Urbanisation is associated with air and soil pollution, particularly from heavy metals. One of the tissues most exposed to such pollutants is the midgut epithelium as insects may ingest these pollutants with food. Bees are one of the most important urban insects, providing important ecosystem services such as pollination. However, to the best of our knowledge, no studies have investigated the possible histological alterations to the midgut epithelium of bees caused by urbanisation. We sampled workers of the ground-nesting, primitively eusocial bee Halictus scabiosae in a large metropolis (Milan), with the aim to test if individuals from areas characterised by higher urbanisation and consequently higher pollution levels-defined here by a greater proportion of roads-exhibit greater histological tissue and cellular alterations in the midgut epithelium. We obtained semi-thin sections of the midgut through histological techniques, and then adopted a semi-quantitative approach to assess morphological damage. The midgut presented a range of histological alterations including epithelium disorganisation, vacuolisation, and nucleus karyorrhexis (one of the stages of cellular death). We found higher histological damage score (calculated taking into account all found alterations) and frequency of karyorrhectic nuclei in sites with a higher proportion of roads (i.e. more urbanised). The observed alterations may underline a potential impairment of the digestive function in highly urbanised areas.

BACKGROUND: Spodoptera litura is one of the most harmful lepidoptera pests in China, and is difficult to control due to its strong resistance to the current frequently used insecticide species. The requirement to develop pesticides with novel toxicology mechanisms to control S. litura is urgent. The quassinoid of bruceine D display outstanding systemic properties and strong insecticidal activity against S. litura, which possess notable application potential for integrative management of S. litura, but the mechanism of toxicity remains unclear. RESULTS: In this study, we found that bruceine D exerts potent growth inhibitory activity against S. litura, disrupting the ecdysone and juvenile hormone titers, and causing long-term adverse effects. Association analysis between transcriptomics and metabolomics suggested that bruceine D affected the digestion and absorption capacity of S. litura larvae by inducing a strong oxidative stress response and cell apoptosis in the intestine. Further analysis demonstrated that bruceine D can inhibit the activities of digestive and antioxidant enzymes and induce malondialdehyde (MDA) and reactive oxygen species (ROS) overaccumulation in the midgut. Moreover, the protein level of Bax, cleavage caspase 3, and cytochrome c expressed in cytoplasm (cyto) were up-regulated by bruceine D, while Bcl-2 and cytochrome c expressed in mitochondria (mito) were down-regulated. In addition, there was a noticeable increase in caspase-3 protease activity. Histopathological observations revealed that bruceine D damages the structure of midgut epithelial cells and activates lysosomes, which subsequently disrupts the midgut tissue. CONCLUSION: Overall, our findings suggested that bruceine D induced excessive ROS accumulation in midgut epithelial cells. The resulting cell apoptosis disrupted midgut tissue, leading ultimately to reduced nutrient digestion and absorption in the midgut and the inhibition of larval growth. (c) 2024 Society of Chemical Industry.