Insect Control: Biological and Synthetic Agents - Index of

98 3: Neonicotinoid Insecticides
chemicals. They have a high reproductive potential,
and extremely short life cycle allowing for numerous
generations in a growing season. Combined
with frequent applications of insecticides that are
usually required to maintain aphid populations
below economic thresholds, resistance development
is facilitated in these species, resulting in control
failures. Such control failures have been reported
for organophosphorus compounds for many decades,
and more recently also for pyrethroids
(Foster et al., 1998, 2000; Devonshire et al., 1999;
Foster and Devonshire, 1999).
One of the major aphid pests is the green peach
aphid, M. persicae. Resistance of M. persicae to
insecticides is conferred by increased production of
a carboxylesterase, named E4 or FE4, which provides
cross-resistance to carbamates, organophosphorus,
and pyrethroid insecticides (Devonshire
and Moores, 1982; Devonshire, 1989). This esterase
overproduction was shown to be due to gene amplification
(Field et al., 1988, 2001). It was the sole
resistance mechanism reported in M. persicae for
more than 20 years, and only recently an insensitive
(modified) acetylcholinesterase was described as
a contributing factor in carbamate resistance in
M. persicae (Moores et al., 1994a, 1994b; Nauen
et al., 1996). The insensitive acetylcholinesterase in
M. persicae confers strong resistance to pirimicarb,
and a little less to triazamate (Moores et al., 1994a,
1994b; Buchholz and Nauen, 2001). Due to the
improvement of molecular biological techniques,
Martinez-Torres et al. (1998) recently showed
that knockdown resistance to pyrethroids, caused
by a point mutation in the voltage-gated sodium
channel, is also present in M. persicae. In summary,
resistance to all major classes of aphicides occurs
in M. persicae; however, the only class of insecticides
not yet affected by any of the mechanisms
described above are the neonicotinoids, including
its most prominent member imidacloprid (Elbert
et al., 1996, 1998a; Nauen et al., 1998a; Horowitz
and Denholm, 2001).
The most comprehensive studies on the biochemical
mechanisms of resistance to neonicotinoid
insecticides using an agriculturally relevant pest species
were performed in whiteflies, B. tabaci (Nauen
and Elbert, 2000; Nauen et al., 2002; Rauch and
Nauen, 2003; Byrne et al., 2003). Biochemical
examinations revealed that neonicotinoid resistance
in Q-type B. tabaci collected in 1999 was not associated
with a lower affinity of imidacloprid to
nAChRs in whitefly membrane preparations (Nauen
et al., 2002). This was confirmed more recently by
testing strains ESP-00, GER-01, and ISR-02 obtained
in the years 2000–2002 by Rauch and Nauen
(2003). Although neonicotinoid resistance was very
high in these strains (up to 1000-fold), the authors
found just a 1.3-fold and 1.7-fold difference in binding
affinity between strains, and concluded that target
site resistance is not involved in neonicotinoid
resistance in those strains investigated. Piperonyl
butoxide, a monooxygenase inhibitor, is generally
used as a synergist to suppress insecticide resistance
conferred by microsomal monooxygenases. Experiments
with whiteflies pre-exposed to piperonyl
butoxide suggested a possible involvement of
cytochrome P-450 dependent monooxygenases in
neonicotinoid resistance (Nauen et al., 2002).
Rauch and Nauen (2003) biochemically confirmed
that whiteflies resistant to neonicotinoid insecticides
showed a high microsomal 7-ethoxycoumarin
O-deethylase activity, i.e., up to eightfold higher
compared with neonicotinoid susceptible strains.
Furthermore, this enhanced monooxygenase activity
could be correlated with imidacloprid, thiamethoxam,
and acetamiprid resistance. Significant
differences between glutathione S-transferase and
esterase levels were not found between neonicotinoid
resistant and susceptible strains of B. tabaci
(Rauch and Nauen, 2003). Several metabolic investigations
in plants and vertebrates showed that
imidacloprid and other neonicotinoids undergo
oxidative degradation, which may lead to insecticidally
toxic and nontoxic metabolites (Araki et al.,
1994, Nauen et al., 1999b; Schulz-Jander and
Casida, 2002). Metabolic studies in B. tabaci
in vivo revealed that the main metabolite in neonicotinoid-resistant
strains is 5-hydroxy-imidacloprid,
whereas no metabolism could be detected in the
susceptible strain (Figure 23) (see Section 3.7.1.1).
One can therefore suggest that oxidative degradation
is the main route of imidacloprid detoxification
in neonicotinoid resistant Q-type whiteflies (Rauch
and Nauen, 2003). Compared to imidacloprid, the
5-hydroxy metabolite showed a 13-fold lower binding
affinity to whitefly nAChR. This result was in
accordance with previous studies with head membrane
preparations from the housefly (Nauen et al.,
1998). The binding affinity expressed as the IC50
was highest with olefine (0.25 nM) > imidacloprid
(0.79) > 5-hydroxy (5 nM) > 4-hydroxy (25 nM) >
dihydroxy (630 nM) > guanidine and urea
(>5000 nM). The biological efficacy in feeding
bioassays with aphids correlated also with the relative
affinities of the metabolites towards the housefly
nAChR (Nauen et al., 1998b). The lower binding
affinity of 5-hydroxy-imidacloprid compared to
imidacloprid coincides with its lower efficacy
against B. tabaci in the sachet test (17-fold). These
data show that differences between binding to the