Pesticides rarely come alone

In their latest publication, scientists from the Helmholtz Centre for Environmental Research quantified the relevance of pesticide mixtures using monitoring data from pesticide application schemes and Germany-wide stream samplings. The research concluded that environmental risk assessment of pesticides requires refinements.

A complex problem

A single agricultural field faces multiple pesticide treatments per season. A single pesticide treatment mostly comprises multiple plant protection products (PPPs). Finally, a single PPP also often contains multiple pesticidal active substances, resulting in highly complex pesticide mixtures in the environment. However, the current pesticide risk assessment almost exclusively considers single PPPs. By analyzing more than 800 pesticide application schemes (all plant protection and plant growth regulation measures on a field within one growing season) and over 800 water samples from Germany-wide monitoring of small streams (“Kleingewässer-Monitoring” – KgM), the authors of the study confirmed that pesticide mixtures were omnipresent in the environment. When analyzing the actual application patters, the researchers also found that four out of five pesticide applications featured multiple pesticides.

Figure 1: One of the several stream monitoring sites of the Kleingewässer-Monitoring (photo by O. Weisner)

A toxic mixture

Single applications contained up to 12 pesticides. Yet, pesticide mixtures in water samples were  even more complex, featuring 30 pesticides on average and a maximum of 57 pesticides. The toxicity of the mixtures, however, was clearly dominated by one to few alternating pesticides. The authors estimated a factor of 3.2 to be required to extrapolate single PPP risk to environmental pesticide mixture risk. This means that the actual risks in environment is commonly three times higher than what is accounted for during registration and assessment of pesticides. Noteworthily, this only accounts for simple additive effects of these mixture components, disregarding potentially synergistic interactions among pesticides or with other environmental stressors.

Figure 2: While pesticide risk assessment focuses on the exposure and effects of single plant protection products, pesticides that are applied  occur in complex mixtures in reality. (Graphic by O. Weisner)

A frequent issue

High frequency of exposure of agricultural fields and streams to pesticide pulses is another aspect highlighted in the research. The intervals between pulses are considered too short to allow non-target organisms to fully recover or for pesticide residues to degrade. In majority of agricultural fields and streams, the pulse intervals were, at least once during the 3-month monitoring, shorter than 8 weeks. This corresponds to the  time period after exposure, in which recovery renders adverse effects acceptable in environmental risk assessment.

The authors concluded that a re-adjustment to more realistic conditions, considering both simultaneous and sequential exposure is required in pesticide risk assessment . Further research is needed to elaborate concepts which enable a quantification of the additional ecological risk due to sequential exposure to improve the environmental risk assessment of pesticide mixtures.

The paper titled Risk from pesticide mixtures – The gap between risk assessment and reality was authored by Oliver Weisner, Tobias Frische, Liana Liebmann, Thorsten Reemtsma, Martina Roß-Nickoll, Ralf B. Schäfer, Andreas Schäffer, Björn Scholz-Starke, Philipp Vormeier, Saskia Knillmann and Matthias Liess. It was published in ‚Science of the Total Environment‘.