Insect Control: Biological and Synthetic Agents - Index of

Figure 13 Binding to putative cationic subsite in insect
nAChR (Kagabu, 1997a).
(1997) reported that a nitroenamine isomer (N 3 -free
imidacloprid) posseses similar insecticidal activity to
imidacloprid (Boëlle et al., 1998).
3.3.4. Physicochemistry of Neonicotinoids
The physicochemical properties of the ring system
and noncyclic neonicotinoids played an important
role in their successful development. In addition,
photostability is a significant factor in field performance
of this class of insecticides (see Section 3.2).
As described (Kagabu and Medej, 1995; Kagabu,
1997a; Kagabu and Akagi, 1997), the energy gap
for the different functional groups [›X w Y], from
the ground state to the single state, excited in the
order [›N w CN] > [›N w NO2] > [›CH w NO2].
For practical application of neonicotinoids, such as
for soil and seed treatment, as well as for foliar
application, the uptake and translocation in plants
is crucial for their insecticidal activity. Thereby, not
only the bioisosteric segments of neonicotinoids (see
Section 3.3.3.3) but also the whole molecular shape
(Figure 12), including the resultant water solubility,
has to be considered. Neonicotinoid insecticides are
push–pull olefins made up of conjugated electron
donating and accepting groups (Kagabu, 1997a).
Such polar, nonvolatile molecules have a high
water solubility (Table 3) and low POW values compared
with other nonpolar insecticidal classes. The
following characteristics were described from
Kagabu (1997a):
1. Noncyclic neonicotinoids are less lipophilic than
the corresponding ring system neonicotinoids.
2. Concerning the functional group [›X w Y] as
part of the pharmacophore [ w N w C(E)›X w Y]
(iii), water solubility increases in the order of
[›CH w NO2] > [›N w CN] > [›N w NO2].
3. Regarding E, the lipophilicity increased in the
order of S > C > O > NH (Kagabu, 1996).
Somewhat more lipophilic neonicotinoids should be
better for seed treatment application, because
uptake by roots is more effective than in the case
3: Neonicotinoid Insecticides 75
of more hydrophilic compounds (Briggs et al., 1982)
(Figure 14, Table 3). Due to the excellent systemic
properties of neonicotinoids conferred by the moderate
water solubility, these insecticides are effective
against their main target pests, which are sucking
insects, like aphids, leafhoppers, and whiteflies.
3.3.4.1. Physicochemical properties of commercialized
neonicotinoids The physicochemical
properties of a compound are related to its structural
formula. However, it is not possible to predict the
behavior of a substance, with sufficient certainity,
only on the basis of its formula and physicochemical
properties (Stupp and Fahl, 2003).
3.3.4.1.1. Imidacloprid Due to the special moieties
like the CPM residue and the 2-(N-nitroimino)imidazolidine
5-ring system, imidacloprid has very
weak basic properties under environmental conditions
(see Section 3.8.1.2). Water solubility and
low partition coefficient in octanol–water are not
influenced by pH values between 4 and 9, and at
20 C (Krohn and Hellpointer, 2002) (Table 3). The
low partition coefficient of imidacloprid indicates
that it has no potential to accumulate in biological
tissues and further enrichment in the food chain.
The occurrence of imidacloprid in the air is determined
by its low vapor pressure of 4 10 10 Pa
which excludes volatilization from treated surfaces.
The rapid uptake and translaminar transport of imidacloprid
from the treated upper leaf side to the
lower surface is excellent, as observed in cabbage
leaves (Elbert et al., 1991), and in rice and cucumber
(Ishii et al., 1994). However, imidacloprid has a
considerable acropetal mobility in xylem of plants.
In contrast, its penetration and translocation in cotton
leaves was less pronounced, as qualitatively
reflected by phosphor-imager autoradiography
(Buchholz and Nauen, 2001). This xylem mobility
makes imidacloprid especially useful for seed treatment
and soil application, but it is equally effective
for foliar application (Elbert et al., 1991).
Due to its lack of any acidic hydrogen, the pKa of
imidacloprid is >14, and therefore its transport
within the phloem is unlikely, as been shown in
several studies (Stein-Dönecke et al., 1992; Tröltzsch
et al., 1994). Systemic properties were examined
using 14 C-labeled imidacloprid. The translocation
of imidacloprid in winter wheat and its uptake and
translocation from treated cotton seeds into the
growing parts of the cotton plant is described by
Elbert et al. (1998). Registered patterns of use of
imidacloprid in agriculture now include traditional
foliar spray application as well as soil drench application,
drip irrigation, trunk (injection) application,