Insect Control: Biological and Synthetic Agents - Index of

192 5: Azadirachtin, a Natural Product in Insect Control
though the latter contained the same amount of
azadirachtin (Bomford and Isman, 1996).
5.4.5. Systemic Action
The discovery by Gill and Lewis (1971) that azadirachtin
was effective systemically was largely ignored
for some years, but recently, that valuable aspect
of its properties has again received attention. The
uptake of azadirachtin from solution by cabbage
leaves was demonstrated with the cabbage white
butterfly Pieris brassicae (Arpaia and van Loon,
1993). Tobacco (Nicotiana clevelandii) seedlings
also absorbed azadirachtin from solution through
their roots and its subsequent translocation to leaves
caused disturbances in the feeding behavior of the
aphid M. persicae, resulting in a reduced ability of
the aphids to acquire phloem-located viruses from
infected seedlings (Nisbet et al., 1993, 1996a).
Sundaram et al. (1995) showed that neem extract
watered around the roots of young aspen (Populus
tremuloides) was taken up within 3 h and translocated
to the stem and foliage within 3 days. Soil
drenching of bean plants (Phaseolus vulgaris) with
neem extract controlled the pea leaf miner Lyriomyza
huidobrensis (Weintraub and Horowitz,
1997). More recently it has been shown that systemic
dispersion of neem extract can be used to control
Ips pini bark beetle in lodgepole pine (Pinus contorta)
logs (Duthie-Holt et al., 1999). No works
appears to have been done to show whether all the
azadirachtin-like compounds are translocated, or
whether some more readily than others.
5.5. Antifeedant Effects of
Azadirachtin
Azadirachtin has marked antifeedant activity against
a large number of insect species, its effects being
mediated through contact chemoreception (primary
antifeedancy) and internal feedback mechanisms (secondary
antifeedancy); the latter related to toxic effects
on the gut, e.g., enzyme production, cell proliferation,
and motility (Timmins and Reynolds, 1992;
Nasiruddin and Mordue (Luntz), 1993a; Trumm and
Dorn, 2000). Insects vary markedly in their behavioral
sensitivity to azadirachtin. The desert locust
Schistocerca gregaria and many species of Lepidoptera,
are among the most sensitive, being deterred
by as little as 0.007 ppm (in diets) whereas the
Hemiptera and Coleoptera are much less sensitive
with EC 50 values of around 100 ppm or more (e.g.,
Isman, 1993; Mordue (Luntz) and Blackwell, 1993).
Whereas Schistocerca gregaria prefers to starve to
death rather than ingest azadirachtin, the primary
antifeedant effect on aphids of azadirachtin applied
systemically in plants occurs at levels far higher than
those causing IGR and sterility effects (Lowery and
Isman, 1993; Nisbet et al., 1993, 1994; Mordue
(Luntz) et al., 1996). Hematophagous insects such
as Culex mosquitoes are also less sensitive to the
antifeedant effects of azadirachtin than to its IGR
effects (Su and Mulla, 1998, 1999).
Whereas azadirachtin alone shows toxic IGR and
sterilant actions against insects, several compounds
in the plant biosynthetic pathway leading to azadirachtin
have antifeedant effects against phytophagous
insects (Aerts and Mordue (Luntz), 1997).
The antifeedant role is an important phenomenon
in the overall toxic effects of azadirachtin in those
insects that are sensitive to it behaviorally and this
phenomenon has been utilized in bioassays to explore
the structure–activity relationships of the azadirachtin
molecule. Both the decalin and the dihydrofuranacetal
fragments of azadirachtin are important
in eliciting antifeedant activity. Methylation of the
hydroxy substitutions on the molecule and the
addition of bulky groups to the dihydrofuran ring
decrease antifeedant activity (Blaney et al., 1990;
Blaney and Simmonds, 1994; Govindachari et al.,
1995; Simmonds et al., 1995).
The primary behavioral antifeedant response of
insects to azadirachtin is mediated via neural input
from the contact chemoreceptors. Inhibition of feeding
behavior results from stimulation of deterrent
receptors by azadirachtin often coupled with an
inhibition of sugar receptors (Simmonds and Blaney,
1984). In Spodoptera littoralis, Helicoverpa armigera,
and H. assulta, cells sensitive to sucrose and
azadirachtin are mainly in the lateral sensillum styloconicum
with azadirachtin evoking high impulse
discharges (Simmonds et al., 1995; Tang et al.,
2000). The firing of sucrose sensitive cells is reduced
in the presence of azadirachtin and in some species
the firing of the deterrent cells to azadirachtin is also
reduced in the presence of sucrose (Simmonds and
Blaney, 1996). This phenomenon, known as peripheral
interaction, varies among insects and occurs in
S. littoralis and S. gregaria but not in L. migratoria
(Simmonds and Blaney, 1996). Investigations of the
antifeedant mode of action by both electrophysiological
recordings and behavioral analysis have
shown that both polyphagous and oligophagous
insects are behaviorally responsive to azadirachtin
with the most responsive species being able to differentiate
extremely small changes in the parent
molecule (Blaney et al., 1990, 1994; Nasiruddin
and Mordue (Luntz), 1993b; Simmonds et al.,
1995). In Lepidoptera the antifeedant response is
also correlated with increased neural activity of
the chemoreceptors. A comparison of antifeedant