Insect Control: Biological and Synthetic Agents - Index of

Barley yellow dwarf virus vectors such as R. padi
and S. avenae are controlled and the secondary
spread of the disease is inhibited. In contrast to
one or several sprays, seed dressing as the only
treatment assures prolonged protection during the
critical period, when virus transmission is of importance.
Furthermore pyrethroid sprays with a broad
spectrum of activity can be substituted by only one
application of imidacloprid, which has virtually no
effects on beneficials and other nontarget organisms.
High yield varieties achieve their optimum
yield potential only if they are sown early, which
implies an enhanced risk of infestation with aphids
and, consequently, with barley yellow dwarf virus.
As Gaucho Õ controls aphids and suppresses the
infection with this virus, early-sown cereals are
especially well protected against insects and diseases.
Therefore, a better tillering is achieved and
15–25% of the seed can be saved.
Both imidacloprid seed treatment and pyrethroid
spray application have very good to excellent
effects against the vector R. padi. The acute effect of
the pyrethroid was better due to its quick knockdown
activity. Imidacloprid alternatively acts more
slowly, so, it takes more time to kill invading aphids,
but its residual effect is much more pronounced.
Thus imidacloprid use results in long-lasting control
of aphids and, as a consequence, considerably better
prevention of virus symptoms. A yield increase
of 26% over untreated controls was achieved in
comparison with, 10% achieved by pyrethroid
treatment.
Field trials over 4 years in France confirmed that
both wheat and barley are well protected against the
above-mentioned pests and diseases. In an average
of 76 trials in wheat and 66 trials in barley a yield
increase of 3.1 dt ha 1 and 4.4 dt ha 1 , respectively;
was obtained with the Gaucho Õ treatment when
compared with untreated.
3.4.3. Foliar Application
Spray applications are especially used against pests
attacking crops such as cereals, maize, rice, potatoes,
vegetables, sugar beet, cotton, and deciduous
trees. Table 4 shows the acute activity (estimated
LC 95 (lethal concentration at which 95% of insects
are killed) in ppm a.i.) of imidacloprid against a
variety of pests, following foliar application (dip
and spray treatment) of host plants under laboratory
and greenhouse conditions. Imidacloprid was very
active to a wide range of aphids. The most susceptible
was the damson hop aphid Phorodon humuli
(LC 95 0.32 ppm), which is often highly resistant
against conventional insecticides (Weichel and
Nauen, 2003). Imidacloprid was highly effective
against some of the most important rice pests, such
as leafhoppers and planthoppers, rice leaf beetle
L. oryzae and rice water weevil L. oryzophilus.
Although imidacloprid is generally less effective
against biting insects, its efficacy against the Colorado
potato beetle Leptinotarsa decemlineata is
relatively high (LC95 40 ppm). The LC95 values
for the second or third instar larvae of some of the
most deleterious noctuid pest species of the order
Lepidoptera, Helicoverpa armigera, Spodoptera
frugiperda, and Plutella xylostella, were approximately
200 ppm, and higher than those for the
species mentioned above (Elbert et al., 1991).
3.4.4. Soil Application and Seed Treatment
Typical soil insect pests such as Agriotes sp., Diabrotica
balteata, or Hylemyia antiqua were controlled
by incorporation of 2.5–5 ppm a.i. into the
soil. Higher concentrations of imidacloprid are
necessarytocontrolReticulitermes flavipes (7 ppm)
and Agrotis segetum (20 ppm). However, imidacloprid
activity is much more pronounced against earlyseason
sucking pests, which attack the aerial parts of
a wide range of crops. Soil concentrations as low
as 0.15 ppm a.i. gave excellent control of Myzus
persicae and Aphis fabae on cabbage in greenhouse
experiments (Elbert et al., 1991). A good residual
activity is essential for the protection of young plants.
3.5. Mode of Action
3: Neonicotinoid Insecticides 83
The biochemical mode of action of neonicotinoid
insecticides has been studied and characterized
extensively in the past 10 years. They act selectively
on insect nAChRs, a family of ligand-gated ion
channels located in the CNS of insects and responsible
for rapid neurotransmission. The nAChR is a
pentameric transmembrane complex, and each subunit
consists of an extracellular domain containing
the ligand binding site and four transmembrane
domains (Nauen et al., 2001; Tomizawa and
Casida, 2003). Neonicotinoid insecticides bind to
the acetylcholine binding site located on the hydrophilic
extracellular domain of a-subunits. Their
ability to displace tritiated imidacloprid from its
binding site correlates well with their insecticidal
efficacy (Liu and Casida, 1993a; Liu et al., 1993b).
[ 3 H]imidacloprid binds with nanomolar affinity to
nAChR preparations from insect tissues, and next to
the less specific a-bungarotoxin, it is the preferred
compound in radioligand competition studies (Lind
et al., 1998; Nauen et al., 2001). Furthermore,
electrophysiological studies revealed that neonicotinoid
insecticides act agonistically on nAChR, and
this interaction is again very well correlated with