© Benaki Phytopathological Institute
Persistence of insecticides in qat leaves
55
of pesticides in plants, which is longer in the
dry season than in the wet season, is influ-
enced by different factors, such as the target-
ed plants, the physico-chemical properties
of the pesticide, and its application methods
(Itoiz
et al
., 2012; Gupta
et al.,
2008; Fujita
et
al
., 2014). However, a variety of similar studies
have been realized to determine the dissipa-
tion of imidacloprid using different applica-
tion techniques on different plants. The half-
life of imidacloprid residues in qat leaves was
relatively long at 12.2 days (Table 3), which is
consistent with another study showing that
the half-lives of imidacloprid in sugarcane
leaves were 8.1–9.7 days in two different ap-
plication doses (Sharma and Singh, 2014). In
contrast, imidacloprid was reported to dis-
sipate more rapidly in other plants, such as
tea shoots, with a half-life of 1.09–1.25 days
(Gupta
et al.,
2008), 4.4 days in lettuce leaves
(Itoiz
et al.,
2012), and 1.7–2.3 days in chickpea
pods and leaves (Chahil
et al.,
2014).
Until these days, there is no legislation
in the qat production countries for the rec-
ommended minimum pre-harvest inter-
vals (PHIs) or for the maximum residue lim-
its MRLs of pesticides in qat despite its high
consumption by about 10 million people of
different countries. Consequently, no val-
ue has yet been established with respect to
the maximum permissible intake of imida-
cloprid, acetamiprid and methomyl in qat.
Due to the lack of some scientific informa-
tion on the persistence of pesticides in qat
and the absence of MRL values in its leaves
in the major guidance documents i.e. Codex
(2015) and EU (2005), we used the MRL val-
ues already established for lettuce leaves to
compare the preharvest intervals of investi-
gated insecticides because both lettuce and
qat leaves are chewed fresh and uncooked
among the qat consumers and approxi-
mately at the same quantity weekly. The
MRL for imidacloprid in lettuce is 3.5 mg kg
-1
and the recommended PHI is 7 days (Global
MRL Database, 2015). In our study, the resi-
due concentrations of imidacloprid were be-
low the MRL at 3.1 mg kg
-1
(Table 2) 7 days af-
ter treatment. In contrast, Chahil
et al.
(2014)
reported that the residue concentrations of
Table 2.
Residues (mg kg
-1
) of imidacloprid, acetamiprid and methomyl, and the percentage
of remaining residues (%) in leaves of qat at different time intervals after application.
Time
(days)
Imidacloprid
Acetamiprid
Methomyl
Residues
%
Residues
%
Residues
%
0 (1h)
6.2 ± 0.41
100
3.0 ± 0.28
100
12.0 ± 0.63
100
1
5.2 ± 0.33
83.87
2.6 ± 0.30
86.67
2.6 ± 0.43
21.67
3
3.9 ± 0.19
62.90
2.1 ± 0.24
70.00
2.3 ± 0.39
19.17
7
3.1 ± 0.22
50.00
2.0 ± 0.11
66.67
1.1 ± 0.22
9.17
12
1.8 ± 0.12
29.03
1.3 ± 0.08
43.33
0.54 ± 0.18
4.50
19
1.8 ± 0.16
29.03
1.0 ± 0.09
33.33
0.31 ± 0.04
2.58
26
1.1 ± 0.09
17.74
0.89 ± 0.10
29.67
0.17 ± 0.06
1.42
37
0.66 ± 0.07
10.65
0.25 ± 0.05
8.33
0.03 ± 0.01
0.25
Table 3.
Dissipation rate parameters for the fit of imidacloprid, acetamiprid and methomyl
in qat leaves,
to a first order kinetic model.
Pesticide
r
2
K
(day
-1
)
C
0
(mg kg
-1
)
Std. error
t
1/2
(days)
Imidacloprid
0.9542
0.057a
6.2
0.0021
12.2
Acetamiprid
0.9482
0.059a
3.0
0.0017
11.7
Methomyl
0.9290
0.135b
12
0.0009
5.1
r
2
: determination coefficient;
K
: rate constant;
C
0
: initial concentration of residue; Std. error: Standard error;
t
1/2
: half-
life;
a,b
: significant difference between treatment using a Student T test.
