Insect Control: Biological and Synthetic Agents - Index of

96 3: Neonicotinoid Insecticides
Table 8 Baseline susceptibility of some high risk pests to imidacloprid
Species Diagnostic dose Bioassay system/assessment time Reference
Myzus persicae 15 ppm Leaf dip (6-well plate) Nauen and Elbert (2003)
Myzus persicae 2.25 ng per aphid Topical Foster et al. (2003)
Aphis gossypii 13 ppm Leaf dip (6-well plate) Nauen and Elbert (2003)
Phorodon humuli 13 ppm Leaf dip (6-well plate) Weichel and Nauen (2003)
Bemisia tabaci 16 ppm Systemic bioassay Cahill et al. (1996)
Bemisia tabaci 1 ppm Leaf dip Rauch and Nauen (2003)
Leptinotarsa decemlineata 8 ppm Artificial diet (larvae) Olson et al. (2000)
Leptinotarsa decemlineata 0.2 mg per beetle Topical (adult) Nauen (unpublished data)
from 48 h to 72 h, and also after maintaining such
strains under laboratory conditions for some weeks
(Nauen and Elbert, 1997). More recently Foster
et al. (2003b) demonstrated that tolerance to imidacloprid
in M. persicae from different regions in
Europe also provided cross-tolerance to acetamiprid.
The authors were able to show a clear correlation
between ED50 values of acetamiprid and imidacloprid
for strains with a different degree of tolerance.
However, tolerance factors compared to a susceptible
reference population never exceeded factors
of 20, and field failures were not seen (Foster
et al., 2003b).
One species of major concern over the last decade
is the tobacco or cotton whitefly, B. tabaci; this is
a serious pest in many cropping systems worldwide
and several biotypes of this species have been
described (Perring, 2001). The most widespread
biotype is the B-type, which is also known as
B. argentifolii Bellows & Perring. The B-type whitefly
is a common pest, particularly in cotton, vegetables,
and ornamental crops, both by direct feeding
and as a vector of numerous plant pathogenic viruses.
In southern Europe, it coexists with another
biotype, the Q-type, which was originally thought
to be restricted to the Iberian peninsula, but which is
now also known to occur in some other countries
throughout the Mediterranean area, including Italy
and Israel (Brown et al., 2000; Palumbo et al., 2001;
Nauen et al., 2002; Horowitz et al., 2003). The biotypes
B and Q can easily be distinguished by randomly
amplified polymorphic DNA polymerase chain
reaction (RAPD-PCR) or native polyacrylamide gel
electrophoresis (PAGE) and subsequent visualization
of their nonspecific esterase banding pattern
(Guirao et al., 1997; Nauen and Elbert, 2000).
As a consequence of extensive exposure to insecticides,
B. tabaci has developed resistance to a wide
range of chemical control agents (Cahill et al., 1996).
The need for a greater diversity of chemicals for
whitefly control in resistance management programs
has been met by the introduction of several
insecticides with new modes of action, which are
unaffected by mechanisms of resistance to organophosphates
or pyrethroids. Since the introduction
of imidacloprid, the neonicotinoids have been the
fastest-growing class of insecticides. Imidacloprid
exhibits an excellent contact and systemic activity
and therefore has been largely responsible for the
sustained management of B. tabaci in horticultural
and agronomic production systems worldwide.
Beside imidacloprid, there are other neonicotinoids
with good efficacy against whiteflies, e.g., acetamiprid
and thiamethoxam.
In Israel, monitoring of resistance in B. tabaci to
imidacloprid and acetamiprid was initiated in 1996
in cotton and greenhouse ornamental crops. After
2 years of use in cotton, no apparent resistance
to imidacloprid and acetamiprid was reported
(Horowitz et al., 1998). However, 3 years of acetamiprid
use in greenhouses in Israel resulted in a
5–10-fold decrease in susceptibility of B. tabaci to
acetamiprid (Horowitz et al., 1999). In the past only
a few cases of lowered neonicotinoid susceptibility
in B-type B. tabaci have been described, among
them strains from Egypt and Guatemala that were
recently reported (El Kady and Devine, 2003; Byrne
et al., 2003). In Arizona, where imidacloprid has
been used since 1993, monitoring of B. tabaci populations
from cotton fields, melon fields, and greenhouse
vegetables suggested reduced susceptibility to
imidacloprid from 1995 to 1998, but subsequent
monitoring showed that these populations had
actually regained and sustained susceptibility to imidacloprid
in 1999 and 2000 (Li et al., 2000, 2001).
Furthermore, imidacloprid use in Arizona and
California remains high, but no signs of reduced
control in the field have been reported yet (Palumbo
et al., 2001). B-type whiteflies have been shown to
develop resistance to imidacloprid under selection
pressure in the laboratory (Prabhaker et al., 1997).
There was a moderate increase of resistance of up
to 17-fold in the first 15 generations, but 82-fold
resistance after 27 generations. However, resistance
was not stable and disappeared after a few generations
without insecticide pressure. Resistance to