Bulletin of Insectology 56 (1): 35-40, 2003
ISSN 1721-8861

Uptake, translocation and metabolism of
imidacloprid in plants
Robin SUR, Andreas STORK
Bayer CropScience AG, Metabolism/Environmental Fate, Monheim, Germany

Abstract
The insecticide imidacloprid is mainly applied as a seed dressing formulation. After root uptake, imidacloprid is translocated acropetally within the xylem and degraded quickly in the plants. Three principal metabolic pathways have been identified in various
plant metabolism studies with different crops and types of application showing a nearly uniform quantitative and qualitative pattern in all test systems.
Key words: imidacloprid, metabolism, nectar, pollen, residue analysis, sunflower, translocation, uptake.

Introduction
Imidacloprid is the first commercially available representative of a new chemical class, the chloronicotinyl or
neonicotinoid insecticides. It was synthesized in 1985 andthe first registration was achieved in France (1991) insugar beet. It is a systemic broad-spectrum insecticideand acts as a contact and stomach poison against sucking
and some biting insects (rice hoppers, aphids, thrips,
whitefly, termites etc.). It can be applied for seed, soil or
foliar treatment. The molecule exhibits a novel mode ofaction as it is an agonist of the nicotinic acetylcholine receptor leading to paralysis and death of pest organisms(Bai et al., 1991; Nauen et al., 1998; Schmuck et al.,2003). The mammalian toxicity is about 700-fold lower
than that of nicotine having a similar chemical structure
and the same mode of action (Elbert et al., 1998).
Experiments have been carried out to determine uptake, translocation and metabolic pathways of imidacloprid after different types of application (foliar, soil andseed treatment, stem injection, painting application) on
various plant species including apples (Vogeler et al.,1992), corn (Vogeler and Dräger, 1989), cotton (Vogeler and Brauner, 1993), eggplant (Yoshida, 1992), hop(Reckmann, 1998), potatoes (Dräger et al., 1992; Vogeler et al., 1991), rape (Clark, 1997), rice (Kurogochi
and Araki, 1989; Kurogochi et al., 1989; Sakamoto,1991), sunflower (Stork, 1999; Schmuck et al., 2001),
tobacco (Clark and Brauner, 1994), tomatoes (Dräger etal., 1989), wheat and sugar beet (Stein-Dönecke et al.,1989). In addition, the behaviour in cell suspension
cultures (Köster et al., 1988; Köster, 1990) and rotational crops (Vogeler et al., 1992) was investigated.
The objective of these investigations was to determine
the nature of the residue in different crops and cropparts and to set up a uniform residue definition for all
crops as a basis for succeeding pre-registration residue
studies as well as for post-registration residue enforcement and monitoring purposes.
Materials and methods
As this article reviews a lot of metabolism studies per-formed with imidacloprid over a span of a decade, only
general information about the used instrumentation,equipment and materials is given. During that time anddepending on crop, nature of the residues and author
different types of instrumentation and analytical procedures may have been applied. A more detailed insightcan be taken from the individual studies cited above.
Radioactively labelled [methylene-14C]imidacloprid
was applied mainly as a spray, seed dressing or granular
formulation in all greenhouse trials. The use of a 14Clabelled active substance (a.s.) allowed very sensitiveand selective analyses. Certified non-radiolabelled compounds were used as reference substances for metabolite
identification by co-chromatography. Extraction of plant
samples was performed with different solvents or solvent mixtures, containing methanol, acetone, acetonitrile, ethyl acetate or water. Radioactivity in extract solutions and solid samples was determined by liquid
scintillation counting (LSC) and combustion/LSC, respectively. For separation, detection and quantitation ofparent compound and metabolites within extracts, thinlayer chromatography (TLC) in combination with a BioImaging Analyser or high-performance liquid chromatography (HPLC) with a radioactivity flow-through detector were used. Where identification with referencecompounds was not possible, isolation, purification andsubsequent structure elucidation was performed bymeans of mass and/or nuclear magnetic resonancespectroscopy.
Results and discussion
Following seed dressing or granular application of
imidacloprid on cotton, eggplant, potato and rice, uptakeof the applied activity ranged from 1.6 to 4.9% (Kurogochi et al., 1989; Sakamoto, 1991; Vogeler andBrauner, 1993; Vogeler et al., 1991; Yoshida, 1992; Kurogochi and Araki, 1989). After seed treatment of corn
the uptake amounted to 20% (Vogeler and Dräger,1989). The ratio of the activity in the reproductive organs compared to the activity of the whole plant wasvery low in corn, cotton, eggplant and rice ranging from

