The use of pesticides continues to be the most cost-effective and efficient approach to controlling pests in farming systems. The growing global population has placed significant pressure on agricultural systems, driving increased demand for pesticides to maintain both the quality and quantity of food crops. Pesticides, which are chemical agents specifically designed to eliminate plant and animal pests, play a crucial role in enhancing crop yields and ensuring food security in many nations [1] - [3] .

Pesticides are intentionally used in agriculture and public health to safeguard crops and livestock from diseases caused by pests [4] - [6] . In Zimbabwe, pesticides like organophosphates, carbamates, and neonicotinoids are commonly used to control various pests. Farmers typically apply pesticides to farmland through spraying and aerial drifts disperse and transports these agrochemicals long distances. Consequently they end up in aquatic and terrestrial environments and exert toxic effects on living organisms in these ecosystems. Most farmers in Zimbabwe, particularly those in the small-scale category, practice monoculture, which promotes the extensive use of pesticides to avoid productivity losses due to diseases.

Extensive use of agrochemicals negatively impacts living organisms in their environments. Al-Ghanim et al. [5] demonstrated that fenvalerate induces oxidative stress in zebrafish, significantly reducing the activity of antioxidant enzymes such as superoxide dismutase and catalase. Similarly, organophosphorus insecticides have been found to be toxic to fish, causing internal organ damage, immunotoxicity, and endocrine dysfunction [7] . Wee and Aris, [8] reported developmental defects and oxidative stress effects in zebrafish exposed to chlorpyrifos. Despite the significant increase in pesticide use in Zimbabwe’s agro-land, there is insufficient research on the effects of these agricultural chemicals on both aquatic and terrestrial ecosystems.

Research on the effects of agrochemicals, including thiamethoxam a pesticide widely used in Zimbabwe for its effectiveness during drought conditions is highly limited. Regular monitoring of pesticide levels and their impacts on terrestrial and freshwater ecosystems is crucial to safeguard the health of both aquatic and terrestrial organisms. This study aimed to evaluate the effects of three commonly used pesticides in Zimbabwe on freshwater ecosystems, using esterase activity in the water snail Helisoma duryi as a biomarker of exposure.

All the pesticides tested caused significant time-dependent reductions in the activities of non-cholinesterase enzymes ( Figure 1 ) and cholinesterase enzymes ( Figure 2 ) in the freshwater snails Helisoma duryi exposed to the organophosphate, carbamate, and neonicotinoid. For all pesticides, cholinesterase inhibition observed in snails exposed for 28 days was greater than the inhibition recorded in snails exposed for 14 days. Organophosphates are well-known to reduce acetylcholinesterase activity in both aquatic and terrestrial organisms [13] - [15] . In fact, cholinesterase inhibition is widely used as a biomarker of exposure to organophosphorus pesticides. In this study, profenofos inhibited cholinesterase activity by up to 70% and 80% after 14 and 28 days of exposure, respectively ( Figure 4 ). Similar findings were reported by Gagnaire et al. [16] who observed reduced cholinesterase activity in the freshwater snails Potamopyrgus antipodarum and Valvata piscinalis exposed to chlopyrifos. Kumar et al. [17] found a 30% - 50% reduction in cholinesterase activity in Paratya australiensis exposed to profenofos, while Nillos et al. [18] reported decreased acetylcholinesterase activity in Daphnia magna exposed to the pesticides profenofos, fonofos and crotoxyphos. Khalil [19] observed a 78% reduction in acetylcholinesterase activity in snails exposed to 0.29 ppb of the organophosphate chlorpyrifos for 28 days. Additional confirmation of the anticholinesterase effects of organophosphates is provided by Duarte-Restrepo et al. [20] who noted reduced acetylcholinesterase activity in shrimps exposed to chlorpyrifos.

Neonicotinoids, which are neurotoxic chemicals targeting the central nervous systems of pest insects, also inhibit cholinesterase [21] [22] . Shaker et al. [23] reported significant cholinesterase inhibition in snails exposed to thiamethoxam and imidacloprid. Similarly, Vehovszky et al. [24] documented reduced cholinergic neurotransmission in Lymnaea stagnalis exposed to neonicotinoids. In this study, thiamethoxam caused up to a 74% reduction in cholinesterase activity in Helisoma duryi. Ewere et al. [25] also reported significant cholinesterase inhibition in snails exposed to neonicotinoids.

Carbamates have been identified as anticholinesterase agents [13] [26] . Khalil [19] observed reduced cholinesterase activity in the land snail Eobania vermiculata exposed to the carbamate methomyl lannate. Gohary et al. [27] reported similar reductions in cholinesterase activity in Monacha cantiana and Eobania vermiculata exposed to Mesurol 75-W.

