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Mason bee
Mason bee
Mason bee
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Mason bee
Osmia bicornis
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Hymenoptera
Family: Megachilidae
Subfamily: Megachilinae
Tribe: Osmiini
Genus: Osmia
Panzer, 1806
Type species
Apis bicornis
Linnaeus, 1758
Species

>300 species, including

Homemade nest block showing full occupancy
Mason bee nest cell with egg on pollen bed
Worksheet cycle of larvae to cocoon

Mason bee is a name now commonly used for species of bees in the genus Osmia, of the family Megachilidae. Mason bees are named for their habit of using mud or other "masonry" products in constructing their nests, which are made in naturally occurring gaps such as between cracks in stones or other small dark cavities. When available, some species preferentially use hollow stems or holes in wood made by wood-boring insects.[1]

Species of the genus include the orchard mason bee O. lignaria, the blueberry bee O. ribifloris, the hornfaced bee O. cornifrons, and the red mason bee O. bicornis. The former two are native to the Americas, the third to eastern Asia, and the latter to the European continent, although O. lignaria and O. cornifrons have been moved from their native ranges for commercial purposes. Over 300 species are found across the Northern Hemisphere. Most occur in temperate habitats within the Palearctic and Nearctic realms, and are active from spring through late summer.[2]

Osmia species are frequently metallic green or blue, although many are blackish and at least one rust-red. Most have black ventral scopae which are difficult to notice unless laden with pollen.[1] They have arolia between their claws, unlike Megachile or Anthidium species.[1]

Historically, the term mason bee has also been used to refer to bees from a number of other genera under Megachilidae such as Chalicodoma, most notably in "The Mason-Bees" by Jean-Henri Fabre and his translator Alexander Teixeira de Mattos in 1914.[3]

Life cycle

[edit]

Unlike honey bees (Apis) or bumblebees (Bombus), Osmia species are solitary; every female is fertile and makes her own nest, and no worker bees for these species exist.[1]

Eastern snail shell mason bee (Osmia conjuncta)
Hornfaced bee (Osmia cornifrons)

When the bees emerge from their cocoons, the males exit first. The males typically remain near the nests waiting for the females, and some are known to actively extract females from their cocoons. When the females emerge, they mate with one or several males. The males soon die, and within a few days the females begin provisioning their nests.

Osmia females typically nest in narrow gaps and naturally occurring tubular cavities.[1] Commonly, this means in hollow twigs but can be in abandoned nests of wood-boring beetles or carpenter bees, in snail shells, under bark, or in other small protected cavities.[4] They do not excavate their own nests. The material used for the cell can be clay, mud, grit, or chewed plant tissue. The palearctic species O. avosetta is one of a few species known for lining their nest burrows with flower petals.[5] A female might inspect several potential nests before settling in.

Within a few days of mating, the female has selected a nest site and has begun to visit flowers to gather pollen and nectar for her nests; many trips are needed to complete a pollen/nectar provision mass.[6] Once a provision mass is complete, the bee backs into the hole and lays an egg on top of the mass.[7] Then, she creates a partition of "mud", which doubles as the back of the next cell.[7] The process continues until she has filled the cavity.[7] Female eggs are laid in the back of the nest and male eggs toward the front.

Once a bee has finished with a nest, she plugs the entrance to the tube, and then may seek out another nest location.[7]

Within weeks of hatching, the larva has probably consumed all of its provisions and begins spinning a cocoon around itself and enters the pupal stage, and the adult matures either in the fall or winter, hibernating inside its insulatory cocoon.[8][9] Most Osmia species are found in places where the temperature drops below 0 °C (32 °F) for long durations and they are well-adapted to cold winters; chilling seems to be a requirement for maturation.[2] Some species of mason bees are semi-voltine, meaning that they have a two-year maturation cycle, with a full year (plus) spent as a larva.[1]

Anatomy and morphology

[edit]

Osmia share a basic anatomy with all bees and most insects; the main functional regions being the head, thorax, and abdomen. On the head, Osmia have three small ocelli, two large compound eyes, antennae, and a mouth. On the thorax, Osmia have six legs and four wings. The abdomen of females contains a scopa for pollen-collecting, absent in males. Although the scopa is usually located on the legs in most bees, it lies underneath the abdomen for Osmia and other genera in the family Megachilidae.[10]

Pollination

[edit]

Osmia can pollinate very efficiently, which is largely attributed to their anatomy and behavior. Unlike most other bee species that collect pollen from their hind legs, female Osmia and other bees in the family Megachilidae use pollen-collecting hairs from their abdominal scopa. When Osmia transfer pollen to flowers, dry pollen falls from the scopa onto the flower's stigma, facilitating pollination at nearly every visit. Osmia typically pollinate early spring flowers in the family Rosaceae, and will even forage under poor weather conditions.[11]

