Ground-nesting Bees Research Articles

Digging below the surface: Hidden risks for ground-nesting bees.

Hidden risks for ground-nesting bees.

A century of wild bee sampling: historical data and neural network analysis reveal ecological traits associated with species loss.

We analysed the wild bee community sampled from 1921 to 2018 at a nature preserve in southern Michigan, USA, to study long-term community shifts in a protected area. During an intensive survey in 1972 and 1973, Francis C. Evans detected 135 bee species. In the most recent intensive surveys conducted in 2017 and 2018, we recorded 90 species. Only 58 species were recorded in both sampling periods, indicating a significant shift in the bee community. We found that the bee community diversity, species richness and evenness were all lower in recent samples. Additionally, 64% of the more common species exhibited a more than 30% decline in relative abundance. Neural network analysis of species traits revealed that extirpation from the reserve was most likely for oligolectic ground-nesting bees and kleptoparasitic bees, whereas polylectic cavity-nesting bees were more likely to persist. Having longer phenological ranges also increased the chance of persistence in polylectic species. Further analysis suggests a climate response as bees in the contemporary sampling period had a more southerly overall distribution compared to the historic community. Results exhibit the utility of both long-term data and machine learning in disentangling complex indicators of bee population trajectories.

Pollinator response to livestock grazing: implications for rangeland conservation in sagebrush ecosystems.

World food supplies rely on pollination, making this plant-animal relationship a highly valued ecosystem service. Bees pollinate flowering plants in rangelands that constitute up to half of global terrestrial vegetation. Livestock grazing is the most widespread rangeland use and can affect insect pollinators through herbivory. We examined management effects on bee abundance and other insect pollinators on grazed and idle sagebrush rangelands in central Montana, USA. From 2016 to 2018, we sampled pollinators on lands enrolled in rest-rotation grazing, unenrolled grazing lands, and geographically separate idle lands without grazing for over a decade. Bare ground covered twice as much area (15% vs. 7) with half the litter (12% vs. 24) on grazed than idle regardless of enrollment. Bee pollinators were 2-3 times more prevalent in grazed than idle in 2016-2017. In 2018, bees were similar among grazed and idled during an unseasonably wet and cool summer that depressed pollinator catches; captures of secondary pollinators was similar among treatments 2 of 3 study years. Ground-nesting bees (94.6% of total bee abundance) were driven by periodic grazing that maintained bare ground and kept litter accumulations in check. In contrast, idle provided fewer nesting opportunities for bees that were mostly solitary, ground-nesting genera requiring unvegetated spaces for reproduction. Managed lands supported higher bee abundance that evolved with bison grazing on the eastern edge of the sagebrush ecosystem. Our findings suggest that periodic disturbance may enhance pollinator habitat, and that rangelands may benefit from periodic grazing by livestock.

The effects of plant invasion, floral resources and soil characteristics on ground-nesting bees

Ground-nesting bees are susceptible to disturbances that affect foraging resources, vegetation structure and soil characteristics. We studied the effects of 12 invasive alien plant species on environmental variables and directly on ground-nesting bees to reveal their specific importance. We sampled bees along transects; flowers, vegetation, bare ground and litter in quadrats, as well as soil characteristics. Our results suggested that plant invasion reduced overall and flowering plant richness and positively affected the height of native vegetation. However, this did not affect ground-nesting bees’ abundance or species richness. Although we found invaded plots to have altered soil chemical characteristics compared to control plots, there was no correlation between soil characteristics and ground-nesting bees. The large explanatory potential of the random factors suggests that there were large differences in the abundance and species richness of ground-nesting bee species among the different habitat types and before and during the flowering of the invasive species. Although our study lacked a direct sampling of nesting bees, our results show the adverse effects of plant invasion on foraging and some nesting characteristics of ground-nesting bees. Future studies should put effort into the investigation of the number of nest holes and additional soil characteristics to get a better picture of the effects of plant invasion on ground-nesting activity.