In this study, the highest cholinesterase and carboxylesterase inhibition was observed in snails exposed to the carbamate carbofuran. The degree of enzyme inhibition decreased over time (Figures 1 and 2), likely due to the depletion of unbound enzymes. A comparison of the three pesticides revealed the order of potency in inhibiting enzyme activities to be carbofuran > profenofos > thiamethoxam. In contrast, Yadav et al. [28] found chlorpyrifos to be more potent than carbofuran in reducing acetylcholinesterase activity in Eisenia fetida earthworms, suggesting differing trends among pesticides.

All snails transferred to pesticide-free water after exposure showed signs of recovery, as evidenced by reduced inhibition of enzyme activities ( Figure 3 and Figure 4 ). Snails exposed to carbofuran displayed 75% inhibition after 14 days, increasing to 90% after 28 days of exposure, and only recovered less than 8% in the 28 day-duration in pesticide-free water ( Figure 3 and Figure 4 ). The slower recovery of enzymatic activity observed in snails exposed to carbofuran, even after a 28-day period in pesticide-free water, compared to the recovery of enzymatic activity in those exposed to profenofos or thiamethoxam, may be attributed to the specific mode of action and persistence of carbofuran as a carbamate pesticide. In Helisoma duryi, the carbamylation of enzymes likely results in the formation of more stable bonds that take longer to dissociate, thereby delaying enzymatic recovery. Also, the Helisoma snails probably have a higher sensitivity and affinity for carbofuran such that even at low concentrations of 0.5 ppb, extended of 28 days led to extensive binding, which may have overwhelmed the snails’ detoxification and repair mechanisms resulting in the markedly low recovery of acetylcholinesterase and carboxylesterase activities observed.

Recovery in non-carbamate pesticide-exposed snails was nearly complete, with less than 10% inhibition observed after 28 days in clean water. The recovery patterns observed in organophosphate-exposed snails align with studies on aquatic mollusks [29] and fish [30] [31] , where acetylcholinesterase activity inhibited by organophosphates required 2 - 4 weeks to return to near-control levels. This recovery likely results from de novo synthesis of the enzyme, as de-phosphorylation at the enzyme’s active site is slow and considered essentially irreversible. In contrast, Dhainaut and Scaps [32] attributed recovery from carbamate or organophosphate exposure to the gradual release of free enzymes through aging of the enzyme-pesticide complex.

Despite recovery, snails exposed to carbamates exhibited 60% and 85% inhibition of carboxylesterase and cholinesterase activities, respectively, after 28 days. The slow recovery suggests a highly stable enzyme-pesticide complex that decarbamylates slowly, necessitating de novo enzyme synthesis. Recovery may also involve adaptations such as increased production of detoxifying enzymes like glutathione S-transferase, as suggested by Hernandez et al. [33] . Declining pesticide concentrations in clean water may further facilitate recovery by diluting residual pesticide levels within the snails.

Pesticides commonly used in agricultural pest control often enter aquatic ecosystems through aerial drift and leaching, where they can significantly impact the health of aquatic organisms. In this study, the pesticides profenofos, carbofuran, and thiamethoxam commonly used in Zimbabwean agriculture were found to significantly inhibit carboxylesterase and cholinesterase activities in the freshwater snail Helisoma duryi, with the degree of inhibition increasing over time. Among the tested pesticides, carbofuran (a carbamate) exhibited the strongest anti-esterase effects, followed by the organophosphate profenofos and the neonicotinoid thiamethoxam. Notably, Helisoma duryi snails showed partial to substantial recovery of enzyme activity after being transferred to pesticide-free water, indicating some capacity for physiological recovery and detoxification over time.

These findings highlight the ecological risks associated with pesticide contamination in freshwater ecosystems, particularly in light of their widespread use and the potential for runoff from agricultural areas. The study also supports the use of esterase activity in Helisoma duryi as a valuable biomarker for detecting pesticide exposure and toxicity in aquatic environments. However, the study had several limitations: First, it was conducted under controlled laboratory conditions, which may not fully capture environmental complexities such as temperature variations and pollutant levels. Second, the study focused solely on one biomarker endpoint (sublethal enzymatic effects) without evaluating other important biological parameters such as survival, behaviour and reproduction, which are critical for assessing population-level impacts. Third, only one freshwater snail species was used, limiting the generalizability of the findings to other aquatic organisms.

Future research could address these limitations by:

1) Conducting field studies to assess real-world exposures and validate laboratory findings under natural conditions, including additional biomarkers such as antioxidant enzyme activities, genotoxicity, histopathological changes, and reproductive toxicity.

2) Investigating the effects of pesticide mixtures and examining chronic and multigenerational impacts of pesticide exposure on Helisoma duryi and other representative aquatic species.

This research was supported by the International Programme in Chemical Sciences (IPICS), Uppsala University, Sweden and the Research Board, NUST, Bulawayo, Zimbabwe.