0.7 to 1.4%; in potato this ratio amounted to 12% (table1).
Uptake and translocation have also been studied after
spray application of imidacloprid 25 WP on apple andtomato (Vogeler et al, 1992; Dräger et al., 1989). Fourteen days after the treatment of apple and tomato fruits
28 and 21% of the applied activity, respectively, wasrecovered in or on the fruits; between 65 and 76% of

this recovered activity was located on the surface of the
fruits (table 2). In additional translocation experiments
the a.s. was applied only onto apple and tomato leaves,
while the fruits were covered with plastic foil during theapplication to prevent contamination. At harvest, 14
days later, the amount of radioactivity in the fruit compared to the activity applied was 0.1% at maximum(Vogeler et al., 1992; Dräger et al., 1989).

Table 1. Uptake, distribution and main metabolites of imidacloprid after soil application/seed treatment (TRR: total
radioactive residue, 6-CNA: 6-chloronicotinic acid, 6-CPA: 6-chloropicolyl alcohol, *Nitrosimine<<1% of TRR in
reproductive organs).
Crop

Harvest[days after treatment/seeding]

Uptake ofappliedactivity [%]Activity in reprod. organ
relative to the activity inwhole plant [%]

Corn

134

20

1.2

Cotton

211

4.9

1.4

Eggplant

69

1.6

1.0

Potato

129

2.5

12

Rice

79

4.5

1.1

Rice

124

4.4

0.7

Nature of the Residue(Compounds >1% of TRR)
Imidacloprid, Guanidine, 5-Hydroxy,
Olefine, Nitrosimine*, Ring-openguanidine, 6-CNA, 6-CPA, Dihydroxy
6-CPA-conj., Guanidine, 6-CPAGlucoside, Imidacloprid, 6-CNA, 6CPA, Olefine, Nitrosimine*
Guanidine, Imidacloprid, , 6-CPAGlucoside, 5-Hydroxy, Olefine, 6-CNA
Imidacloprid, 6-CNA, Guanidine, 5Hydroxy, Olefine, Nitrosimine*, 6CPA-Glucoside
Guanidine, Imidacloprid, 6-CNA, 5Hydroxy, Nitrosimine*
Guanidine, Imidacloprid, 5-Hydroxy, 6CNA, Olefine

Table 2. Uptake and main metabolites after spray application of imidacloprid.
Crop
Apple fruit

Harvest [days after
treatment]

Uptake of applied
activity [%]% TRRon surface

14

28

65

21

76

Tomato fruit
14
TRR: total radioactive residue

Nature of the Residue(Compounds >1% of TRR)
Imidacloprid, Olefine, 5-Hydroxy, Guanidine, 6-CPA-Glucoside, Urea, Dihydroxy
Imidacloprid, Guanidine, Urea, 5-Hydroxy

Table 3. Residues of imidacloprid, hydroxy- (4- and 5-hydroxy) and olefine-metabolites in nectar and pollen samples of corn, sunflower and summer rape (field trial results).
Crop

Formulation

Corn
WS 70
1 trial (France)
1 mg a.s./seed
Sunflower
WS 70
2 trials (Germany)
0.7 mg a.s./seed
Summer Rape
FS 5003 trials
0.04 mg a.s./seed(France, Sweden, UK)a.s.: active substance (=imidacloprid)

36

SampleMaterial

Residues in treated and control samples [mg/kg]
Imidacloprid

Hydroxy-metab.

Olefine-metab.

