Insect Control: Biological and Synthetic Agents - Index of

190 5: Azadirachtin, a Natural Product in Insect Control
in ethanol or acetone and carefully diluting with
water to avoid formation of the glassy mass when
the solid is wetted with water, and which then dissolves
very slowly. Solutions in organic solvents are
stable almost indefinitely (Jarvis et al., 1998) but
solutions in water at pH 7 or higher are unstable.
Aqueous solutions are most stable from pH 4 to 6
(Jarvis et al., 1998).
While degradability is one great advantage in agricultural
application of neem triterpenoids, the rate
of disappearance is a little too high according to
some authors. One report says azadirachtin is stable
for less than 1 week under ambient conditions
(Sundaram, 1996), another says its stability is less
than 3 months (Brooks et al., 1996). Scott and
Kaushik (2000) gave a half-life of 36–48 h in water
exposed to sunlight. The DT50 (time for 50% of
azadirachtin to disappear) in soil at 25 C was
20 and 31.5 days in unautoclaved and autoclaved
soil respectively (Stark and Walter, 1995). Probably
the most comprehensive studies on the environmental
behavior and stability of azadirachtin have
been carried out by the Canadian Forest Service for
its use on young trees (Sundaram, 1996). Its persistence
on balsam fir and red oak foliage; its
dissipation in forest nursery soils; absorption, leaching,
and desorption from sandy loam forest soil;
photostability on foliage; and rate of hydrolysis in
natural waters have all been measured (Sundaram,
1996; Sundaram et al., 1997). They have also
made efforts to formulate azadirachtin against
hydrolysis and UV degradation, but no large-scale
study of formulation for stability appears to have
been made.
5.4. Neem Insecticides in Pest Control
5.4.1. Background
Today, after 40 years of discovery, research, and
development of neem as a natural pesticide there
are well-established products in the organic agriculture
and niche markets in both North America
and Western Europe. Azadirachtin, the main active
ingredient of neem seeds, is an extremely effective
antifeedant to many phytophagous insects, and an
IGR and sterilant to all insects tested. Other constituents
of neem seeds, including the oil, have been
shown also to be effective control agents against
plant nematodes and fungal pathogens.
The complexity of the azadirachtin molecule and
our inability, as yet, to synthesize it has precluded
its production as a synthetic pesticide, hence use of
the natural product is the only choice. Neem as a
botanical pesticide has many excellent attributes
that include its broad-spectrum IGR effects, systemic
action in some plants, minimal disruption of natural
enemies and pollinators, rapid breakdown
in the environment, and lack of toxicity to vertebrates.
However, its initial promise has not been
realized. Problems have arisen during the research
and development phase and these relate to its mode
of action, where (as with most IGRs) azadirachtin
acts slowly and although feeding and crop damage
ceases shortly after treatment, pest insects remain
alive on treated crops, reducing acceptance by
growers and consumers. Neem pesticides have
limited persistence on plants and require multiple
applications against certain pests when used as a
stand-alone treatment. This, together with the cost
of production of a consistent, high-quality product
with 1% or more of azadirachtin by weight from
annual supplies of seeds, results in the cost of neem
pesticides to be some three times greater than
synthetic pyrethroids (Isman, 2004). Regulatory
issues have also featured large in the development
of neem and have been a formidable barrier to the
successful commercialization of neem-based pesticides.
The Environmental Protection Agency of the
USA chose to simplify the approval process of neem
insecticides by recognizing azadirachtin as the sole
active ingredient and deeming the remaining chemical
components present in neem kernels to be ‘‘inert
ingredients’’ (Isman, 2004). However, Europe and
Japan, for example, have required identification
and toxicological studies for all major constituents
of neem, processes that have greatly delayed its
introduction into these and other countries.
Despite these problems neem insecticides are
established as valuable components in integrated
crop management systems and accepted by organic
producers. In California neem insecticides are used
on over 60 food crops, particularly lettuce and tomato,
and account for 0.25% of insecticide use on
tomato (California EPA, 2001, in Isman, 2004). In
Europe, Neemazal insecticides (Trifolio-M GmbH)
have been cleared for use in Germany after the
parent company completed toxicological studies
on all compounds present in their neem products.
Neem is expected to show continued sales growth in
highly developed countries as new markets continue
to be developed. Growth in organic food production
and the gradual reduction, and ultimate elimination,
of synthetic neurotoxic insecticides will both favor
expansion of the market for neem. The introduction
of neem products specifically formulated for particular
crops and crop pests will also aid expansion, as
would intensive education of growers.
In developing countries, simple inexpensive formulation
of neem extracts must be encouraged as an