Review of the Role of Bees as Ecosystem Engineers in Nature

Bees are a highly important group, providing a multitude of services to ecosystems, most notably through pollination. While much of the research on bees has traditionally focused on their role in pollinating a variety of wild and cultivated crops, this review paper expands the discourse by examining their functions as ecosystem engineers. Traditional definitions of ecosystem engineers exclude pollinators due to their nonphysical modifications of habitats for other species. However, contemporary studies challenge this perspective, recognizing pollinators as integral ecosystem engineers who offer a range of direct and indirect ecological services. For instance, bees’ pollination activities lead to the formation of dry fruits that subsequently serve as shelters for various organisms. In addition, ground-nesting bees modify soil composition through mechanisms such as aeration and bioturbation. This review aims to elucidate the transformative impact bees have as ecosystem engineers, thereby enhancing our understanding of their ecological importance.

Harnessing community science to conserve and study ground-nesting bee aggregations

Protecting diverse solitary ground-nesting bees remains a pivotal conservation concern. Ground-nesting bees are negatively impacted by anthropogenic land use change that often removes suitable nesting habitat from the landscape. Despite their enormous ecological and agricultural contributions to pollination, solitary, ground-nesting bees are often neglected, partly due to the significant obstacle of discovering exactly where these bees establish their nests. To address this limitation, we have developed a ‘community science’ project to map aggregations of ground-nesting bees globally. In certain locations, their abundances reach astounding levels, sometimes in the millions, but are scarcely known. Utilizing the iNaturalist platform, which permits geo-referencing of site observations and bee identification, we are providing public education and seeking public engagement to document bee aggregations in order to understand the nesting requirements of diverse species and open new opportunities for their conservation. Conservation priorities may then unequivocally be directed to areas of high species richness, nest densities, and nesting sites of rare bees. Such community-led efforts are vital for successful long-term management of native bees and the biotic and abiotic landscape data from nest-site localities can allow modeling to predict nest-site suitability and to readily test such predictions on the ground. Here, we summarize the progress, current limitations, and opportunities of using a global mapping project (GNBee) to direct conservation efforts and research toward solitary ground-nesting bees.

Diversity and biological traits of bees visiting flowers of Cucurbita maxima var. zapallito differ between biodiversity-based and conventional management practices.

Low diversity of pollinators and the modified composition of functional groups of bees have been proposed as the causes of pollination deficiency in cultivated Cucurbitaceae species. Functional groups of bees are determined by traits, such as body size, nesting site, and social behavior. The presence of bees with specific traits can be differentially affected by agricultural management practices. This work aimed to assess how management types (agroecological and conventional) in Cucurbita maxima var. zapallito crops affect the abundance of bees with different biological traits. The study was conducted on four farms located in horticultural areas of central-eastern Santa Fe province, Argentina. A total of 108 10-min censuses were conducted to record bee species abundance in flowers. The species were assigned to categories for each of the three biological traits. A total of 552 individuals, belonging to 16 bee species, were recorded. Honey bees were more abundant under conventional management, whereas the native bees Eucera fervens and other species were more abundant under agroecological management. Species of the categories analyzed (body size: small, medium, and large; nesting site: above-ground cavities or ground-nesting; and social behavior: solitary or social) were present on farms under both management types. We found that management type affected bees, and their effects differed among bees with specific biological traits. Medium-sized and small bees, ground-nesting bees, and solitary bees were found in greater abundance on agroecological farms than on conventional farms. Our data allowed us to explain the diversity and abundance of bees relative to the management type and biological traits of the species. Implications for insect conservation: This study suggests that incorporating biodiversity-based management strategies might increase abundance and richness of native bees with different biological traits, ensuring the free pollination service they provide and a taxonomically and functionally diverse assemblage.

Urbanisation reduced body size but potentially improved flight performance in bees and wasps