Pollen<<0.005<<0.005<<0.01

NectarPollen<<0.005<<0.005<<0.005<<0.005<<0.01<<0.01

NectarPollen<<0.005<<0.005<<0.005<<0.005<<0.01<<0.01

Translocation experiments examine the mobility anddistribution of chemical compounds within the vascular system and the tissues of plants. The results ofthese experiments for imidacloprid with different application types show, that there is a good acropetal
translocation of the a.s. to shoots and leaves (excellent
xylem mobility) on the one hand and on the other handa poor basipetal translocation to sinks, i.e. storage organs, roots and fruits (negligible phloem mobility).
Consequently, highest residues are expected to occur
in the older leaf parts of the plants. The systemic properties of a molecule are a function of its physicochemical properties, mainly water solubility, octanol/water-partition coefficient (log POW) and dissociation constant (pKa) determining e.g. its ability topenetrate through biomembranes. These properties are
responsible for the kinetics of root uptake and translocation into the xylem, which is a prerequisite for entering the phloem and in turn to be taken up by sucking insects. According to the BRIGGS model (Briggs etal., 1982) the loading of the xylem by organic compounds due to root-systemic uptake can be describedby a bell-shaped curve with maximum xylem mobility
between log POW of 1.0 and 2.5. As a measure of xylem mobility the transpiration stream concentration
factor (TSCF) has been introduced relating the concentration of the compound in the transpiration stream(xylem) to the exposure concentration. This model
predicts for imidacloprid with log POW = 0.51 a considerable high xylem mobility of about TSCF = 0.6,

which was qualitatively confirmed in the translocation
experiments mentioned above.
Xylem and phloem have different pH values of about
5 and 8, respectively. Therefore, especially weak acids
with pKa values of about 5.0-5.5 and log POW between
1.0 and 2.5 (molecules with this lipophilicity properties
can cross membranes very easily), e.g. 6-chloronicotinicacid, tend to accumulate in the phloem sieve tubes (iontrap mechanism). Imidacloprid with its high pKa of 14 isnonionized and therefore these pH differences do notaffect the distribution between the compartments andthe a.s. moves freely between phloem and xylem according to its biomembrane permeability. However,since there is no active loading of imidacloprid to thephloem and due to the far greater water flow (50- to
100-fold) imidacloprid is predominantly transported
within the xylem vessels (Bromilow and Chamberlain,1989). Once imidacloprid has entered the leaves it willbe trapped by this counter current principle in the leaf
and not re-transported into the plant stem.
Despite the wide variety of crops and application
types having been investigated a rather uniform pictureof the metabolic behaviour of imidacloprid in plants wasfound consisting of three principal biotransformation
pathways (figures 1-3). Especially after soil applicationor seed treatment a quick degradation of the a.s. was observed after root uptake of the a.s. In the case of spray
application only a part of the a.s. is translocated into theplant and metabolized there, so the degree of metabolism tends to be lower in this case.

N
Cl

NH
N

N

NO2

Olefine

HO
N
Cl

N

NH
N

Imidacloprid

NO2

N

N

OH
N

NH

N
Cl

HO

(OH)

Cl

N

N

NO2

NH
NO2

4,5-Dihydroxy5(4)-Hydroxy
conj.
O
N
Cl

N

NH
N

NO2

5-Hydroxy-conjugate

Figure 1. Metabolism of imidacloprid (I): ethylene-bridge hydroxylation of the imidazolidine ring and elimination ofwater.

37

NH
N
Cl

NH

N

N
NO2
Imidacloprid(Nitroguanidine)
N

Cl

N

N

Cl

NO

Nitrosimine

N
Cl

N

NH

NH

N

N

Cl

NH2

Cl

N

NH
Bound residues

Urea

O

N

N

N

O

N

NH2

Ring-open-guanidine

N

Aminoguanidine

NCl

Guanidine

NH

N

N
H

NH

N

Triazinone

Figure 2. Metabolism of imidacloprid (II): nitro-group reduction to nitrosimine and further loss of NO to form guanidine.

HO
O
Cl

O
OH

N

HO

OH
OH

O
Cl

O

O

N
Gentiobioside

Glucoside

OH
OH
HO

OH
OH

O
OH

O
N
Cl

N

N

Imidacloprid

Cl
NO2

OH

OH

NH

Cl

N

6-Chloropicolyl alcohol(6-CPA)OCl

Bound residues

N

6-Chloronicotinic acid
(6-CNA)

conj.