Some farmers currently manage populations of Osmia to facilitate efficient pollination on their farms. However, using non-native Osmia species as managed pollinators has ignited the spread of disease, introducing invasive bee species that increase competition for native bees. In some areas, native Osmia species are in decline as of 2020; practices to minimize the impact of non-native pollinators on wild species include prioritizing the use of native bee species, raising local bee populations, and enforcing parasite/disease screening.[12]

Management

[edit]
A Mason Bee working on the terminal wall of a piece of bamboo in which it has laid its eggs

Solitary bees produce neither honey nor beeswax. They are immune from acarine and Varroa mites, but have their own unique parasites, pests, and diseases. The nesting habits of many Osmia species lend themselves to easy cultivation, and a number of Osmia species are commercially propagated in different parts of the world to improve pollination in fruit and nut production.[13] Commercial pollinators include O. lignaria, O. bicornis, O. cornuta, O. cornifrons, O. ribifloris, and O. californica. They are used both as an alternative to and as an augmentation for European honey bees. Mason bees used for orchard and other agricultural applications are all readily attracted to nesting holes – reeds, paper tubes, nesting trays, or drilled blocks of wood; in their dormant season, they can be transported as intact nests (tubes, blocks, etc.) or as loose cocoons.[14] As is characteristic of solitary bees, Osmia species are very docile and rarely sting when handled (only under distress such as when wet or squeezed); their sting is small and not painful, and their stinger is unbarbed.

See also

[edit]

References

[edit]

Further reading

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Mason bee

View on Grokipedia
from Grokipedia
Mason bees (Osmia spp.) are a diverse group of solitary bees in the family , renowned for constructing partitioned nests using mud in pre-existing cavities such as hollow stems, reeds, or drilled holes. Native to various regions worldwide, including where approximately 150 occur, they are highly efficient pollinators of early-spring blooming plants, particularly fruit trees like apples, cherries, and plums. Unlike social bees, mason bees do not form colonies or produce but nest gregariously, often building adjacent to one another without aggressive interactions. Physically, mason bees are robust and fly-like, measuring 0.2 to 0.8 inches in length with round, broad heads and wide abdomens that typically lack hair bands or markings. Coloration varies across species, ranging from metallic green and dark blue to black; for instance, the common blue orchard bee () exhibits a shiny dark blue-green sheen, while females are roughly the size of honey bees and males are slightly smaller with longer antennae (13 segments versus 12 in females) and light-colored . These bees are non-aggressive and sting only if roughly handled, posing minimal risk to humans. Mason bees follow a univoltine life cycle, completing one generation annually and overwintering as adults within protective cocoons. Adults emerge in early spring—typically mid-March to mid-May in temperate regions when temperatures reach 50–55°F—with males appearing first to await females for . Each female then seeks a nesting site, provisions brood cells with balls of and , lays an egg in each, and seals the chambers with mud partitions, often marking the entrance with a unique scent to deter intruders. Larvae develop through summer, pupate by fall, and remain dormant until the following spring. Their nesting habits make mason bees amenable to human management; they readily occupy artificial nests like drilled wood blocks or paper tubes (ideally 5/16-inch diameter and 6 inches deep) placed 3 feet or higher, facing or southeast near sources of moist clay and early-blooming flowers. In natural settings, they utilize cavities in wood, banks, or plant stems. As pollinators, mason bees are exceptionally effective, visiting more flowers per minute than honey bees and transferring to up to 95% of the flowers they contact, which enhances fruit set in orchards. Species like are commercially reared for this purpose, contributing significant economic value to fruit production while supporting as native, non-stinging alternatives to managed honey bees.

Taxonomy

Etymology

The term "mason bee" derives from the ' characteristic nesting behavior, in which they use , clay, or similar materials to build partitions and seals within their nests, mimicking the constructive work of a human mason. The English first appeared in the late , with the earliest recorded use dating to in the writings of naturalist . The scientific genus name Osmia, to which most mason bees belong, originates from the ancient Greek word osme meaning "odor," a reference unrelated to their masonry-like habits and likely alluding to the lemony scent produced by some species to mark nests. This theme of construction is echoed in common names across languages; for instance, murarbi (mason bee) in Swedish and abeja albañil (mason or builder bee) in Spanish.

