticide because it is
relatively abundant in neem kernels and has shown biological
activity on a wide range of insects. Azadirachtin is actually a
mixture of seven isomeric compounds labeled as azadirachtin-A to
azadirachtin-G with azadirachtin-A being present in the highest
quantity and azadirachtin-E regarded as the most effective insect
growth regulator (Verkerk et al., 1993). Many other compounds
have been isolated that show antifeedant activity as well as
growth regulating activity on insects. Polar and non-polar
extractions yield about 24 compounds other than azadirachtin that
have at least some biological activity (Schmutterer, 1990b;
Jacobson, 1990). This cocktail of compounds significantly
reduces the chances of tolerance or resistance developing in any
of the affected organisms. However, only four of the compounds
in neem have been shown to be highly effective in their activity
as pesticides: azadirachtin, salannin, meliantriol, and nimbin
(Jacobson, 1990; National Research Council, 1992).
These compounds can be extracted by many methods. Leaching
with water is the oldest method and is still used by some firms
to selectively extract azadirachtin. On the other hand, most
companies are using more non-polar solvents to obtain a more
varied mixture of chemicals. Hexane, pentane, ethanol, methanol,
esters, and dichloromethane are used in extractions as well as
mixtures of these solvents with water (Lee et al., 1988; National
Research Council, 1992; Schmutterer, 1990b). Once extracted,
several separation techniques are often incorporated to isolate
compounds. For instance, in the isolation and identification of
7-deacetyl-17á-hydroxyazadiradione, researchers used insect
bioassays to guide reverse phase HPLC fractionation, IR spectrum
analysis, 13C NMR and 1H NMR spectrum analysis, and mass spectrum
to determine the structure of the active compound (Lee et al.,
1988).
By using laboratory techniques, it is possible to closely
mimic azadirachtin as it has been identified from the neem tree.
Anderson et al. (1990) and Kolb et al. (1991) each describe
processes in which they synthesized roughly half of separate ends
of the azadirachtin molecule. These subunits form a compound
that has similar but less activity than the natural molecule.
The activity of synthetic azadirachtin compares close enough to
natural products to verify that azadirachtin is the primary toxic
compound in neem (Verkerk et al., 1993). Because of the great
number of reactions involved in each process, synthetic
azadirachtin will be very costly to produce. For this reason,
companies developing azadirachtin as a commercial pesticide are
working with natural products. W. R. Grace & Co., NPI, and Safer
Ltd. are all trying to produce low cost, yet effective, neem-
based pesticides (Isman et al., 1990; Walter et al., 1990; Wood,
1990). Research is discovering that initial by-products of
azadirachtin extraction have significant efficacy on pests also.
Neem seed oils have detrimental effects on viruses, mites, and
early larval stages of some insects, while the solid seed residue
has enough residual chemical content to have activity on
soilborne fungal pathogens and plant parasitic nematodes (Larew,
1990; Locke, 1990; Schmutterer, 1990b).
NEEM EFFECTS
The mode of action of neem extracts is not understood very
well. It is quite possible that the different chemicals or
different ratios of chemicals found in neem trees have varied
effects on insects. There is also evidence given in many
research studies, a few of which will be cited later, that insect
species react quite differently to compounds from the neem tree.
More research has been conducted to find the primary mode of
action of azadirachtin than of any other chemical in the neem
tree. This is because of interest in it as a product for
commercial use. Azadirachtin alone probably has several modes
and sites of action (Koul, 1991). Primary of which is an
interference with the neuroendocrine system in insects which
controls the synthesis of ecdysone and juvenile hormone. It has
been indicated by Schmutterer (1988) that interference involves
the inhibition of the release of these hormones. Indication of
this was an accumulation of large quantities of stainable
neurosecretory material in the corpora cardiaca of Locusta
migratoria. In this insect, azadirachtin regulated juvenile
hormone titer to prevent vitellogenin production in females,
causing sterility. This and other research has convinced many
people that azadirachtin definitely has antihormonal activity.
However, other evidence indicating that control of hormone
concentrations is controlled indirectly leads to the conclusion
that azadirachtin is not a true antihormone.
The effect of azadirachtin as an antihormone on juvenile
hormone titer was also investigated in the variegated cutworm by
Koul et al. (1991). Their goal was to either eliminate or
reproduce the effect of azadirachtin on metamorphic abnormalities
by artificially raising the concentrations of juvenile hormones I
and II or BEPAT, a juvenile hormone esterase inhibitor. They
were unable to achieve any desired outcome, but ligation
experiments did indicate that the region of activity was in the
head capsule. The possibility was proposed that an inhibition of
the synthesis of a neurosecretory protein could alter titer
levels (Koul et al., 1991). So while azadirachtin has activity
on hormone levels, it may be an indirect relationship indicating
that azadirachtin is not a true antihormone. Evidence at this
time is not conclusive on the matter of primary mode and site of
action, and researchers involved admit that much more
investigation is necessary to unwind the mystery (Schmutterer,
1988; Schmutterer, 1990a).
Other research has indicated a more direct role in the
inhibition of molting. Direct cytotoxic effects on imaginal
discs and epidermal cells result in primary lesions that prevent
molting (Koul et al., 1991). Azadirachtin has also been proved
to be a chitin synthesis inhibitor, but the role of this
inhibition as the primary mode of action has not been
investigated (Schmutterer, 1988).
Neem extracts have many effects on insects. The antifeedant
and growth regulating effects are the most valued in pest
management as these are the most intense effects on the widest
range of insects. Other secondary effects that have been studied
include repellency, antioviposition, sterility, fecundity
reduction, loss of flying ability, disrupting sexual
communication, and reducing guttural motility (National Research
Council, 1992; Schmutterer, 1990a).