N

Figure 3. Metabolism of imidacloprid (III): Oxidative cleavage of the methylene bridge to form 6-chloropicolyl alcohol and subsequent oxidation to 6-chloronicotinic acid.
In figure 1 the ethylene-bridge hydroxylation at theimidazolidine (dihydroimidazole) ring of the a.s. leads
to the formation of the hydroxy-metabolite, whichmainly undergoes a subsequent dehydration to form theolefine-compound. Secondly, as depicted in figure 2, a
nitro-group reduction takes place to form the nitrosimine compound. In a further step, after loss of the
NO-group the cyclic guanidine and urea metabolites areformed. Starting with nitrosimine a cyclisation with en38

dogeneous pyruvic acid (created in respiration during
glycolysis) to the triazinone metabolite via a supposed
but not found aminoguanidine intermediate occurs.
However, minute amounts of this compound were foundonly in potato leaves. Thirdly, imidacloprid is oxidized
to 6-chloropicolyl alcohol and in turn to 6chloronicotinic acid. In addition, alcohol conjugateswith glucose and isomaltose to glucopyranoside andgentiobioside, respectively, are also formed (figure 3).

In the reproductive organs of seed-treated crop plants,
only very low amounts of imidacloprid metabolites
were detected. Based on the quantitative aspect the 5hydroxy-, olefine-, dihydroxy-, urea- and 6-CNAmetabolite were most commonly found in these parts ofseed-dressed crop plants.
It has to be mentioned that the findings of the metabolism and translocation experiments reflect worst-casescenarios and that under practical field conditions lower
amounts of metabolites and a more rapid degradation
are observed. This is on the one hand due to UVphotolysis and weather conditions being only of negligible importance in greenhouse trials and the lowamount of soil in the plant boxes compared to the root
mass causing exaggerated uptake of imidacloprid.
As far as bee-relevant matrices are concerned, a metabolism study in sunflower after seed-dressing withimidacloprid WS 70 was carried out (Stork, 1999;
Schmuck et al., 2001). The applied amount was 0.79 mg
a.s./seed. During flowering, the nectar was collected
after sampling the female florets from the inflorescenceby tweezers and extracting the nectar using glass capillaries. Pollen samples were collected in plastic boxes
fixed below the inflorescences before flowering. The
total radioactive residue (TRR) in nectar and pollen wasvery low and amounted to 0.0019 and 0.0039 mg/kg,
expressed as parent compound equivalents, respectively.
In nectar 100% of the TRR consisted of imidacloprid. In
pollen, the whole extractable radioactivity (85.8%,
0.0033 mg/kg) represented imidacloprid. The remaining
radioactivity in pollen (= non-extractable residues) was
not further analysed due to the very low absoluteamount. As only imidacloprid was found in the metabolism study, this was considered as the only relevant
residue in those sample materials and consequently, theresidue definition was expressed as “parent only”.
Field-residue trials with non-labelled imidacloprid on
sunflower, corn and rape were additionally carried out
to confirm the findings of the metabolism study underpractical conditions (Schmuck, 1999). The samples ofthese studies were not only analyzed for imidacloprid
but also for hydroxy- and olefine-metabolites, as thesecompounds also show biological activity against certain
insect species (Nauen et al., 1999; Nauen et al., 2001).
The results of all six field trials are compiled in table 3.
Neither in pollen nor in nectar samples of corn, sunflower and summer rape any residues of imidacloprid
and the two other metabolites have been determined
above the limit of quantitation (LOQ of imidacloprid
and hydroxy-imidacloprid = 0.005 mg/kg, LOQ of olefine metabolite = 0.01 mg/kg).
Conclusions
More than 15 studies with imidacloprid have been carried out concerning uptake, translocation and metabolism in various plant species mainly after foliar, soil orseed treatment. The uptake after soil or seed treatment isabout 5% of the applied dose and the a.s. shows good
acropetal mobility within the xylem and poor basipetal
translocation within the phloem. Three principal meta-

bolic pathways of imidacloprid in plants were identified
showing a quick degradation of the a.s., especially after
seed or soil application. The findings of the metabolism
studies show a clear and consistent picture. It can beconcluded that in nearly all crops the metabolic pathwayof imidacloprid runs via the same three routes and results in qualitative and quantitative similar compositionof the metabolic spectrum. All identified metabolites
still contain the 6-chloropicolyl moiety of imidacloprid.
Hence, the relevant residue to be analysed in field residue trials can be defined as the sum of imidacloprid andits metabolites containing the 6-chloropicolyl moiety,expressed as imidacloprid.
The nature of the residue in bee-relevant matrices ofoilseeds (sunflower nectar and pollen) was determined
to consist of the parent compound imidacloprid only.
Field residue trials with imidacloprid after seed-dressingof sunflower, corn and rape revealed that no residues
above the limit of quantitation of the residue analyticalmethod were present in pollen and nectar.
Acknowledgements
Thanks are expressed to all persons having been involved in the successful development of imidacloprid.

