Commercial formulations of copper-based conventional and nano-formulated pesticides affect chironomid-associated microbiota and hosts traits

Copper-based plant protection products (PPPs) are widely used in agriculture to control fungal and bacterial diseases. However, concerns regarding copper accumulation in the environment have led to restrictions on its use. Copper-based nano-formulations have been proposed as an alternative, offering improved efficiency and reduced application rates. However, their environmental risks remain poorly understood, particularly their effects on aquatic ecosystems. This study evaluates the ecotoxicity of conventional and nano-formulated copper pesticides using Chironomus riparius as a holobiont model, considering both host life history traits and microbiota responses. Larvae were exposed to environmentally relevant concentrations (5, 50, and 500 µg Cu/L) of conventional and nano-formulated products under controlled laboratory conditions. Exposure to the highest concentration resulted in 100 % mortality. Following exposure to 50 µg Cu/L, sublethal effects were observed, including alteration of the larval microbiota composition, apparently shifting towards a copper-tolerant profile following exposure to the conventional formulation. Despite the distinct shifts in bacterial taxa between conventional and nano-formulated treatments, similar decreased adult emergences are noticed following exposure to both formulations. Despite comparable toxicity profiles, nano-formulations resulted in higher copper bioaccumulation in emerging adults, raising concerns about trophic transfer and ecosystem impacts. Our findings highlight the need for a holistic approach integrating host-microbiota interactions in ecotoxicological assessments. The comparable toxicity of nano-formulated and conventional copper pesticides, coupled with increased bioaccumulation potential, questions the assumption that nano-formulations represent a safer alternative. These results emphasize the importance of comprehensive risk assessments to ensure sustainable agricultural practices while minimizing environmental harm.