Urbanisation is a main driver of land-use change, leading to rising in temperatures and fragmentation and reduction of green areas. Bees and wasps, which are important insect groups due to the ecosystem services they provide, may respond to this disturbance via changes in morphological traits which are functionally relevant. To date, studies focusing on this aspect only investigated few social bee species, and often gave contrasting results even at intra-generic level. Here, we studied how body size, wing loading, wing aspect ratio and wing fluctuating asymmetry vary in a social ground-nesting bee (Halictus scabiosae), a solitary hole-nesting bee (Osmia cornuta) and a social paper wasp (Polistes dominula) along an urbanisation gradient within Milan (Italy). By assessing the effects of temperature, green areas fragmentation and vegetation productivity on the above-listed functional traits, we found the three species to variably respond to increasing urbanisation, albeit the driving environmental parameters differed among species. More specifically, smaller individuals were sampled in warmer (for bees) and in less productive (for wasps) areas along the urbanisation gradient. Furthermore, greater wing aspect ratio values were recorded at warmer locations for H. scabiosae, lower wing loading was recorded at more fragmented sites for O. cornuta, and greater wing loading was recorded at locations with greater productivity for P. dominula. H. scabiosae and P. dominula showed greater wing fluctuating asymmetry at more fragmented sites. Although distinct species seemed sensitive to different driving factors, our results point toward a consistent response: smaller body but potentially improved flight performance in more urbanised environments.

Morphology and temporal evolution of ground-nesting bee burrows created by solitary and social species quantified through X-ray imaging

Most research on wild bees has focused on their role as pollinators, while their importance as soil ecosystem engineers has been largely overlooked, despite the fact that most species nest in the soil. There is limited quantitative knowledge regarding the architecture of nests created by wild bees and the temporal evolution of bee burrows. The aim of this study was to evaluate the feasibility of repeatedly scanning ground-nesting bee nests using X-ray computed tomography to quantify the morphology and temporal evolution of burrow systems created by both solitary and social species. We installed eleven large cylinders at locations with ongoing nesting activity of ground-nesting bees, and repeatedly scanned these soil columns with a medical X-ray scanner over a period of 16 months. From the X-ray images, we extracted bee burrows to visualize and quantitatively characterize their morphology and temporal evolution. The architecture and temporal evolution of burrows strongly differed between the studied social and solitary ground-nesting bee species. Burrows created by the solitary species were simple, linear and unbranched burrows, which were not reused and decayed with time. The burrow systems created by the social species were more complex, with highly branched networks of horizontal and vertical burrows, which increased in complexity and size over time during the bee activity period. The persistence of burrows created by ground-nesting bees varied greatly, with some decaying within a few weeks and others remaining mostly intact for the entire 16-month study period. This study demonstrates the potential of X-ray imaging to provide new insights into the underground life of ground-nesting bees, and highlights the locally important role of ground-nesting bees as soil ecosystem engineers.

The Extraordinary Alkali Bee, Nomia melanderi (Halictidae), the World's Only Intensively Managed Ground-Nesting Bee.

Among the ground-nesting bees are several proven crop pollinators, but only the alkali bee (Nomia melanderi) has been successfully managed. In <80 years, it has become the world's most intensely studied ground-nesting solitary bee. In many ways, the bee seems paradoxical. It nests during the torrid, parched midsummer amid arid valleys and basins of the western United States, yet it wants damp nesting soil. In these basins, extensive monocultures of an irrigated Eurasian crop plant, alfalfa (lucerne), subsidize millions of alkali bees. Elsewhere, its polylectic habits and long foraging range allow it to stray into neighboring crops contaminated with insecticides. Primary wild floral hosts are either non-native or poorly known. Kleptoparasitic bees plague most ground nesters, but not alkali bees, which do, however, host other well-studied parasitoids. Building effective nesting beds requires understanding the hydraulic conductivity of silty nesting soils and its important interplay with specific soil mineral salts. Surprisingly, some isolated populations endure inhospitably cold climates by nesting amid hot springs. Despite the peculiarities and challenges associated with its management, the alkali bee remains the second most valuable managed solitary bee for US agriculture and perhaps the world.

Occurrence of Bees and Bumblebees in Bark Beetle Slit Traps from Spruce and Fir Woodlands of Central Dinaric Alps

The paper analysed bees by-catch collected in 259 bark beetle slit traps, from eleven localities in Bosnia and Herzegovina. Sampling was carried out in spruce and fir forests in 2020 and 2021. As a by-catch from bark beetle slit traps 84 bee individuals from four families and 13 genera were collected. In the bark beetle slit traps sample, out of 29 bee taxa, 22 species were identified at the species level and eight specimens were left at the genus/subgenus level. The most dominant genera were Megachile with 34 specimens and Osmia represented by 20 specimens in the total sample. The research identified 14 bee species new to the fauna of Bosnia and Herzegovina. The bee species collected in the bark beetle slit-traps were dominated by nesters in cavities, above the ground-nesting bees.

