Assessment of Dissipation and Pre-harvest Interval of Alpha-cypermethrin and Acetamiprid Pesticide Residues in Kales and Spinach in Njoro, Nakuru County, Kenya

Abstract

This study evaluated the dissipation rate of alpha-cypermethrin and acetamiprid when applied to kales and spinach leaves in relation to their specified pre-harvest intervals as recommended by their manufacturers. The objectives of the study were; to determine the pre-harvest interval for alpha-cypermethrin and acetamiprid pesticides in the kales and spinach leaves; to determine the dissipation rate of alpha-cypermethrin and acetamiprid pesticides residue levels in kales and spinach leaves; and to determine the dissipation rate of alpha-cypermethrin and acetamiprid pesticides residue levels in soil from kales and spinach cultivated farms. A supervised field trial was conducted in Njoro Sub-County on measured 3x1.5 m2 area plots. The pesticides formulations were purchased from licensed agrochemical shops and were under the list of Pest Control Products Board (PCPB) registered products in Kenya. Pesticide application was done with respect to the manufacturers recommended rates in dry and wet seasons 2021. After the application of pesticides, leaves samples of the spinach and kales vegetables were collected 30 minutes after the application, this represented day 0 and the rest after an interlude of one day, three days, seven days and fourteen days. The soil samples were also collected 30 minutes after the pesticide’s application and at similar time intervals. Sample preparation and extraction of the pesticide residues from the vegetable leaves and the soil were done using EPA method 1699. Analysis was done thereafter using Gas Chromatography/Mass Spectrometry (GC/MS) and Liquid Chromatography/ Mass Spectrometry (LC/MS) to ascertain the precise concentration of alpha-cypermethrin and acetamiprid respectively, on different days following application. The half-life of the alpha-cypermethrin enantiomers {(1R-cis) S and (1S-cis) R} were calculated using Single First Order (SFO) kinetic equations. (1R-cis) S dissipated faster during the wet season, in comparison to the dry season in the different matrices (kales, spinach, and soil). In the rainy season, the amount of (1R-cis) S in kales greatly reduced from day 0 to day 1. On day 1, (1R-cis) S residue was 1.47±1.22 ng/g which was 99.17% reduction from day zero concentration (177.18±1.67 ng/g). From day 3 onwards, no (1R-cis) S residues were detected, up to day 14 in the wet season. (1R-cis) S was more persistent in the kale samples, during the dry season. There were residues (8.61±1.69 ng/g) on day 3. No (1R-cis) S residues were detected on day 7 and 14 for both seasons in the kale samples. (1S-cis) R dissipated faster during the rainy season, in comparison to the dry season. In the wet season, the half-life of (1S-cis) R was 0.19, 0.16 and 0.33 days for the kale, spinach and soil samples respectively. In the dry season, the half-life of (1S-cis) R isomer was 1.5 days for the kale samples, 0.71 days for the spinach and 0.36 days for the soil. The alpha-cypermethrin levels in both kale and spinach were within the established Codex Alimentarius MRL value of 0.7 mg/kg for both the wet and dry seasons. Alpha-cypermethrin residues on vegetables exceeding the set MRL values is not likely to arise when farmers implement good agricultural practices. Hence the need to sensitize farmers on good agricultural practices. The dissipation rate of alpha-cypermethrin enantiomers in soil was slower in the dry season than the wet season which could be attributed to surface runoff and leaching