Insect Control: Biological and Synthetic Agents - Index of

nAChR are comparable to the efficacy differences in
adult bioassays (Rauch and Nauen, 2003).
Daborn et al. (2001) described D. melanogaster
mutants exhibiting an eight- and sevenfold resistance
to imidacloprid and DDT, respectively,
which showed an overexpression of the P-450 gene
Cyp6g1. However, it was not tested whether recombinant
Cyp6g1 itself can metabolize imidacloprid;
potentially it could metabolize a trans-acting factor
which then upregulates presently uncharacterized
P-450 genes. However, it was demonstrated that
the recombinant cytochrome P-450 isozyme
CYP3A4 from human liver can selectively metabolize
the imidazolidine moiety of imidacloprid,
resulting in 5-hydroxy-imidacloprid as the major
metabolite (Schulz-Jander and Casida, 2002). It
would therefore be fruitful to investigate if homologs
of Cyp6g1 and Cyp3a4, or similar genes, are
present in the B. tabaci genome, and if overexpression
of these genes is associated with neonicotinoid
resistance.
Although considerable resistance to neonicotinoid
insecticides was also reported for populations
of the Colorado potato beetle from Long Island,
NY, the underlying mechanisms associated have
not been very well studied. Pharmacokinetic investigations
on susceptible and resistant strains did not
reveal any differences in the uptake and excretion of
14 C-labeled imidacloprid, nor were there any differences
in metabolic conversion found (Hollingworth
et al., 2002). The authors suggested a modification
of the target site to be responsible for the observed
resistance. But, preliminary investigations revealed
no differences between resistant and susceptible
strains in receptor binding studies using [ 3 H]imidacloprid
(Nauen and Hollingworth, unpublished
data).
Resistance development to imidacloprid would
be disastrous in many regions, so there is a demand
for an effective resistance management program,
and interest in finding and developing new active
ingredients for pest control.
3.9.3. Resistance Management
Monitoring and detection of insecticide resistance in
order to recommend and implement effective resistance
management strategies is currently one of the
most important areas of applied entomology. This
program is necessary to sustain the activity of as
many active ingredients with different modes of
action as possible by using alternate spray regimes,
rotation, and sophisticated application techniques
(Denholm and Rowland, 1992; McKenzie, 1996;
Denholm et al., 1998, 1999; Horowitz and
Denholm, 2001). Historically, the problem of
insecticide resistance was tackled by continuously
introducing new active ingredients for pest insect
control. The number of insecticides available is
high, but the biochemical mechanisms targeted by
all these compounds is rather limited (Casida and
Quistad, 1998; Nauen and Bretschneider, 2002).
A pronounced decline in the introduction of new
insecticides since the late 1970s revealed the limitations
of a strategy relying on the continuous
introduction of new active compounds (Soderlund,
1997).
This problem has been recognized by the
regulatory authorities; and the European Plant
Protection Organization (EPPO) recently published
guidelines outlining the background work on
resistance required for registration of a new active
ingredient (Heimbach et al., 2000). These requirements,
which are provided by agrochemical companies
as an essential part of the registration dossier,
include
. baseline susceptibility studies (testing several
strains of a target species known to be prone to
resistance development);
. monitoring (continuous studies on the development
of resistance of target species by simple bioassays
after the launch of a new compound); and
. possible resistance management strategies (how
the new compound should be combined with
others in order to expand its life time in the field).
3.10. Applications in
Nonagricultural Fields
3: Neonicotinoid Insecticides 99
Because of the high insecticidal efficacy together
with its nonvolatility and stability under storage
conditions, imidacloprid (Premise Õ ) has been successfully
applied as a termiticide ( Jacobs et al.,
1997a; Dryden et al., 1999), and has also been
used for control of turf pests such as white grubs
(Elbert et al., 1991). Furthermore, imidacloprid
is the active ingredient of the insecticide Merit Õ ,
and is commonly incorporated into fertilizers for
early control of grubs in turf (Armbrust and Peeler,
2002). In addition, imidacloprid was the first neonicotinoid
to be used in a gel bait formulation
for cockroach control. The imidacloprid gel showed
outstanding activity even after 27 months’ under
various conditions (Pospischil et al., 1999). As an
endoparasiticide imidacloprid exerted activity
against the gastrointestinal nematode Haemonchus
contortus in sheep only at higher concentrations
(Mencke and Jeschke, 2002). Synergistic mixtures
containing imidacloprid are patented and these
are useful against textile-damaging insects, such as