Location and Creation of Nest Sites for Ground-Nesting Bees in Apple Orchards.

Wild ground-nesting bees are key pollinators of apple (Malus domestica). We explored, (1) where they choose to nest, (2) what influences site selection and (3) species richness in orchards. Twenty-three orchards were studied over three years; twelve were treated with additional herbicide to increase bare ground with the remainder as untreated controls. Vegetation cover, soil type, soil compaction, nest number and location, and species were recorded. Fourteen species of ground-nesting solitary/eusocial bee were identified. Most nests were in areas free of vegetation and areas treated with additional herbicide were utilised by ground nesting bees within three years of application. Nests were also evenly distributed along the vegetation-free strips underneath the apple trees. This area was an important ground-nesting bee habitat with mean numbers of nests at peak nest activity of 873 per ha (range 44-5705), and 1153 per ha (range 0-4082) in 2018 and 2019, respectively. Increasing and maintaining areas of bare ground in apple orchards during peak nesting events could improve nesting opportunities for some species of ground-nesting bee and, combined with flowers strips, be part of a more sustainable pollinator management approach. The area under the tree row is an important contributor to the ground-nesting bee habitat and should be kept bare during peak nesting.

The North American bees of the genus Ptilothrix Cresson, 1878 (Hymenoptera, Apidae, Emphorini), with the description of two new species

Ptilothrix Cresson is a genus of New World bees with an amphitropical distribution. Like other genera in the tribe Emphorini, Ptilothrix have narrow pollen preferences. These solitary ground-nesting bees exhibit a remarkable nesting behavior in which females carry water from ponds to facilitate the excavation of the hard soil where they nest. With 16 described species, there are few taxonomic studies and, before this work, a lack of taxonomic treatments for the species in North America. Thus, in this study we revised and recognized four species for the region: Ptilothrix bombiformis Cresson, Ptilothrix sumichrasti Cresson, Ptilothrix chiricahua Florez-Gomez &amp; Danforth, sp. nov. and Ptilothrix zacateca Florez-Gomez &amp; Danforth, sp. nov. We describe and illustrate males and females of the two new species. We also present diagnoses for the four species, a key to identify them, and a map of their geographic distributions.

Invasive grass and litter accumulation constrain bee and plant diversity in altered grasslands

Ecologists consider biological invasions one of the primary drivers of global change. Many remaining grasslands in North America have extensive invasions of exotic grass species that have replaced native plant species. In the Northern Great Plains, exotic cool-season grasses Kentucky bluegrass (Poa pratensis) and smooth brome (Bromus inermis), paired with human alterations to the landscape and historical disturbance regimes, have resulted in functionally and structurally altered grassland plant communities. These changes may extend to impact species that rely on these plant communities, such as bees. Bees are ecologically diverse and serve important pollinator roles but are at risk from the loss and change of floral and nesting resources in plant communities. Our objectives were to determine whether Kentucky bluegrass and smooth brome alter the bee and forb species richness in invaded Northern Great Plains grasslands and how litter accumulation, grass cover, the amount of bare ground, and forb species richness interact with bee functional traits. To do this, we surveyed 67 grassland sites from 2017 to 2020 with two bee-sampling methods (268 netting and bee bowl surveys total) and vegetation cover surveys at each site. We collected 20,559 bees from 201 bee species and observed 249 forb and shrub species in vegetation surveys. Bee richness and Shannon diversity were associated with greater forb richness while forb richness was significantly lower with thicker litter depths and higher with a greater coverage of all grasses other than Kentucky bluegrass and smooth brome. Bee trait analyses showed varying relationships with plant community variables. Of these, litter depth and Kentucky bluegrass cover were associated with lower ground-nesting bee abundance while small-bodied bee abundance was positively associated with increasing bare ground. While our results support the close relationship between bee and plant diversity, we also found litter depth, in particular, contributed to the structure of these two communities. Specifically, Kentucky bluegrass and smooth brome are two exotic grass species associated with thatch-forming litter layers, especially under idle management that appear to simplify bee and forb communities. Our results make apparent the importance in maintaining structural and compositional diversity in invaded grasslands to support diverse bee communities.

