imidacloprid (ISO); 1-(6-chloropyridin-3-ylmethyl)-N-nitroimidazolidin-2-ylidenamine

Biodegradation in soil

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

biodegradation in soil: simulation testing

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Qualifier:

according to guideline

Guideline:

other: Guidelines for the Official Testing of Pesticides, Part IV, 4-1 (1986)

Version / remarks:

Version of 1986

GLP compliance:

yes (incl. QA statement)

Test type:

laboratory

Radiolabelling:

yes

Oxygen conditions:

aerobic

Soil classification:

DIN 19863 (Deutsche Industrie-Norm)

Soil no.:

#1

Soil type:

Silt

% Clay:

7.5

% Silt:

89.4

% Sand:

3.1

% Org. C:

1.23

pH:

5.5

CEC:

10.5 meq/100 g soil d.w.

Details on soil characteristics:

SOIL COLLECTION AND STORAGE
- Geographic location: Soil "Höfchen" was collected at a slope of the Bayer AG experimental farm Höfchen, West Germany
- Sampling depth (cm): The soil originated from the top layer.
- Pesticide use history at the collection site: The field had been in use as grassland for a period of more than 10 successive years with regular mulching. No pesticides were applied during this period.
- Storage conditions: In order to maintain the original level of biological activity the soil was kept in a wooden container with permanent vegetation (grass) until the start of the experiment. The biological activity of the soil was determined at the start and end of the experiment. The method is based on the determination of the soil generated carbon dioxide after addition of glucose.
- Storage length:
- Soil preparation (e.g., 2 mm sieved; air dried etc.): Prior to use the soil was carefully dried to allow it to pass through a 2-mm-sieve. Immediately before application of the test substance the moisture content of the soil was determined by drying several portions at 105°C for a period of 4 h.

PROPERTIES OF THE SOILS (in addition to defined fields)
- Moisture at 1/3 atm (%): 38.1
- The soil was classified as silt soil and "Schluff" according to the USDA and DIN 19682 texture analysis, respectively.

Soil No.:

#1

Duration:

100 d

Soil No.:

#1

Initial conc.:

0.36 mg/kg soil d.w.

Based on:

test mat.

Parameter followed for biodegradation estimation:

radiochem. meas.

Soil No.:

#1

Temp.:

20 ± 2 °C

Microbial biomass:

697 mg microbial carbon/kg

Details on experimental conditions:

EXPERIMENTAL DESIGN

Test material application
- Volume of test solution used/treatment: A portion of 860 µL of the above test solution (~ 3.1 MBq, ~ 0.56 mg a. i.) was added to 35 g of finely ground quartz sand in a mortar. This subsample was added to 2000 g of soil Höfchen and mixed well on a tumbling mixer.
- Application method (e.g. applied on surface, homogeneous mixing etc.): Portions of 100 g (dry weight) of the treated soil were weighed into sixteen 300 mL-Erlenmeyer flasks.

Experimental conditions (in addition to defined fields)
- Moisture maintenance method: The soil moisture was adjusted to 40 % of the maximum water capacity in all flasks. This level was maintained during the investigation.
- Continuous darkness: Yes. The samples were incubated at 20 ± 2 °C in a dark incubation chamber until sampling.


SAMPLING DETAILS

- Sampling intervals: Duplicate samples were taken out of the incubation chamber 1, 3, 7, 14, 30, 59 and 100 days post treatment. The sampling scheme and soil extraction procedure are outlined in Appendix 6 (see Table 1 in section "Any other information on materials and methods incl. tables").
- Method of collection of CO2 and volatile organic compounds: The flasks were connected to an appropriate carbon dioxide trapping system. The glass tube contained from bottom to top a plug of quartz wool soaked with paraffin oil (1 g of 2 % paraffin in hexane), a layer of 10 g soda lime, another plug of quartz wool and a layer of 2 g soda lime topped by quartz wool.

Soil No.:

#1

% Recovery:

99.5

Remarks on result:

other:

Remarks:

Recovery adter an incubation time of 100 days in replicate 1

Soil No.:

#1

% Recovery:

99.3

Remarks on result:

other:

Remarks:

Recovery adter an incubation time of 100 days in replicate 2

Soil No.:

#1

% Degr.:

6.3

Parameter:

CO2 evolution

Sampling time:

100 d

Remarks on result:

other:

Remarks:

replicate 1

Soil No.:

#1

% Degr.:

6.5

Parameter:

CO2 evolution

Sampling time:

100 d

Remarks on result:

other:

Remarks:

replicate 2

Soil No.:

#1

DT50:

248 d

St. dev.:

50

Type:

second order

Temp.:

20 °C

Soil No.:

#1

DT50:

526.5 d

St. dev.:

106.1

Type:

second order

Temp.:

12 °C

Remarks on result:

other:

Remarks:

Calculated DT50 based on experimental data at 20 °C

Transformation products:

not specified

Remarks:

Seven metabolites were found in small amounts. They were not further identified and quantified.

