Insect Control: Biological and Synthetic Agents - Index of

80 3: Neonicotinoid Insecticides
Figure 19 Modification of the imidacloprid bridging chain and pI 50 values for the nicotinic acetylcholine receptor (nAChR)
from housefly head membranes. (Reproduced from Nauen, R., Ebbinghaus-Kintscher, A., Elbert, A., Jeschke, P., Tietjen, K.,
2001. Acetylcholine receptors as sites for developing neonicotinoid insecticides. In: Iishaaya, I. (Ed.), Biochemical Sites of
Insecticide Action and Resistance. Springer, New York, pp. 77–105.)
affinity (8 nM), and successfully identified a 66 kDa
polypeptide in Drosophila head membranes. Several
cholinergic ligands and neonicotinoid insecticides,
like imidacloprid and acetamiprid, strongly inhibited
this photoaffinity labeling. Thus, the labeled
polypeptide is pharmacologically consistent with the
ligand- and insecticide-binding subunit of Drosophila
nAChR (Tomizawa and Casida, 1997, 2001).
3.3.5.3. Active metabolites of neonicotinoids From
the metabolic pathway (see Section 3.7.1) of imidacloprid
it is known that hydroxylation of the
imidazolidine ring leads in general to the mono-
(R 0 ¼ w H; R 00 ¼ w OH) and the bishydroxylated (R 0 ,
R 00 ¼ w OH) derivatives, both of which have reduced
affinity (Figure 19). Alternatively, the mono (R 0 ¼
w OH; R00 ¼ w H) derivative reflects a higher level of
efficacy (pI 50 value 8.5) (Nauen et al., 2001; Sarkar
et al., 2001). Interestingly, the olefin metabolite
showed a higher pI50 value for nAChR from
housefly head membranes than imidacloprid, and
provides superior toxicity to some homopterans
after oral ingestion (Nauen et al., 1998b, 1999b).
This result suggests that for the central ring system
the exact rearrangement of the ring atoms, and not
the electronic effect of the ring system, seems to be
necessary for insecticidal activity. A similar phenomenon
has been described for the conjugated
pyridone derivatives (Kagabu, 1999).
3.4. Biological Activity and
Agricultural Uses
The biological activity and agricultural uses of
neonicotinoid insecticides are enormous’ and these
insecticides are continuing to see new uses (Elbert
and Nauen, 2004). It is definitely beyond the scope
of this chapter to provide a full overview of the
agronomic and horticultural cropping systems that
use neonicotinoid insecticides and readers interested
in these aspects should refer to many articles
and book chapters published during the past
decade, e.g., Elbert et al. (1991, 1998), Yamamoto
(1999), Kiriyama and Nishimura (2002), and Elbert
and Nauen (2004). In order to provide a flavor of
the agricultural uses of neonicotinoids, and the
affected target pests, a few examples considering
imidacloprid are given below. Imidacloprid is presently
the most widely used neonicotinoid insecticide
worldwide.
3.4.1. Efficacy on Target Pests
Due to their unique properties – high intrinsic acute
and residual activity against sucking and some
chewing insect species, high efficacy against aphids,
whiteflies, leafhoppers and planthoppers, and the
Colorado potato beetle, and excellent acropetal
translocation – neonicotinoids can be used in a variety
of crops (Figure 20). These uses include: aphids
on vegetables, sugar beet, cotton, pome fruit, cereals,
and tobacco; leafhoppers, planthoppers, and water
weevil on rice; whiteflies on vegetables, cotton, and
citrus; lepidopteran leafminer on pome fruit and
citrus; and wireworms on sugar beet and corn
(Table 4). Termites and turf pests such as white
grubs can also be controlled by imidacloprid (Elbert
et al., 1990, 1991).
Neonicotinoids such as imidacloprid and thiamethoxam
also control important vectors of virus
diseases, thereby impairing the secondary spread
of viruses in various crops. This control has been
observed, e.g., for the persistent barley yellow dwarf
virus (BYDV) transmitted by Rhopalosiphum padi
and Sitobion avenae (Knaust and Poehling, 1992).
Seed treatments proved highly effective in controlling
barley yellow dwarfvirus vectors and the
subsequent infection, in a series of field trials in
southern England. Sugar beet seed, pelleted with
imidacloprid, was well protected especially against
infections with beet mild yellow virus transmitted
by the peach potato aphid, Myzus persicae (Dewar
and Read, 1990).