Ants as bioindicators of urban traffic pollution.

Ants can serve as valuable bioindicators due to their ecological ubiquity, stable presence due to perennial nests, and direct interaction with soil and airborne particulate matter. This study investigates the potential of the arboreal ant species Crematogaster scutellaris as a sentinel organism for traffic-related magnetic pollution in urban environments. Sampling was conducted across 30 sites in the city of Parma (Italy) characterized by varying levels of vehicular traffic, alongside four control sites. Magnetic susceptibility measurements and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) were employed to quantify and characterize magnetic particles accumulated on the ants' exoskeleton and surrounding soil. Results revealed a strong correlation between vehicular traffic intensity and the quantity of magnetic particulate matter deposited on ants (Spearman's ρ = 0.825, p < 0.001), while soil samples showed no significant relationship (ρ = 0.167, p = 0.346). SEM-EDS analysis identified iron oxide-rich particles doped with heavy metals such as Zn, Cu, and Ti on the ants' surface, particularly concentrated on the thorax. These particles exhibited morphologies consistent with traffic-derived ultrafine particulate matter. Soil samples, collected at 5 cm depth, predominantly contained clay minerals with minimal anthropogenic influence. The relationship becomes linear on a logarithmic scale (R2 = 0.66, p < 0.001); each additional 1,000 vehicles per day was associated with a 1.41-fold increase in ant magnetic susceptibility, corresponding to an approximately 700-fold increase across the traffic gradient sampled. The study proposes a novel biomonitoring protocol integrating magnetic and morphological analyses to assess urban air quality, offering a scalable alternative to traditional fixed monitoring stations.