Volatile metabolites:

no

Remarks:

In all samples volatiles accounted for < 0.1% of the applied radioactivity

Details on results:

- Considering the average of duplicate samples the total recovery of radioactivity ranged from 98.7 to 103.6 % of the applied amount during the incubation period of 100 days.

- The rate of mineralization decreased slightly with time as was to be expected from the decrease in biomass to 57 % of the original value. During the total incubation period an equivalent of 6.4 % of the applied radioactivity was degraded to carbon dioxide. Only traces of volatiles other than carbon dioxide were detected (< 0.1 % of the applied radioactivity).

- The data gathered in the current laboratory investigation showed that NTN 33893 was degraded slowly in the silt soil Höfchen under aerobic conditions. The degradation and mineralization rate diminished with time as the microbial biomass of the soil decreased. The half life (DT-50) was calculated to be 248 ± 50 days. The major portion of the applied radioactivity was extracted at all sampling times and unchanged a. i. accounted for most of the extracted radioactivity. One hundred days post treatment an average of 66.8 % of the applied radioactivity (0.24 mg/kg soil) was identified to be NTN 33893. Several metabolites occurred at very low levels. Increasing amounts of radioactivity were bound to the soil during the incubation period. After 100 days 21.6 % of the applied radioactivity could not be extracted using the standard extraction procedure at room temperature. A portion of 8.5 % was released by refluxing with acetonitrile, 8.1 % of which were identified as NTN 33893. The data show that the insecticide Imidacloprid is degraded in soil to the final metabolite carbon dioxide. Due to the known limitations of the laboratory experiment the degradation rate reported here is not representative for the natural environment.

Table 1: Recovery of Radioactivity

 

	% of the applied radioactivity (198.978 kBq)
incubation time/days (sample no)	acetonitrile extract	dichloro-methane extract	water extract	not extracted residues	(reflux extract)*	volatiles	carbon dioxide	Total
0 (1) 0 (2)	91.2 89.5	4.7 5.0	1.2 1.3	3.4 3.6	n. a. n. a.	n. a. n. a.	n. a. n. a.	100.5 99.4
1 (3) 1 (4)	85.3 93.3	4.1 4.1	1.2 1.8	3.7 3.7	n. a. n. a.	< 0.1 < 0.1	0.1 0.1	94.4 103.0
3 (5) 3 (6)	89.0 90.8	4.4 4.4	0.7 0.8	5.8 5.9	n. a. n. a.	< 0.1 < 0.1	0.2 0.2	100.1 102.1
7 (7) 7 (8)	85.0 84.5	4.7 4.4	1.2 1.2	8.0 7.8	(2.3) (2.3)	< 0.1 < 0.1	0.7 0.7	99.6 98.6
14 (9) 14 (10)	86.4 84.9	5.0 4.3	1.1 1.1	11.0 10.3	(4.8) (4.5)	< 0.1 < 0.1	1.5 1.5	105.0 102.1
30 (11) 30 (12)	75.0 75.8	5.2 4.8	2.0 1.8	14.8 14.7	(3.8) (4.6)	< 0.1 < 0.1	2.7 2.6	99.7 99.7
59 (13) 59 (14)	70.7 70.7	5.5 6.1	1.0 0.9	18.3 18.2	(7.8) (8.7)	< 0.1 < 0.1	4.6 4.6	100.1 99.5
100 (15) 100 (16)	63.8 63.0	5.7 5.7	2.3 2.4	21.4 21.7	(8.2) (8.8)	< 0.1 < 0.1	6.3 6.5	99.5 99.3

*            this value is included in the not extracted residues

n. a.      not analyzed

Reference 2

Endpoint:

biodegradation in soil: simulation testing

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Qualifier:

according to guideline

Guideline:

other: Guidelines for the Official Testing of Pesticides, Part IV, 4-1

Version / remarks:

1986

GLP compliance:

yes (incl. QA statement)

Test type:

laboratory

Radiolabelling:

yes

Oxygen conditions:

aerobic

Soil classification:

other: DIN 19682

Remarks:

Textural class: sandy loam

Soil type:

sandy loam

% Clay:

13.2

% Silt:

30.7

% Sand:

56.1

% Org. C:

1.31

pH:

6.76

Parameter followed for biodegradation estimation:

radiochem. meas.

% Degr.:

2.7

Parameter:

radiochem. meas.

