Are tyre wear particles a potential stressing contaminant for urban ants? First data on Tetramorium immigrans

Tire wear particles, generated by the mechanical abrasion of tires, may be one of the largest sources of microplastics. As of today, most of the studies on this matter investigated the toxicity of these particles on marine or freshwater fauna, while there is little if any studies on the effects of such particles on terrestrial fauna. To fill this knowledge gap, we investigated the effects of tire wear particles on Tetramorium immigrans, a native European ant (Hymenoptera) commonly inhabiting anthropized environments, even often nesting in concrete and asphalt cracks within cities. We simulated different tire wear-exposed conditions of urban nests in the laboratory, maintaining groups of ant workers for several days. We hypothesised that these particles adhere to the ants’ cuticle and might be ingested through contaminated food or grooming, having a potential effect on the midgut epithelium. Ants from three colonies were given a different treatment for five days (food contaminated with tire dust; aerial tire dust; food contaminated with tire dust and aerial tire dust), using three different concentrations of tire wear particles and controls. We used Scanning Electron Microscopy (SEM) coupled with X-ray energy dispersive (EDX) analysis to detect whether particles adhered to the cuticle of the head and thorax and to determine if they were ingested, whereas to see the effects on the midgut epithelium we used histology techniques on transverse sections of the abdomen. SEM analyses revealed that little to no particles adhered to the cuticle, suggesting that these particles simply don’t stick to the cuticle or that ants groom each other. Furthermore, no particles were identified in the gut lumen, suggesting that ants neither eat the contaminated food nor ingest particles in the grooming process. Histological analyses support the latter hypothesis, given that no obvious damage to the digestive tissue or cell nuclei was detected. More experiments on different species, and under different conditions (e.g. using isolated ants) are necessary to determine if the potential ingestion of tire wear particles can cause damage at the molecular level (e.g. oxidative and metabolic stress).