Use of Plant-Protection Products and Aquatic Ecotoxicity Risk

When harmful organisms infest crops, major harvest losses can occur. Consequently, efforts are made to protect these crops, inter alia with the use of chemical plant-protection products (PPPs). The active substances contained in PPPs, however, can also have undesirable effects on humans and non-target organisms. Because of this, in 2017 the Swiss Federal Council adopted the Action Plan for Risk Reduction and Sustainable Use of Plant Protection Products, with the aim of reducing both PPP use and the associated risks.

Calculation, key figures, risks

Calculating PPP Indicators

In the SAEDN, farmers record which PPPs they use on which crop, at what dose and when. From this, three key figures for PPP use are calculated: the number of interventions (i.e. the number of applications), the total amount of the applied active substances, and the active-substance ranking (i.e. the frequency with which individual active substances are used). All key figures are evaluated separately for each crop as well as for each plant-protection product type (herbicides = weed-control agents; fungicides = agents for controlling fungal diseases; insecticides = agents for controlling insect pests).

With the information on PPP application in hand, the risk for aquatic organisms can be modelled in a subsequent step. Here, it is not just the toxicity, soil accumulation characteristics and decomposition rates of the active substances used that are taken into account, but also the directions for use of the products and the location characteristics typical of Switzerland for the various land-use types (arable farming, fruit production, viticulture, etc.). Unlike the other agri-environmental indicators, the plant-protection product indicators are not calculated for different farm types, but rather per crop.

Key Figures on the Evolution of PPP Use

The three key figures on the evolution of PPP use in Switzerland supply different information (de Baan et al. 2015). The number of interventions tells us how often plant-protection products are used on a crop. For this, the number of spraying passes performed per year is calculated for each field. The amount of active substances records the quantity of all plant-protection product active substances applied on a field per hectare and year.The active-substance ranking tells us which active substances are mainly used.

Overall, there are major differences between crops: plant-protection products are used both more frequently and in larger amounts in special crops (fruit, wine, vegetables) than in field crops. To date, the SAEDN’s Special Crops database has been too small, and should be expanded in future in order to ensure a more stable basis for calculating indicators on PPP use and the associated risks.

Risks for Aquatic Organisms

The risks of PPPs for aquatic organisms are calculated using the SYNOPS model (Gutsche and Strassemeyer, 2007, de Baan et al. 2020). Because SYNOPS was adapted for Switzerland, it models the transport of PPPs in surface waters via various entry routes, viz., surface runoff, erosion, tile drainage and aerial transport of spray mist based on typical Swiss location factors (de Baan, 2020). In this context, the amount of PPPs transported depends on the conditions of use (amount of active substances, timing, crop, risk-mitigation requirements), active-substance characteristics (degradability, mobility) and location characteristics (distance between field and water bodies, slope, soil type, precipitation, temperature, etc.). Here, the expected surface-water concentration (exposure) for each active substance is calculated and weighed against the toxicity of the active substance in order to yield the risk:

Risk = Exposure / Toxicity

The toxicity of each active substance is known from standardised laboratory experiments. For acute effects, toxicity is often given as the concentration at which 50% mortality was observed within a specific time period. In addition to acute effects, long-term effects are also taken into account, for example, a possible reduced reproduction rate of the organisms. Toxicity is considered for five groups of organisms that are representative of the surface-water ecosystem: fish, daphnia (water fleas), chironomid (non-biting midge) larvae, algae and duckweed.

Using SYNOPS, the risks can be calculated for individual applications or entire spraying sequences (i.e. for all active substances applied on a field over the course of a year). This also allows the effects of mixtures of active substances to be taken into account.