References
BAI D., LUMMIS S. C. R., LEICHT W., BREER H., SATTELLE D.
B., 1991.- Actions of imidacloprid and a related nitromethylene on cholinergic receptors of an identified insect motor
neurone.- Pestic. Sci., 33: 197-204.
BRIGGS G. G., BROMILOW R. H. AND EVANS A. A., 1982.- Relationships between lipophilicity and root uptake and translocation of non-ionised chemicals by barley.- Pestic. Sci.,
13: 495-504.
BROMILOW R. H., CHAMBERLAIN K., 1989.- Designing molecules for systemicity.- In: Mechanisms and regulation ofTransport Processes (ATKIN R.K., CLIFFORD D.R., Eds),
Monograph 18, British Plant Growth Regulator Group.
CLARK T., 1997.- Uptake of NTN 33893 in Phacelia andSummer Rape.- Unpublished report no. 4293, Bayer CropScience AG, Metabolism/Environmental Fate, Monheim,
Germany.
CLARK T., BRAUNER A., 1994.- Metabolism of NTN 33893 in
Tobacco.- Unpublished report no. 3997, Bayer CropScience
AG, Metabolism/Environmental Fate, Monheim, Germany.
DRÄGER G., BRAUNER A., BORNATSCH W., 1989.- NTN
33893: Metabolism in tomatoes.- Unpublished report no.
3257, Bayer CropScience AG, Metabolism/Environmental
Fate, Monheim, Germany.
DRÄGER G., BORNATSCH W., BRAUNER A., 1992.- Study on the
Metabolism of NTN 33893 after Spray Application to Potatoes.- Unpublished report no. 3678, Bayer CropScience AG,
Metabolism/Environmental Fate, Monheim, Germany.
ELBERT A., NAUEN R., LEICHT W., 1998.- Imidacloprid, anovel chloronicotinyl insecticide: biological activity and agricultural importance.- In: Insecticides with novel modes ofaction: Mechanism and application (ISHAAYA I., DEGHEELE
D., Eds), Springer Verlag, Berlin Heidelberg, Germany, 5074.
KÖSTER J., BORNATSCH W., BRAUNER A., 1988.- Metabolismof [pyridinyl-14C-methyl]NTN 33893 in Potato, Wheat and39

Corn Cell Suspension Cultures.- Unpublished report ID M
1710181-9,
Bayer
CropScience
AG,
Metabolism/Environmental Fate, Monheim, Germany.
KÖSTER J., 1990.- Comparative Metabolism of [pyridinyl14C]NTN 33893 in plant cell suspension cultures.- Unpublished report no. 3667, Bayer CropScience AG, Metabolism/Environmental Fate, Monheim, Germany.
KUROGOCHI S. AND ARAKI Y., 1989.- Isolation and Identification of Metabolites of NTN 33893 in Rice by Water Culture.- Unpublished report no. 1282, Nihon Bayer Agrochem
K.K., Yuki Research Center, Environmental Science Research, Yuki, Ibaraki, Japan.
KUROGOCHI S., MARUYAMA M., ARAKI Y., 1989.- Absorption
and Translocation of 14C-NTN 33893 in Eggplants and Rice
Plants.- Unpublished report no. 1273, Nihon Bayer Agrochem K.K., Yuki Research Center, Environmental Science
Research, Yuki, Ibaraki, Japan.
NAUEN R., TOLLO B., TIETJEN K., ELBERT A., 1998.- Antifeedant effect, biological efficacy and high affinity bindingof imidacloprid to acetylcholine receptors in Myzus persicae
and Myzus nicotianae.- Pestic. Sci., 51: 52-56.
NAUEN R., RECKMANN U., ARMBROST S., STUPP H.-P., ELBERT
A, 1999.- Whitefly-active metabolites of imidacloprid: biological efficacy and translocation in cotton plants.- Pestic.
Sci., 55: 265-71.
NAUEN R., EBBINGHAUS-KINTSCHER U., SCHMUCK R., 2001.Toxicity and nicotinic acetylcholine receptor interaction of
imidacloprid and ist metabolites in Apis mellifera (Hymenoptera: Apidae).- Pest. Manag. Sci., 57: 577-586.
RECKMANN U., 1998.- Translocation of 14C-imidacloprid in
hop after stem application (translated title; report available
in German language).- Unpublished report no. RMU 501,
Bayer CropScience AG, Formulation Development, Monheim, Germany.
SAKAMOTO H., 1991.- Metabolism of [pyridyl-14C-methyl]
NTN33893 in Rice Plants (Nursery Box Application).- Unpublished report no. 1284, Nihon Bayer Agrochem K.K.,
Yuki Research Center, Environmental Science Research,Yuki, Ibaraki, Japan.
SCHMUCK R., 1999.- No causal relationship between Gauchoseed dressing in sunflowers and the French bee malady.Pflanzenschutz-Nachrichten Bayer, 52(3): 267-309.
SCHMUCK R., SCHÖNING R., STORK A., SCHRAMEL O., 2001.Risk posed to honeybees (Apis mellifera L, Hymenoptera)by an imidacloprid seed dressing of sunflowers.- Pest.
Manag. Sci., 57: 225-238.

