Research indicates that natural biopesticides impact agricultural soil

Blind trust in botanical pesticides can compromise soil health. A study warns of the environmental impacts caused by plant-based compounds, often seen as safe alternatives to synthetic pesticides. The research led by Verónica Pereira, from the University of Madeira, Portugal, analyzed the effects of several biopesticides on soil chemistry and biology. The result points to a scenario that is more complex than previously imagined.

Natural compounds such as rotenone, nicotine and pyrethrins, although extracted from plants, accumulate in the soil, interfere with microbial and enzymatic activity and, in some cases, maintain significant toxicity to non-target organisms.

In the laboratory, many of these compounds have shown prolonged half-lives. Rotenone, for example, in addition to being associated with Parkinson's-like symptoms in animal studies, has shown variable persistence depending on soil type and temperature.

The European Union’s proposal to reduce the use of conventional pesticides by 50% by 2030 has boosted the biopesticide market. The expectation is that this sector will reach US$30 billion by the end of the decade. However, the Portuguese study suggests caution. The belief that “natural is always better” does not stand up to scientific evidence.

Several factors affect the degradation of botanical pesticides: soil composition, pH, moisture, depth and presence of microorganisms. In clayey soils or soils rich in organic matter, compounds such as carvone and thymol tend to degrade more slowly. The absence of light and low microbial activity in deeper layers also favor their persistence.

In addition, the effects on beneficial organisms are a cause for concern. Mycorrhizal fungi, nitrogen-fixing bacteria and earthworms—essential for soil balance and fertility—can suffer from continued exposure to these compounds. Enzymes such as phosphatase, urease and dehydrogenase, which are crucial for nutrient cycling, are also inhibited in some tests.

Although some extracts have shown temporary or reversible effects, others, such as essential oil of Thymbra capitata, caused a prolonged impact on microbial populations. Encapsulations with maltodextrin, a technique used to control the release of the compounds, also interfered with bacterial and fungal growth.

Beneficial insects such as ladybugs, predatory mites and lacewings, were the targets of toxicity tests. Products based on orange and thyme oils showed different effects, mainly affecting larvae. In earthworms of the genus Eisenia, some oils attracted animals, while others caused genetic changes over time.

From a chemical point of view, plant pesticides comprise a wide range of substances: terpenes, flavonoids, alkaloids and phenols. The decomposition of these molecules in the soil follows complex and little-known pathways, with the possibility of generating even more toxic or persistent by-products. In many cases, current analytical techniques are insufficient to accurately map the fate and effects of these secondary metabolites.

Given the evidence, the authors of the study call for a review of regulatory protocols for plant-based pesticides. Currently, the standards follow similar parameters to those for synthetic compounds, which may not be appropriate. The natural variability of plant extracts, which depends on climate, soil and extraction method, requires specific guidelines.

The research also highlights the urgency of standardizing extraction and soil analysis methods, in addition to expanding toxicological testing with a greater diversity of organisms. Predictive techniques, such as computational models, are under development, but still require validation given the chemical diversity of biopesticides.

More information can be found at mdpi.com/2077-0472/15/10/1053