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Residual Behaviour of Imidacloprid in the Country Bean Growing Soil

H. Rahman
Department of Entomology, Bangabandhu Sheikh Mujibur Rahman Agricultural University (BSMRAU)

T. Akter
Department of Entomology, Bangabandhu Sheikh Mujibur Rahman Agricultural University (BSMRAU)

H. K. Mishu
Department of Entomology, Bangabandhu Sheikh Mujibur Rahman Agricultural University (BSMRAU)

R. U. Miah
Department of Entomology, Bangabandhu Sheikh Mujibur Rahman Agricultural University (BSMRAU)

M. S. Alam
Department of Soil science, BSMRAU, Bangladesh

Abstract/ Executive Summary:

The assessment of residual behavior of imidacloprid (Admire 20SL) in the soil of country bean agroecosystem and its risk assessment for consumption was studied. QuEChERS method was used for the extraction and clean-up of samples and the residues of imidacloprid was estimated using Gas Chromatography. The dissipation studies in the soil system were carried out by application of imidacloprid at five different dosages i.e. 100, 200, 300, 400, and 500 g a.i. ha^-1. Average initial deposits of imidacloprid were found to be 0.99, 1.33, 1.62, 1.83 and 2.20 mg.kg^-1. The residues reached below determination limit (BDL) of 0.01 mg kg^-1 in 9 days for recommended dose and 12 days for remaining higher dosages. Half-life (T1/2)) of imidacloprid in the soil was observed to be 1.88, 1.74, 1.73, 1.56 and 1.52 days for 100, 200, 300, 400, and 500 g a.i. ha^-1, respectively. The chemodynamics study of imidacloprid spray indicated that only 27.20 -28.40% sprays were deposited to the target site (plant canopy) and 71.60-72.80% were lost to non-target site such as soil and air. The drift of imidacloprid to soil (39.47-40.20%) was higher than the air (32.13- 32.60%). Based on degradation pattern and maximum, the recommended preharvest interval (PHI) might be eight 9 days and a waiting period of two days might be suggested for reapplication of the imidacloprid in country bean agroecosystem.

Keywords: Imidacloprid, Residue, Soils, Country bean, Agroecosystem

Location: The pesticide and environmental toxicology laboratory of Bangabandhu Sheikh Mujibur Rahman Agricultural University (BSMRAU), Gazipur

Start Date: 00-11-2016

End Date: 00-10-2017

Program Area: Resource Development and Management

Commodity: Country bean, Soil fertility

Objectives:

The present investigation has been planned and designed to carry out in depth study on degradation pattern and risk assessment of imidacloprid in the soils of country bean agroecosystem.

Methodology:

The study was conducted during November 2016 to October 2017. Field experiment was done in the Entomological farm and laboratory analysis was done in the pesticide and environmental toxicology laboratory of Bangabandhu Sheikh Mujibur Rahman Agricultural University (BSMRAU), Gazipur. The experimental site is located at Madhupur Tract (24°09' N latitude and 90°26' E longitude) having an elevation of 8.2 m from sea level.
Field experiment: Country bean crop was raised following recommended agronomic practices (Mondal, 2015). Imidacloprid (Admire 20 SL) was applied in the soil during flowering stage of the plant @ 100, 200, 300, 400 and 500 g a.i. ha-1 with a control. About 250 g soil sample was collected randomly at 0 (1h), 1, 3, 6, 9 and 12 days after the application of the insecticide. The country bean grown soil samples were collected from each plot separately, packed in polyethylene bags and brought to the laboratory for processing. Samples were extracted and cleaned up immediately after sampling.
Chemodynamics study
Determination of Imidacloprid in soil: For determining insecticide application loss in soil, a polythene sheet (2.25 m²) was initially placed randomly in the 1.5 m² plot. Then the foam sheet (1.25 cm thickness) of the same size (weighed) was placed on the polythene sheet to avoid loss of applied insecticide to the ground. Immediately after application of insecticide, the foam was weighed by electric balance. Insecticide application loss in soil was determined by subtracting the initial weight from the final weight of the foam sheet.
Determination of Imidacloprid loss in air: The plots (1.5 m²) were housed by the polythene sheet. Underside of the roof of the house, 2.25 m² foam sheet (weighed) was placed randomly. Immediately after application of insecticide, final weight of the foam sheet was taken by electrical balance and loss of insecticide in the air was measured from the difference between final and initial weight of foam sheet.
Preparation of standard solution: A standard stock solution of imidacloprid (1 mg. L^-1) was prepared in GC grade acetonitrile. The standard solutions required for constructing a calibration curve (10, 20, 30 ngL^-1) were prepared from stock solution by serial dilutions with GC grade acetonitrile. All standard solutions were stored at 4 °C before use.
Instruments: The quantification of imidacloprid residues was done by using Gas Chromatograph (GC) equipped with Electron Capture Detector (ECD), The residues of imidacloprid were confirmed using the standard chromatogram and retention time.

An aliquot was injected into the GC with auto injector. Tentative identification of the suspected insecticide was carried out in relation to retention times of the pure analytical standard.
Sample preparation, extraction and cleanup: A standardized analytical method with slight modification was followed for extraction of imidacloprid residues as reported by Sharma (2013). The soil samples were prepared following QuEChERS method for the determination of imidacloprid residues.
The collected soil samples (250g) were dried under the sun. A sub sample of 10 g of soil was weighed into a 250-mL conical flask and then 20 mL acetonitrile was dispensed into it. The sample was homogenized using high speed homogenizer (Heidolph Silent Crusher-M®) for 2-3 min at 1400-1500 rpm to homogenize soil sample. 4 g MgSO₄ and 1 g Sodium chloride (NaCI) was added to homogenize the sample for phase separation. Then the sample solution was covered with parafilm as acetonitrile is volatized in nature. The contents were centrifuged at 3300 rpm for 5 min. An aliquot of 20 ml acetonitrile layer was transferred in a vial and then stored in 4°C. The acetonitrile extract was then subjected to suspension mixture of primary secondary amine (PSA) + MgSO₄ to clean up the sample. An aliquot of acetonitrile was transferred into a separatory funnel and n- hexane was added to the aliquot and vigorously shaken for 1 minute. The layers were allowed to separate. Then acetonitrile was drained out and n-hexane remained in the separatory funnel. Then sulfuric acid was added to hexane so that microorganisms were killed. After 1-1.5 minutes, sulfuric acid drained out of the separatory funnel and distilled water added to the hexane to remove sulfuric acid from the aliquot. The aliquot was drained out through the bed of anhydrous sodium sulphate with a filter paper.

Source of Information: Bangladesh J. Agril. Res. 44(1): 89-101, March 2019 ISSN 0258-7122 (Print), 2408-8293 (Online)

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Achievement/Findings:

The results also indicated that the rate of imidacloprid deposit, out of the target site varied differently. The percentage (%) loss of Imidacloprid in air was much lower than that of soil. But their loss was not greatly differed with different doses of application. Loss of applied imidacloprid through drifting to the air and soil at maximum mature stages of country bean was found as 32.13-32.60% and 39.47- 40.20%, respectively. Loss of sprayed imidacloprid in soil through drifting was higher than the air and target site (plant canopy).

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