Effect of urbanization and its environmental stressors on the intraspecific variation of flight functional traits in two bumblebee species

Patterns and Drivers of Bumblebee Diversity in Gansu

Understanding the influence of factors responsible for shaping community assemblage is crucial for biodiversity management and conservation. Gansu is one of the richest regions for bumblebee species in the world. We explored the distribution data of 52 bumblebee species collected in Gansu and its surroundings between 2002 and 2022, predicting habitat suitability based on 17 environmental variables using MaxEnt. The factors influencing community assemblage were assessed using canonical correspondence analysis. Net primary productivity, water vapor pressure, temperature seasonality, annual precipitation, and precipitation seasonality were some of the most influential drivers of species distributions. Based on Ward's agglomerative cluster analysis, four biogeographic zones are described: the Southern humid zone, the Western Qilian snow mountain zone, the Eastern Loess plateau zone, and the Western dry mountain zone. In the clusters of grid cells based on beta diversity values, the Southern humid zone comprised 42.5% of the grid cells, followed by the Eastern Loess plateau zone (32.5%), the Western dry mountain zone (20%), and the Western Qilian snow mountain zone (5%). Almost all the environmental factors showed a significant contribution to the assemblages of bumblebees of different groups. Our findings highlight the need for better data to understand species biogeography and diversity patterns, and they provide key baseline data for refining conservation strategies.

Subtle morphological changes in the visual and antennal sensory system of bees and wasps across an urbanisation gradient

Increased temperature and fragmentation of green spaces in urban areas could drive variations in functional traits of insects. Such morphological shifts may occur for sensory systems, which were previously reported to be prone to change with habitat characteristics in non-urban contexts. Here, we measured traits related to the visual and antennal sensory systems in the bees Halictus scabiosae and Osmia cornuta and the wasp Polistes dominula along an urbanisation gradient within Milan (Italy). We hypothesised that fragmentation could filter for better visual properties, and that higher temperature could filter for fewer thermoreceptors and more olfactory hairs. While controlling for body size, results show subtle but appreciable responses to urbanisation in one or more traits in all species, though not always supporting our hypotheses. O. cornuta shows marginally higher ommatidia density and smaller ommatidia diameter (associated with better visual resolution) in more fragmented sites, as well as marginally fewer thermoreceptors in hotter sites, in agreement with our two predictions. On the other hand, H. scabiosae has marginally smaller antennae and P. dominula has smaller eyes at warmer locations, and the wasp also has smaller antennae and 9th flagellomeres in more fragmented areas. Perhaps higher temperatures accelerate development of sensory system at higher speed than the rest of body in these two species. Our results represent the first evidence of urbanisation effects on the visual and antennal sensory systems of bees and wasps and underline how such effects may involve a much broader bouquet of traits then previously observed.

Temperature differently affects body pigmentation of the paper wasp Polistes dominula along an urban and a wider geographical gradient

In insects, different pigments, such as melanins and pterins, are involved in thermoregulation. The degree of melanisation often varies along geographical gradients, according to the so-called thermal melanism hypothesis, i.e. darker forms are found in colder places because they can warm up more quickly. Similarly, pterins work as heat sinks and thus are expected to be more abundant in colder sites. Cities, which are warmer than surrounding areas (Urban Heat Island (UHI) effect), might also be expected to influence pigmentation, although studies are lacking. Here, we sampled workers of the social paper wasp Polistes dominula (Christ, 1791) (Vespidae) across an urbanisation gradient in an Italian metropolis and used iNaturalist pictures of this species across Italy to study pigmentation patterns at both urban and larger geographical scales. We found a lower yellow intensity of abdominal spots at warmer locations. Scanning Electron Microscopy strongly suggested that yellow colouration is due xanthopterin, known to be the heat sink molecule in other social vespids. Thus, wasps from warmer (i.e., urban) environments are likely to have fewer xanthopterin granules, in line with the lack of need for heat storage due to the local thermal gradient (UHI effect). At the country level, we found that wasps at higher latitudes had smaller yellow spots on the thorax and only two spots instead of four at higher altitudes, in full accordance with the thermal melanism hypothesis. In conclusion, climatic conditions seem to affect insect colour patterns both along urban and wider geographical gradients, although colour changes may affect different body parts and pigments likely according to different needs.

