A pilot study to assess carabids (Coleoptera: Carabidae) as potential bioindicators of microplastics contamination in soils

Microplastics are pervasive pollutants across ecosystems, posing environmental risks due to their bioavailability and toxicity. Monitoring microplastics presence in different environments is crucial, and employing simple, cost-effective methods, such as using insects as bioindicators, can be highly effective. In this pilot study, Carabid beetles (Coleoptera: Carabidae), a terrestrial insect group, were used, for the first time, to investigate their potential as bioindicators for microplastics in soils. Their biological and morphological characteristics, along with their broad distribution, make them ideal candidates for assessing the presence of microplastics in terrestrial environments. Monthly sampling was conducted from July to October 2022 using pitfall traps at three sites along the Conero coast (Italy), differing in the degree of human influence. Microplastics were extracted from organisms’ guts using oxidative digestion and vacuum filtration and then characterized through optical microscopy and μFT-IR spectroscopy. Findings revealed that 32% of Carabid beetles ingested microplastics. While no consistent spatial or temporal patterns were observed in the number of particles found per individual, ingestion frequency varied significantly among sites (Pearson's chi-squared test, p = 0.028). Site C, a stony beach subject to intense summer tourism, exhibited the highest ingestion rates (75% in July, 87.5% in August). Most microplastics were primarily fragments of 0.1-1 mm in size, with polyester and silicone being the most prevalent polymers. Given limited information available on microplastics contamination in soils, this pilot study confirms that Carabids are effective bioindicators of the presence of soil microplastics, and demonstrated a link between microplastics pollution and anthropogenic presence. • For the first time Carabids were used as bioindicators for the presence of soil MPs • MPs were analyzed in guts revealing their presence in the 32% of sampled Carabids • Site-dependent variations in the ingestion frequencies were observed • MPs typology reflects local environmental conditions and anthropogenic pressures • This pilot study demonstrates that Carabids are effective bioindicators for the presence of soil MPs