Increasing road cover in urban areas is associated with greater midgut histological damage in a primitively eusocial bee

Agriculture poses many threats to insect pollinators, one of which is the use of synthetic products to improve crops. Recently, several policies have been put in place to reduce the use of synthetic products and promote circular economy practices of recycling, for example the use of sewage sludge as fertilizer. However, sludges can still contain harmful substances such as toxic trace elements. In agricultural environments, bees can come into contact with trace heavy metals through feeding. Although there is evidence of their toxicity to bees, there is little information on the specific effect these substances may have on their digestive system. We therefore investigated, using histology, the effects of trace elements present in sewage sludge on the intestinal epithelium of A. mellifera workers collected in agricultural areas, one where sewage sludge is used and one where its spread is precluded. Soil levels of known toxic elements were also quantified. We hypothesised that bees collected in areas treated with sewage sludge would show more alterations to the midgut epithelium. Soil levels significantly differed between the two areas, with higher concentration of Cr and Ni in the sewage areas. In individuals from treated areas, we observed alterations of the midgut epithelium as well as the presence of a high density of vacuoles containing spherites, one of the bees’ defence systems to isolate toxic metals. Semi-quantitative analyses showed that A. mellifera individuals from treated areas had higher vacuolisation and damage score than those from untreated areas. This suggests that exposure to heavy metals, is a stress factor for the intestinal epithelium of bees and may impair its digestive functions with a consequent impact on their health and survival. This study lays the foundations for further research on the effect of anthropogenic pollutants on the health of bees, focusing on their digestive system.