Classification

Mason bees belong to the order , suborder , superfamily , and family , a group that includes both mason bees and leafcutter bees characterized by their use of plant-derived materials for nesting. The family is the third-largest bee family, comprising predominantly solitary species that construct individual nests rather than communal hives. Within , mason bees are primarily classified in the subfamily Megachilinae and tribe Osmiini, with the genus Osmia serving as the main taxonomic anchor for the group. The "mason bee" designation most commonly applies to Osmia species, which use to partition nest cells, but it occasionally extends to related genera in Osmiini, such as Hoplitis, and in broader contexts to certain Megachile species that incorporate alongside leaf materials in their nests. Unlike the eusocial bees of the family —such as honeybees (Apis spp.) that form colonies with division of labor—mason bees are solitary, with each female provisioning and defending her own nest independently. Mason bees trace their evolutionary origins to ancient bee lineages within , diverging early in the radiation of . Fossil evidence from the Eocene epoch, including an extinct solitary mason bee preserved in 42-million-year-old , reveals mud-nesting behaviors akin to modern species, underscoring their long-standing to solitary, material-based nesting strategies. Phylogenetic analyses confirm Osmiini as a diverse, cosmopolitan tribe within , with mason bees representing a key branch in the family's evolutionary tree.

Diversity

Mason bees exhibit significant diversity within the family , primarily in the tribe Osmiini, where the genus Osmia alone encompasses over 350 species distributed worldwide. These species are characterized by their solitary lifestyles and use of mud or similar materials for nesting, distinguishing them from other megachilid bees like leafcutters. The tribe Osmiini includes approximately 19 genera and at least 1,000 described species, many of which share mason-like nesting habits but vary in geographic range, size, and ecological roles. In , notable species include , commonly known as the blue orchard bee, which is valued for its services in orchards and native to much of the continent. Another key species is Osmia bruneri, or Bruner's mason bee, found across western regions and known for its adaptability in using alternative nesting materials like fruit pulp when mud is scarce. European and Asian regions host prominent species such as Osmia cornuta, the European orchard bee, which is a effective pollinator of early-blooming fruit trees and has recently established populations in . Similarly, Osmia bicornis, the red mason bee, is widespread in and , recognized for its rusty-red hairs and preference for nesting in existing cavities like mortar joints. Variations among mason bee species include differences in voltinism, with most being univoltine—completing one generation per year and overwintering as adults—but some in warmer climates exhibiting bivoltine patterns with two generations annually. Nesting materials also vary; while Osmia species typically use mud to partition and seal brood chambers, related genera within , such as Megachile, employ chewed leaves instead, highlighting adaptive diversity in resource use across the family.

Physical characteristics

Size and coloration

Mason bees in the genus Osmia typically range from 3/8 to 5/8 inch (9.5–16 mm) in length, with females generally larger than males to accommodate egg-laying capabilities. This size variation occurs across species, though many, such as the blue orchard mason bee (Osmia lignaria), are comparable to honey bees in stature, measuring around 10–12 mm. Males tend to be slimmer and slightly smaller, often exhibiting a more streamlined body form. Coloration among mason bees is diverse but predominantly metallic, featuring shades of blue, green, or black that give the a shiny appearance. Some display reddish or brown tones due to dense gingery hairs covering parts of the body, as seen in the red mason bee (Osmia bicornis), while duller, non-metallic variants exist in less vibrant with matte black or subdued green hues. is evident in coloration and structure, with females possessing robust, broader abdomens often equipped with a of colored hairs (yellow, white, brown, or black) for transport, whereas males are slimmer and frequently have distinctive white facial hairs. Mason bees can be distinguished from similar bees like bumblebees by their smaller size and lack of dense fuzziness, presenting a smoother, more metallic sheen instead of the plush pelage typical of bumblebees.

