Insect Control: Biological and Synthetic Agents - Index of

and Hemiptera and was related to effects on the
mouthpart chemoreceptors (Simmonds and Blaney,
1984). Studies on the insect growth regulatory (IGR)
mode of action of azadirachtin in, for example, the
blowfly Calliphora vicinia, the milkweed bug Oncopeltus
fasciatus, the tobacco hornworm Manduca
sexta, and the African migratory locust Locusta
migratoria revealed that ecdysone biosynthesis and
catabolism were affected by azadirachtin treatment
to cause the major insect growth regulatory effects
(Redfern et al., 1982; Sieber and Rembold, 1983;
Schlüter et al., 1985; Dorn et al., 1986; Bidmon
et al., 1987). In adult insects, such as L. migratoria,
effects of azadirachtin on egg development were
shown to be related to changes in both juvenile
hormone (JH) and ecdysone titers (Rembold et al.,
1986). In addition to the effects of azadirachtin on
JH and ecdysone and the interactions between both
hormones, Schlüter (1987) was able to demonstrate
in the Mexican bean beetle Epilachna varivestis
that azadirachtin also affected cells in the process
of proliferation and differentiation. Mitosis in the
wing disc was blocked from metaphase onwards by
azadirachtin injections.
By 1986 azadirachtin was established as an antifeedant,
insect growth regulator, and sterilant with a
5: Azadirachtin, a Natural Product in Insect Control 187
novel mode of action. The basis for its mode of
action was known to involve the neurosecretory–
neuroendocrine axis and perhaps other sites including
particular stages in cell division. Although the
full structure had not yet been elucidated or synthetic
routes explored, the use of neem extracts to produce
insecticides with good market potential was
thought to be feasible. Studies to overcome a number
of problems preventing commercial use of neem
relating to standardization and regulatory matters
were still required, as were studies on its possible
role in integrated pest management strategies.
5.3. Chemistry of Neem Products
Azadirachtin, formula C35H44O16, m.p. 160 C,
[a] 25
D -66 (CHCl3, c ¼ 0.5) (Figure 1) belongs to the
large group of plant triterpenoids, and to the narrower
group of limonoids, which are frequently found
in plants of the Meliaceae. Limonoids are triterpenoids
that have their side chains shortened by four
carbon atoms and the remaining four atoms cyclized
as a furan ring (e.g., nimbin and salannin, and their
desacetyl-derivatives; Figure 1). The molecule of azadirachtin
is unusually highly oxygenated, and in a
molecular model the surface is covered by functional
Figure 1 The structure of azadirachtin, together with the principal limonoids (nimbin and salannin, and their desacetylderivatives)
from the seeds of neem (Azadirachta indica). Nimbin possesses almost no antifeedant effect and that of salannin is
small compared to azadirachtin.