Vegetation complexity and nesting resource availability predict bee diversity and functional traits in community gardens.

Urban gardens can support diverse bee communities through resource provision in resource poor environments. Yet the effects of local habitat and landscape factors on wild bee communities in cities is still insufficiently understood, nor is how this information could be applied to urban wildlife conservation. Here we investigate how taxonomic and functional diversity of wild bees and their traits in urban community gardens are related to garden factors and surrounding landscape factors (e.g., plant diversity, amount of bare ground, amount of nesting resources, amount of landscape imperviousness). Using active and passive methods in 18 community gardens in Berlin, Germany, we documented 26 genera and 102 species of bees. We found that higher plant species richness and plant diversity as well as higher amounts of deadwood in gardens leads to higher numbers of wild bee species and bee (functional) diversity. Furthermore, higher landscape imperviousness surrounding gardens correlates with more cavity nesting bees, whereas a higher amount of bare ground correlates with more ground-nesting bees. Pollen specialization was positively associated with plant diversity, but no factors strongly predicted the proportion of endangered bees. Our results suggest that, aside from foraging resources, nesting resources should be implemented in management for more pollinator-friendly gardens. If designed and managed using such evidence-based strategies, urban gardens can create valuable foraging and nesting habitats for taxonomically and functionally diverse bee communities in cities.

The importance of soil and vegetation characteristics for establishing ground-nesting bee aggregations

Most bee species are ground-nesters, yet knowledge on the nesting behaviour of this diverse group remains sparse. Evidence on the effectiveness of ground-nesting bee species as crop pollinators is growing, but there is limited information on their nesting habits and preferences and how to manage habitats to enhance populations on farms. In this study, artificially prepared plots of bare soil were constructed with the aim to attract ground-nesting bees to nest in a commercial orchard in Kent, UK. Nine soil parameters were measured to determine their preferred soil properties: hydraulic conductivity, soil compaction, soil moisture, soil temperature, soil stoniness, soil organic matter, soil root biomass, soil texture and vegetation cover. Eighteen non-parasitic ground-nesting bee species (7 Andrena, 9 Lasioglossum, 1 Halictus and 1 Colletes spp.) were recorded in the study plots. Soil stoniness and soil temperature at 10cm depth were positively correlated, and vegetation cover and hydraulic conductivity were negatively correlated with the number of ground-nesting bees on the plots. We show that artificially created habitats can be exploited for nesting by several ground-nesting bee species. This study’s findings can inform management practices to enhance ground-nesting bee populations in agricultural and urban areas.

Ground-nesting bees prefer bare ground areas on calcareous grasslands

Although most of the wild bee species are ground-nesting, little is known about their nesting requirements and the conservation measures to promote ground-nesting bees. Calcareous grasslands are one of the most species-rich habitats in Central Europe and therefore essential for protecting bee diversity. The management practices of calcareous grasslands are so far often focused on plant populations, but in order to support declining insect populations, additional measures have to be considered for nature conservation. As wild bees depend on the availability of nesting sites, we studied the effects of locally increased bare ground availability on ground-nesting bees on four large and four small calcareous grasslands in Central Germany. Vegetation cover of 24 experimental plots (1 m²) was removed. The number of bee nests as well as the presence of bees on these plots (here: nesting activity) were compared to 24 control plots during six sampling runs. Results showed that the number of bee nests on experimental plots was fourteen times higher compared to control plots, positively related to the surrounding flower cover and higher on steeper slopes, independent of grassland area. Moreover, the observed nesting activity on experimental plots was 2.5 times higher compared to the control and positively related to the currently prevailing soil surface temperature. Bee abundance quantified during transect walks increased on grasslands with a generally higher bare ground availability and a higher flower cover. In conclusion, our study emphasizes the need to consider the availability of nesting resources to promote the vast majority of wild bees, which are ground-nesting. They benefited from bare ground availability as well as adjacent floral resources, and experimental removal of the vegetation cover appeared to be a major, so far underestimated conservation practice. • Ground-nesting bees prefer artificially created vegetation free patches on calcareous grasslands. • The abundance of (ground-nesting) bees is enhanced by a higher bare ground availability. • Nearby floral resources and steeper slopes increase the nest number of ground-nesting bees. • Higher soil temperatures increased the nesting activity of ground nesting bees. • Habitat size has no influence on bee abundance, the proportion of ground-nesting bees or colonization.