Land use influences the nutrient concentration and composition of pollen and nectar rewards of wildflowers in human-dominated landscapes

Plant biodiversity is crucial to satisfy the trophic needs of pollinators, mainly through nectar and pollen rewards. However, a few studies have been directed to ascertain the intraspecific variation of chemical features and the nutritional value of nectar and pollen floral rewards in relation to the alteration of landscapes due to human activities. In this study, by using an existing scenario of land use gradients as an open air laboratory, we tested the variation in pollen and nectar nutrient profiles along gradients of urbanization and agriculture intensity, by focusing on sugar, aminoacids of nectar and phytochemicals of pollen from local wild plants. We also highlighted bioactive compounds from plants primary and secondary metabolism due to their importance for insect wellbeing and pollinator health. We surveyed 7 different meadow species foraged by pollinators and common in the main land uses studied. The results indicated that significant variations of nutritional components occur in relation to different land uses, and specifically that the agricultural intensification decreases the sugars and increases the antioxidant content of flower rewards, while the urbanization is positively associated with the total flavonoid content in pollen. These effects are more evident in some species than in others, such as Lotus corniculatus L. (Fabaceae) and Malva sylvestris L. (Malvaceae), as shown by the untargeted metabolomic investigation. This study is crucial for understanding the nutritional landscape quality for pollinators in association to different land uses and sets a base for landscape management and planning of pollinator-friendly strategies by improving the quality of plant rewards to provide benefits to pollinator health in various environmental contexts.

Urban habitat fragmentation and floral resources shape the occurrence of gut parasites in two bumblebee species

Urbanization and the expansion of human activities foster radical ecosystem changes with cascading effects also involving host-pathogen interactions. Urban pollinator insects face several stressors related to landscape and local scale features such as green habitat loss, fragmentation and availability reduction of floral resources with unpredictable effects on parasite transmission. Furthermore, beekeeping may contribute to the spread of parasites to wild pollinators by increasing the number of parasite hosts. Here we used DNA-based diagnostics tools to evaluate how the occurrence of parasites, namely microsporidians (Nosema spp.), trypanosomatids (Crithidia spp.) and neogregarines (Apicystis bombi), is shaped by the above-mentioned stressors in two bumblebee species (i.e. Bombus terrestris and Bombus pascuorum). Infection rates of the two species were different and generally higher in B. terrestris. Moreover, they showed different responses towards the same ecological variables, possibly due to differences in body size and foraging habits supposed to affect their susceptibility to parasite infection. The probability of infection was found to be reduced in B. pascuorum by green habitat fragmentation, while increased along with floral resource availability. Unexpectedly, B. terrestris had a lower parasite richness nearby apiaries maybe due to the fact that parasites are prone to be transmitted among the most abundant species. Our finding supports the need to design proper conservation measures based on species-specific knowledge, as suggested by the variation in the parasite occurrence of the two species. Moreover, conservation policies aiming at safeguarding pollinators through flower planting should consider the indirect effects of these measures for parasite transmission together with pollinator biodiversity issues.

Morphological Variation in Bumblebees (Bombus terrestris) (Hymenoptera: Apidae) After Three Decades of an Island Invasion

Introduced social insects can be highly invasive outside of their native range. Around the world, the introduction and establishment of the eusocial bumblebee Bombus terrestris (L. 1758) (Hymenoptera: Apidae) has negatively impacted native pollinators and ecosystems. Understanding how morphological variation is linked to environmental variation across invasive ranges can indicate how rapidly species may be diverging or adapting across novel ranges and may assist with predicting future establishment and spread. Here we investigate whether B. terrestris shows morphological variation related to environmental variation across the island of Tasmania (Australia) where it was introduced three decades ago. We collected 169 workers from 16 sites across Tasmania and related relative abundance and morphology to landscape-wide climate, land use, and vegetation structure. We found weak morphological divergence related to environmental conditions across Tasmania. Body size of B. terrestris was positively associated with the percentage of urban land cover, a relationship largely driven by a single site, possibly reflecting high resource availability in urban areas. Proboscis length showed a significant negative relationship with the percentage of pasture. Wing loading and local abundance were not related to the environmental conditions within sites. Our results reflect the highly adaptable nature of B. terrestris and its ability to thrive in different environments, which may have facilitated the bumblebee's successful invasion across Tasmania.

Signals of adaptation to agricultural stress in the genomes of two European bumblebees

Human-induced environmental impacts on wildlife are widespread, causing major biodiversity losses. One major threat is agricultural intensification, typically characterised by large areas of monoculture, mechanical tillage, and the use of agrochemicals. Intensification leads to the fragmentation and loss of natural habitats, native vegetation, and nesting and breeding sites. Understanding the adaptability of insects to these changing environmental conditions is critical to predicting their survival. Bumblebees, key pollinators of wild and cultivated plants, are used as model species to assess insect adaptation to anthropogenic stressors. We investigated the effects of agricultural pressures on two common European bumblebees, Bombus pascuorum and B. lapidarius. Restriction-site Associated DNA Sequencing was used to identify loci under selective pressure across agricultural-natural gradients over 97 locations in Europe. 191 unique loci in B. pascuorum and 260 in B. lapidarius were identified as under selective pressure, and associated with agricultural stressors. Further investigation suggested several candidate proteins including several neurodevelopment, muscle, and detoxification proteins, but these have yet to be validated. These results provide insights into agriculture as a stressor for bumblebees, and signal for conservation action in light of ongoing anthropogenic changes.