40

SCHMUCK R., NAUEN R., EBBINGHAUS-KINTSCHER U., 2003.Effects of imidacloprid and common plant metabolites ofimidacloprid in the honeybee: toxicological and biochemical considerations.- In: Proceedings of the 8th International
Symposium “Hazards of pesticides to bees”, September 46, 2002, Bologna, Italy (PORRINI C., BORTOLOTTI L., Eds).
Bulletin of Insectology, 56 (1): 27-34.
STEIN-DÖNECKE U., FÜHR F., WIENECKE J., 1989.- Container
tests with the insecticidal active ingredient NTN 33893 concerning uptake, translocation and action in wheat and sugar
beet plants.- Internal report IRA 8/89, Institute for Radioagronomy, Research Centre Jülich.
STORK A., 1999.- Residues of 14C-NTN 33893 (Imidacloprid)in Blossoms of Sunflower (Helianthus annuus) after Seed
Dressing.- Unpublished report no. MR-550/99, Bayer CropScience AG, Metabolism/Environmental Fate, Monheim,
Germany.
VOGELER K., DRÄGER G., 1989.- Investigations on the metabolism of NTN 33893 in corn.- Unpublished report no.
3256, Bayer CropScience AG, Metabolism/Environmental
Fate, Monheim, Germany.
VOGELER K. AND BRAUNER A., 1993.- Addendum to NTN
33893 Cotton Report PF No.: 3675, Metabolism of NTN
33893 in Cotton after Seed Treatment.- Unpublished report
no.
3675,
Bayer
CropScience
AG,
Metabolism/Environmental Fate, Monheim, Germany.
VOGELER K., DRÄGER G., BRAUNER A., 1991.- Investigation of
the metabolism of NTN 33893 in potatoes following granular application.- Unpublished report no. 3628, Bayer CropScience AG, Metabolism/Environmental Fate, Monheim,
Germany.
VOGELER K., CLARK, T., BRAUNER, A., 1992.- Metabolism of[14C]NTN 33893 in Apples.- Unpublished report no. 3676,
Bayer CropScience AG, Metabolism/Environmental Fate,
Monheim, Germany.
VOGELER K., LINKE-RITZER P., BRAUNER A., 1992.- [Pyridinyl-14C-methyl]NTN 33893 Residues in Rotational
Crops.- Unpublished report no. 3674, Bayer CropScience
AG, Metabolism/Environmental Fate, Monheim, Germany.
YOSHIDA H., 1992.- Metabolism of NTN 33893 in eggplant byplanting hole application.- Unpublished report no. 1290, Nihon Bayer Agrochem K.K., Yuki Research Center, Environmental Science Research, Yuki, Ibaraki, Japan.

Corresponding author: Robin SUR, Bayer CropScience
AG, Metabolism/Environmental Fate, Alfred-Nobel Strasse
50, 40789 Monheim, Germany.
E-mail: robin.sur@bayercropscience.com