Sampling time:

100 d

DT50:

341 d

St. dev.:

153

Type:

(pseudo-)first order (= half-life)

Temp.:

20 °C

DT50:

723.9 d

St. dev.:

324.8

Type:

(pseudo-)first order (= half-life)

Temp.:

12 °C

Remarks on result:

other:

Remarks:

Calculated DT50 based on experimental data at 20 °C

Conclusions:

The degradation of NTN 33893 in soil was investigated in a laboratory study according to the general protocol of the respective BBA guidelines.
[Pyridinyl-14C-methylene]NTN 33893 was applied to the sandy loam Monheim 1. The samples were incubated in the dark at 20 ± 2°C and 40 % of the maximum water capacity of the soil under aerobic conditions. The application rate of 0.33 mg/kg was based on the recommended maximum use rate of 200 g/ha. Sampling times were 0, 1, 3, 7, 14, 35, 62, 100, 125, 183, 274 and 366 days.
The amount of radioactivity extracted at room temperature decreased gradually with time. After an incubation time of 100 days an average of 73.2 % of the applied dose was extracted. Parent compound accounted for 69.5 % of the applied radioactivity in the soil extracts 100 days posttreatment.
The statistical interpretation of the degradation behavior was based on the data obtained for the initial 100 days posttreatment. The degradation kinetics was described best by a first order root function. Extrapolation resulted in a half-life (DT-50) of 341 ± 153 days.
Slowly increasing amounts of radioactivity remained bound to the soil with increasing incubation time. After 100 days the bound residues represented an average of 25.1 % of the applied dose.
During the incubation time of 100 days up to 2.7 % of the applied radioactivity were transformed into the final metabolite carbon dioxide.
The average total recovery ranged from 98.6 to 103.1 % of the applied radioactivity in the course of the study.

Reference 3

Endpoint:

biodegradation in soil: simulation testing

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Qualifier:

according to guideline

Guideline:

other: EPA Pesticide Assessment Guidelines, Subdivision N: § 162-1(1982); BBA Ref.: Guideline IV, 4-1 (1986)

GLP compliance:

yes (incl. QA statement)

Test type:

laboratory

Radiolabelling:

yes

Oxygen conditions:

aerobic

Soil classification:

other: DIN 19682

Soil type:

loamy sand

% Clay:

7.6

% Silt:

16.5

% Sand:

75.9

% Org. C:

2.15

pH:

6.3

CEC:

10 meq/100 g soil d.w.

Parameter followed for biodegradation estimation:

radiochem. meas.

% Degr.:

10

Parameter:

radiochem. meas.

Sampling time:

100 d

DT50:

188 d

St. dev.:

25

Type:

second order

Temp.:

20 °C

DT50:

399.1 d

St. dev.:

53.1

Type:

second order

Temp.:

12 °C

Remarks on result:

other:

Remarks:

Calculated DT50 based on experimental data at 20 °C

Conclusions:

In a laboratory study [pyndinyl-14C-methylene]NTN 33893 was applied to the loamy sand soil BBA 2.2 and maintained aerobically in the dark at 20 ± 2°C. The application rate of 0.33 mg/kg was based on the recommended maximum use rate of 200 g/ha. Sampling times were 0, 1, 3, 7, 14, 35, 62 and 100 days.
The amount of radioactivity extracted at room temperature decreased gradually with time and represented 68.6 % of the applied radioactivity after an incubation time of 100 days. Parent compound accounted for 63.2 % of the applied radioactivity in the soil extracts 100 days post treatment. The degradation kinetics could be described best by a reaction order of 2. Extrapolation resulted in a half life (DT-50) of 188 ± 25 days.
The amount of radioactivity bound to the soil increased gradually with time and reached a maximum of 21.6 % of the applied. One hundred days post treatment 7.7 % of the applied radioactivity were released from the soil by reflux extraction, 7.4 % of which were identified as parent compound.
Six metabolites were identified by spectroscopic methods and comparison with authentic reference substances. One additional metabolite was detected by reverse isotope dilution analysis. Neither of them accounted for more than 2.2 % of the applied radioactivity at any time. The degradation of NTN 33893 in soil proceeded via cleavage and oxidation of the dihydro-imidazole ring and via loss of the nitro group from the intact heterocyclic ring to the key intermediate 6-chloronicotinic acid and the final metabolite carbon dioxide. During the incubation time of 100 days the equivalent of 10.0 % of the applied radioactivity was transformed into carbon dioxide.
The total recovery ranged from 99.4 to 103.8 % of the applied radioactivity in the course of the study. Not a single soil borne metabolite surpassed the concentration of 0.01 ppm in the soil: only the parent compound remained at a level above 0.01 ppm.