Artificial Nesting Hills Promote Wild Bees in Agricultural Landscapes.

Simple SummaryThe majority of wild bees nest underground in the soil. Many of these species are threatened by loss of habitat. Adequate nesting sites are particularly limited in agricultural landscapes. We therefore established 20 artificial nesting hills in Germany to promote ground-nesting bees. A detailed construction guide for the hills is presented. On the basis of our study results, we recommend the establishment of hills on sun-exposed sites and to ensure their existence for many years. We constructed hills from local soil with dimensions of 9 × 3 × 1.6 m. During a two-year bee monitoring initiative, we recorded a high diversity of soil-nesting bee species. We conclude that artificial nesting hills can act as a valuable nesting resource for wild bees.The availability of nesting resources influences the persistence and survival of bee communities. Although a positive effect of artificial nesting structures has frequently been shown for aboveground cavity-nesting wild bees, studies on below ground-nesting bees are rare. Artificial nesting hills designed to provide nesting habitats for ground-nesting bees were therefore established within the BienABest project in 20 regions across Germany. Wild bee communities were monitored for two consecutive years, accompanied by recordings of landscape and abiotic nest site variables. Bee activity and species richness increased from the first to the second year after establishment; this was particularly pronounced in landscapes with a low cover of semi-natural habitat. The nesting hills were successively colonized, indicating that they should exist for many years, thereby promoting a species-rich bee community. We recommend the construction of nesting hills on sun-exposed sites with a high thermal gain of the substrate because the bees prefer south-facing sites with high soil temperatures. Although the soil composition of the nesting hills plays a minor role, we suggest using local soil to match the needs of the local bee community. We conclude that artificial nesting structures for ground-nesting bees act as a valuable nesting resource for various bee species, particularly in highly degraded landscapes. We offer a construction and maintenance guide for the successful establishment of nesting hills for bee conservation.

Descriptions of the Mature Larvae of Three Australian Ground-Nesting Bees(Hymenoptera: Colletidae: Diphaglossinae and Neopasiphaeinae)

Fully fed larvae of three Australian bee species formerly classified as Colletidae: Paracolletini are described and compared in light of recent phylogenetic studies. Two of these species, Leioproctus (Goniocolletes) wanni (Leijs and Hogendoorn) and Trichocolletes orientalis Batley and Houston, belong in the Neopasiphaeinae, while the third, Paracolletes crassipes Smith, belongs in Diphaglossinae: Paracolletini (sensu Almeida et al., 2019). We find that larval characters support the separation of Paracolletes from the neopasiphaeines, in particular the spoutlike salivary gland opening associated with cocoon spinning. In addition, we suggest that nest architecture of P. crassipes includes a feature that prevents flooding of open brood cells, a feature common to a number of other large ground-nesting bees.

Honey bee toxicological responses do not accurately predict environmental risk of imidacloprid to a solitary ground-nesting bee species

Honey bees (Apis mellifera) are the current model species for pesticide risk assessments, but considering bee diversity, their life histories, and paucity of non-eusocial bee data, this approach could underestimate risk. We assessed whether honey bees were an adequate risk predictor to non-targets. We conducted oral and contact bioassays for Leioproctus paahaumaa, a solitary ground-nesting bee, and A. mellifera, using imidacloprid (neonicotinoid) and dimethoate (organophosphate). The bees responded inconsistently; L. paahaumaa were 36 and 194 times more susceptible to oral and topically applied imidacloprid than A. mellifera, but showed comparable sensitivity to dimethoate. Furthermore, the proposed safety factor of ten applied to honey bee endpoints did not cover the interspecific sensitivity difference. Our standard-setting study highlights the urgent need for more comparative inter-species toxicity studies and the development of standardized toxicity protocols to ensure regulatory pesticide risk assessment frameworks are protective of diverse pollinators.