Environmental Effects of Pesticides

Sweden’s agricultural landscapes are closely linked to rivers, lakes, and coastal zones. Most cultivated areas are also equipped with drainage systems that rapidly channel runoff from fields into nearby water bodies. Following application, pesticides can leach from agricultural land and enter these watercourses, sometimes reaching concentrations that may pose a risk to aquatic life. National environmental monitoring frequently detects multiple pesticides in surface waters. In many cases, more than ten different pesticides, or their breakdown products, are found simultaneously, as demonstrated in studies such as Lindström et al. (2015).  The simultaneous presence of several substances can result in mixture effects, where the combined toxicity can exceed the predicted effect of each pesticide on its own.

Pesticide concentrations in water can spike shortly after spraying events, especially following rainfall, leading to acute exposure that can harm sensitive aquatic species. In other cases, pesticides leach more slowly into water bodies, resulting in chronic exposure over weeks or even months. In some cases, legacy pesticides that are no longer approved for use still persist in the environment due to their chemical stability and continue to leach into watercourses. Moreover, pesticides rarely occur in isolation. 

Pesticides in water can have a wide range of effects on aquatic organisms, influencing their reproduction, development, behavior, and population dynamics. These impacts vary depending on the type of pesticide and the organisms exposed. For example, insecticides can significantly reduce populations of aquatic invertebrates such as mayflies and caddisflies—species that play a crucial role in freshwater food webs. Herbicides, on the other hand, can disrupt the growth of algae and aquatic plants, leading to changes in habitat structure, nutrient cycling, and oxygen availability.

Although water quality guidelines aim to minimize such risks, they do not capture the complex, real-world scenarios of pesticide behavior, interactions, and accumulation in ecosystems.

In agricultural landscapes, pesticides can easily be dispersed from fields into the surrounding soil and nearby environments, affecting a variety of organisms both directly and indirectly. One of the primary concerns is the impact on pollinators, including honey bees, bumblebees, moths, solitary bees, and other vital species. For example, neonicotinoid insecticides have been shown to impair bee navigation and foraging behavior, reduce colony growth, and increase mortality, raising concerns about their role in pollinator declines. The use of herbicides can reduce the availability of wildflowers and other plants that serve as essential sources of pollen and nectar, making it more difficult for pollinators to find adequate food and impacting the overall quality of the food available to them.

Soil organisms, which play a critical role in nutrient cycling and maintaining soil health, can also be significantly affected by repeated pesticide exposure. Some pesticides disrupt microbial communities, while others reduce the populations of earthworms and beneficial insects. Birds and mammals are indirectly exposed to pesticides through the food chain, often by consuming contaminated insects or seeds. This exposure can lead to subtle but important changes in health, behavior, and reproductive success.

The cumulative impact of these effects is often linked to long-term declines in biodiversity and ecosystem resilience, as the loss of key species can disrupt ecological processes that are vital for maintaining the balance of natural systems. The result is an ecosystem that is less capable of withstanding environmental stressors and less adaptable to change.

The environmental risks of pesticides became clear with the use of earlier generations of chemicals, such as arsenic, mercury compounds, DDT, and organophosphates. These substances were highly toxic and persistent in the environment. They caused direct harm to wildlife, including large-scale bird and fish mortality, and posed serious risks to human health through food and water contamination. These substances were eventually banned, prompting greater public awareness and paving the way for stricter modern pesticide regulations.

Modern pesticides are developed to be more targeted and to break down more rapidly in the environment than their historical counterparts. Only those substances that meet the criteria of an extensive environmental risk assessment under European Union regulations are approved for use. These substances are generally less toxic and less persistent, which reduces some environmental risks.

However, even modern pesticides are designed to affect living organisms, and their use still has ecological consequences. These effects are often more indirect. For example, in aquatic environments, herbicides can alter the composition of algae and aquatic plants, which may affect the entire food web. On land, herbicide use can reduce the availability of flowering plants, making it harder for pollinators like wild bees and butterflies to access food. Such changes may not result in immediate consequences but can still have significant implications for biodiversity and ecosystem health over time.

Determining the ecological effects of pesticides remains a major challenge. Ecosystem health is shaped by many interacting natural and human-induced factors. In agricultural areas, morphology, hydrology, weather, nutrient runoff, and wastewater discharges, to name just a few, all contribute to a complex environmental backdrop and challenge soil and water quality. Pesticides rarely occur in isolation, they coincide with elevated nutrient levels, making it difficult to disentangle the specific impacts of pesticides from other stressors. Researchers, including us from CKB, are actively working to identify robust indicators that can reflect ecosystem-level responses on pesticide exposure and improve environmental risk assessments. A further complication is the presence of pesticide mixtures, whose combined effects, whether additive, synergistic, or antagonistic, are still poorly understood. Additionally, indirect effects, such as shifts in species composition or food web interactions, are difficult to quantify but may have lasting consequences. Together, these challenges underscore the need for more holistic and integrative approaches to assessing pesticide impacts on ecosystems.