Reference 4

Endpoint:

biodegradation in soil: simulation testing

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Qualifier:

according to guideline

Guideline:

other: EPA Pesticide Assessment Guidelines, Subdivision N: § 162-1 (1982); BBA Ref.: Guideline IV, 4-1 (1986)

GLP compliance:

yes (incl. QA statement)

Test type:

laboratory

Radiolabelling:

yes

Oxygen conditions:

aerobic

Soil classification:

USDA (US Department of Agriculture)

Soil type:

sandy loam

% Clay:

6

% Silt:

27

% Sand:

67

% Org. C:

1.4

pH:

6.5

CEC:

16 meq/100 g soil d.w.

Bulk density (g/cm³):

2.6

% Degr.:

7.4

Parameter:

radiochem. meas.

Sampling time:

366 d

DT50:

> 1 yr

Type:

(pseudo-)first order (= half-life)

Temp.:

20 °C

Conclusions:

The metabolism of NTN 33893 in soil was investigated in a laboratory study according to the general protocols of the respective EPA and BBA Guidelines.
[Pyridinyl-14C-methylene]NTN 33893 was applied to a sandy loam from Kansas, USA. The samples were incubated in the dark at 20 ± 2°C and 75 % of 1/3 bar moisture level under aerobic conditions. The application rate of 0.33 mg/kg was based on the recommended maximum use rate of 200 g/ha. Sampling times were 0, 1, 3, 7, 14, 30, 59, 100, 120, 182, 274 and 366 days.
The amount of radioactivity extracted at room temperature decreased gradually with time and represented 66.9% of the applied radioactivity after an incubation time of 366 days. Parent compound accounted for 61.6 % of the applied radioactivity in the soil extracts 366 days posttreatment.
The amount of radioactivity bound to the soil increased gradually with time and attained a maximum of 25.6 % of the applied dose 274 days posttreatment.
A total of 7 metabolites was observed in the soil extracts along with parent compound. Six metabolites were identified by spectroscopic methods and comparison with authentic reference substances. One additional metabolite was detected by reverse isotope dilution analysis. The degradates represented a total of 3.4 % of the applied radioactivity after 366 days. No single degradate accounted for more than 1.7 % of the applied dose (0.006 ppm) at any time.
The degradation of NTN 33893 in soil proceeded via cleavage and oxidation of the dihydro-imidazole-ring and via loss of the nitro group from the intact heterocyclic ring to the key intermediate 6-chloronicotinic acid and the ultimate metabolite carbon dioxide. During the incubation time of 366 days the equivalent of 7.4 % of the applied radioactivity was transformed into carbon dioxide.
The average total recovery ranged from 94.4 to 98.9 % of the applied radioactivity in the course of the study.
The computation of the half-life was based on the initial 100 days of incubation. According to statistical interpretation of the data a first order root function proved to be the best fit. Extrapolation resulted in a half life (DT-50) > 1 year.

Description of key information

DT50 = 188 days to >1 year (399.1 days to >1 year, recalculated to 12 °C)

Key value for chemical safety assessment

Half-life in soil:

526.5 d

at the temperature of:

12 °C

Additional information

Six studies are available for the degradation of the substance in the soil compartment.

In the key study, performed according to GLP and “Guidelines for the Official Testing of Pesticides, Part IV, 4-1 (1986)” the fate of (2E)-1-[(6-chloropyridin-3-yl)methyl]-N-nitroimidazolidin-2-imine was investigated over a period of 100 days. The degradation rate of the substance was determined in silt soil type at 20 °C. The half-life of the substance was accounted for 248 ± 50 days at 20 °C (526.5 ± 106.1 days, recalculated to 12 °C). The substance attained 6.3 – 6.5% mineralization at test termination (100 days).

 

Another study following GLP and “Guidelines for the Official Testing of Pesticides, Part IV, 4-1” investigated the degradation of the substance in a sandy loam soil at 20 °C over a period of 366 days. The determined half-life was 341 ± 153 days (723.9 ± 324.8 days, recalculated to 12 °C). After 100 days 2.7% of the applied radioactivity were mineralized.

 

A third study following GLP and “EPA Pesticide Assessment Guidelines, Subdivision N: § 162-1(1982); BBA Ref.: Guideline IV, 4-1 (1986)” investigated the degradation of the substance in a loamy sand soil at 20 °C over a period of 100 days. The determined half-life was 188 ± 25 days (399.1 ± 53.1 days, recalculated to 12 °C). After 100 days 10.0% of the applied radioactivity were mineralized.

 

A fourth study following GLP and “EPA Pesticide Assessment Guidelines, Subdivision N: § 162-1 (1982); BBA Ref.: Guideline IV, 4-1 (1986)” investigated the degradation of the substance in a sandy loam soil at 20 °C over a period of 366 days. The determined half-life was >1 year (>1 year, recalculated to 12 °C). After 366 days 7.4% of the applied radioactivity were mineralized.

 

Two more studies investigated the fate of the substance in soil. The determined half-life ranged from 147 days to 410 days (recalculated to 12 °C).