Exposure and Fate of Neonicotinoid Insecticides in Cocoa plantations in Ghana

Abstract

Neonicotinoids belong to the most important class of insecticides currently used in crop production and account for about a fourth of the insecticide market. They are considered highly effective against insect pests, safe to mammals, and possess multiple means of application for convenience and widespread usage in diverse crop protection. Neonicotinoids are systemic insecticides - they are absorbed into plants, travel through the vascular tissue, and help protect the plant from piercing and sucking insects. Their mode of action is through agonist activity at the postsynaptic nicotinic acetylcholine receptor (nAChRs) sites in the nervous system. In recent years, reports have emerged on possible harmful effects of neonicotinoid insecticides on bee health. Across Europe, America and Australia, there has been calls for ban and reevaluation of the use of neonicotinoid insecticides. In Europe, a ban has been in place for almost two years following a scientific report by the European Food Safety Authority (EFSA) which identified “high acute risks” for bees. However, the availability of conflicting reports on the effects of neonicotinoids on bees has prompted intense research on these class of insecticides. Quite clearly, understanding the effects of neonicotinoids on organisms in the ecosystem will rely on accurate knowledge of their exposure in the environment. In Ghana, neonicotinoids are one of the most widely used class of insecticides particularly in cocoa production, where they are important for the control of mirids. Insect pests are a major concern in crop production in tropical conditions, due to the prevailing conducive environment for their growth. In cocoa production, insect pests such as mirids contribute to significant loses in yield. To address this concern, the government of Ghana introduced a free mass application policy on insecticides in cocoa farming, which has contributed to substantial improvements in yields of cocoa beans. Under the program, neonicotinoids are the major class of insecticides used. In spite of the remarkable contribution of neonicotinoids to cocoa production, concerns about the environment and food safety have arisen due to widespread and intensive use of these chemicals in cocoa farms. Addressing these concerns requires accurate knowledge of the behavior and fate of these class of insecticides in the Ghanaian environment. The primary goal of this work was to assess the extent of exposure, behavior and fate of neonicotinoids in the Ghanaian environment, particularly in cocoa plantations where they are extensively used. To achieve this, concentrations of neonicotinoid residues in soils across all the cocoa producing regions of the country were studied to examine their environmental exposure. Their fate in soils was studied by investigating their dissipation and sorption behavior using established kinetic models and isotherms. Finally, the exposure to cocoa beans (food) was studied to ascertain concerns for food safety. Analytical methods and instrumentation was an important aspect of this work to ensure accurate and reliable data. To this end, the Quick, Easy, Cheap, Effective, Rugged and Safe (QuEChERS) procedure was optimised and used for the diverse matrixes understudy. Quantification of analytes involved the use of liquid chromatography- tandem mass spectrometry (LC-MS/MS) for high sensitivity and low detection limits in the complex soil and cocoa matrixes used. The findings from the study suggest that, neonicotinoids are persistent in the Ghanaian soils studied and may be found in soils several months to years after application. Sorption studies revealed that, sorption coefficients of neonicotinoids are generally low, with a high potential for leaching into surface and underground water systems. Due perhaps to their systemic nature and high application rates, neonicotinoids may accumulate to high levels in cocoa beans, particularly in cocoa shells. The findings from this study has implications for pesticide application and current policy on pesticide usage, and reveal that, efficient application regimes are needed in cocoa production to ensure food and environmental safety.