EFFICACY STUDIES
Research has shown that many organisms are sensitive to neem
extracts. These include insects from several orders, mites,
nematodes, snails, fungi, and viruses (Bhatnagar et al., 1990;
Locke, 1990; National Research Council, 1992). Insect control is
now the primary use of neem and has been found to be effective
against insects by several methods stated earlier. The growth
regulation and feeding deterrence of azadirachtin are receiving
the most attention, and other effects are studied secondarily as
the experiment enables. This is not so much because these
responses are less important, but not as many insects show
sensitivity.
Insects have shown the most sensitivity to azadirachtin as a
growth regulator. Metamorphic stages are affected in such a way
that death often occurs during the molting process. These
results are not only dose dependent, but also, response increases
with earlier larval stages. Not all species react the same
though. A few insects show no mortality or metamorphic
abnormalities until the final molt to an adult insect, at which
time very high rates of death are observed. Molting inhibition
can be seen at very low topical and ingestion rates, one ppm.
Even though this was in a laboratory, field rates are effective
at rates much lower than those required to elicit other responses
(Isman et al., 1990; Schmutterer, 1990b; Stark et al., 1990; Wood,
1990).
Antifeeding effects have received much attention especially
in crops that suffer from excessive insect damage. Response by
insects to neem extract applications varies greatly across the
spectrum of sensitive insects. Even within an order this can be
seen. The desert locust is believed to be the most sensitive
insect to antifeedant effects of azadirachtin, but the migratory
grasshopper feeds undeterred on cabbage treated with 500 ppm, a
rate that would deter many other insect species. Growth
reduction as a preliminary indication of food refusal can be seen
at 0.1 ppm azadirachtin, but antifeedant activity often requires
higher concentrations, usually over 200 ppm. In Rhodnius prolixus,
antifeedant activity is observed at 600 times the amount needed
to disrupt development. Gustatory and non-gustatory sensilla as
well as reduced guttural motility may contribute to deterrent
responses (Koul et al., 1991; Schmutterer, 1990b; Wood, 1990;
Zehnder et al., 1990).
With a large number of organisms being affected by neem
extracts, concern was expressed for the welfare of beneficial
organisms under management programs using neem tree extracts. It
has been found, however that predator and parasitoid insects are
relatively unaffected when their life cycle involves exposure to
neem extracts. Evidently, azadirachtin does not affect these
insects in the same way or not enough chemical is taken up in
their diet to cause behavioral or metamorphic abnormalities.
Some parasitoids showed slight toxic affects when emerging from
treated mummified hosts, but these parasitoids were likely
exposed to a much higher dose than normal. Further longevity
studies are warranted to determine if extracts have any effects
on reproduction or alter fitness of natural enemies (Hoelmer et
al., 1990).
The effects of neem on other desirable organisms have led to
similar conclusions. In a study conducted by Shapiro et al.
(1994), mortality of the gypsy moth was evaluated in the presence
of a virus pathogen and also when the moth and virus were subject
to neem treatments. Not only did the extracts have no adverse
effect on viral activity but, when applied concurrently, moths
died sooner. A neem product has shown no toxicity to honeybee
workers at rates of 500 ppm. Earthworms actually benefit from
soil application of neem by-products with increased weight gain
and more progeny (Schmutterer, 1990b). And spiders, butterflies,
ants, and ladybugs also show no detrimental effects from exposure
to neem tree extracts (National Research Council, 1992).
APPLICATION PROBLEMS
One of the main problems of using neem treatments is the
durability of azadirachtin in field conditions. The activity of
neem-based products subsides rapidly, lasting four to eight days,
meaning that many applications will likely be needed in a season.
The primary means of this is photodegradation by ultra-violet
light. But leaf pH can also affect detoxification rates, and
rain can wash residue off leaf surfaces. Derivation of natural
product stabilizes azadirachtin and may provide an avenue for
greatly increasing its residual activity (Wood, 1990). Also,
activity can be extended in plants, such as potato and tomato,
that demonstrate systemic activity. This protects azadirachtin
from light and through translocation enables protection of new
growth which is often preferred by insects (Klocke et al., 1991,
Verkerk et al., 1993).
Systemic activity in plants also relates to a greater chance
of phytotoxicity. Potato, onion, cabbage, and chrysanthemum have
demonstrated various types and extent of phytotoxicity. In most
instances this is undesirable, but the stunting that occurs on
chrysanthemums can actually take the place of plant growth
regulators that are sprayed for the same effect on plants grown
in greenhouses (Oetting et al., 1990; Schmutterer, 1990a).
Azadirachtin content in neem kernels and quickness of
activity are further considerations in the commercialization of
neem extracts. To provide a consistent product, refining kernels
with similar levels of compounds is essential. On the contrary,
a Canadian company discovered that samples of neem oil from
Indian sources ranged from undetectable amounts, less than 50
ppm, of azadirachtin to 6,800 ppm (Isman et al., 1990). Farmers
using synthetic pesticides also are used to quick acting
chemicals. They may not be patient enough to wait for the
activity of neem-based products to produce results (Schmutterer,
1990b).
CONCLUSION
While neem tree products have some shortcomings as a
conventional alternative, they fit in well as a tool to be used
in integrated pest management systems. As more and more
synthetic chemicals are being pulled from the market, neem is an
environmentally benign alternative. It has significant effect on
pests without harming beneficial organisms. Toxicology studies
have indicated it to be quite safe to mammals also (Schmutterer,
1990b). Researchers, however, still have much work ahead of them
to characterize the responses of sensitive insects in the field.
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