Anatomy

Mason bees, belonging to the genus Osmia in the family , exhibit a robust body structure divided into three main segments: the head, , and . The is notably hairy, particularly in females, which possess a ventral —a dense brush of specialized setae on the underside of the abdominal segments for collecting and transporting dry . This allows females to carry pollen externally without moistening it, distinguishing them from species like honey bees that use corbiculae on the hind legs. The first abdominal segment (metasoma) is fused to the , with females typically showing six exposed metasomal terga and males seven; the metanotum of the slopes sharply downward posteriorly, contributing to their compact form. The mouthparts of mason bees are adapted for both feeding and nesting activities. They feature a short , typically not extending to the metasoma, suited for accessing in shallow, open flowers such as those of fruit trees. Strong, broadened mandibles with 2–7 apical teeth in females enable the manipulation and mixing of or other materials for cell partitions. These mandibles also assist in shaping provisions during nest construction. Mason bee wings consist of two pairs: clear forewings with two submarginal cells and smaller hind wings featuring a jugal lobe one-third or less the length of the vannal lobe, characteristic of the family. At rest, the wings fold longitudinally under the , lying flush with the body for protection. Reproductive anatomy differs between sexes. Females possess a modified , derived from the seventh and eighth abdominal segments, which functions for egg-laying into provisioned nest cells and doubles as a sting for defense, though mason bees are generally non-aggressive. Eggs are typically laid with one end partially inserted into the pollen-nectar mass. Males lack a but have external genitalia including the gonobase, gonocoxites, gonostyli, volsellae, and valves, used during ; they also feature a on the lateral margin of the sixth tergum. Sensory features include large compound eyes that detect movement and color, aiding in navigation and foraging, alongside three small ocelli sensitive to light intensity. Antennae, with 12 segments in females and 13 in males, serve as primary chemosensory organs for detecting pheromones, volatile organic compounds, and environmental cues during mating and nest location. The eyes are bare, and genal areas are narrow, especially in males.

Distribution and habitat

Geographic range

Mason bees of the genus Osmia are primarily native to the , encompassing over 350 species distributed across the Palearctic and Nearctic realms. In , these bees are widespread, ranging from southern to , with approximately 140 species documented across the continent. For instance, the blue orchard bee () occupies temperate regions throughout most of the (excluding , , and ), southern provinces of from to , and northern states of including and Chihuahua. This species exhibits two , with O. lignaria propinqua predominant west of the 100th meridian and O. lignaria lignaria to the east, spanning elevations from to 2,000 meters. In and , mason bees are similarly abundant in temperate zones. The European orchard bee (Osmia cornuta) is native to southern and (extending north to ), , and western , where it serves as a key in Mediterranean regions. In eastern , species such as the hornfaced bee (Osmia cornifrons) are native to and Korea and have been actively managed for orchard pollination in since the mid-20th century. Human-mediated introductions have further expanded ranges; for example, O. cornuta has established populations in , , with potential spread to the , eastern , and southeastern . Presence in the Southern Hemisphere is limited, with no native Osmia species in Australia but some osmiine bees (closely related mason bees formerly classified under Osmia) occurring naturally in southern Africa, including reclassified species such as Wainia atrorufa and Hoplitis conchophila. As solitary bees, mason bees exhibit no active migration patterns; instead, their distribution expands passively through human activities, such as transporting nests for commercial pollination in orchards.

Preferred habitats

Mason bees, belonging to the genus Osmia, primarily utilize existing natural cavities for nesting, such as woodpecker holes, hollow reeds, borings in dead wood, or even shells. These bees show a strong preference for cavities with diameters ranging from 1/4 to 3/8 inch (approximately 6 to 10 mm) and depths of at least 3 to 6 inches, which provide suitable space for provisioning and sealing their nests. In their surrounding environment, mason bees favor sunny and sheltered locations proximate to sources of damp or clay for mud collection, as well as abundant early-blooming flowers from families like , , and . Common habitats include woodlands, orchards, gardens, prairies, shrublands, and forest edges, where a of dead wood, stems, and open ground supports both nesting and foraging needs. Microhabitat preferences emphasize south- or east-facing exposures to maximize morning warmth, while avoiding windy, excessively shaded, or overly exposed spots that could hinder emergence or activity. Mason bees exhibit adaptations that allow tolerance for a variety of climates, including deserts and forests, but they thrive best in temperate zones featuring distinct springs with temperatures rising to 50–55°F (10–13°C) to trigger emergence. Their ability to at relatively low temperatures and overwinter as dormant adults in cocoons enables exploitation of early-season floral resources, though extreme warming can disrupt these patterns.

Life cycle and behavior

Reproduction and nesting

Mason bees exhibit solitary reproductive behaviors, with no or division of labor among individuals. Adult males typically emerge from nests in early spring, about one week before peak bloom periods such as or apple flowering, and begin patrolling areas near potential nesting sites to locate newly emerged females. These males detect virgin females primarily through sex pheromones emitted by the females, which elicit courtship behaviors including mounting attempts and vibrations. often occurs near nest entrances or on nearby flowers, with females generally mating only once while males may mate multiple times; following copulation, males die shortly thereafter, and females commence nesting activities within 3-4 days. After , each independently selects a pre-existing cavity for nesting, such as hollow stems, reeds, or bored holes in wood, typically 1/4 to 3/8 inch in diameter and at least 3-6 inches deep. She begins by gathering and to form a provision mixed with in the rear of the cavity, upon which she lays a single —fertilized eggs develop into placed deeper in the nest, while unfertilized ones become males nearer the entrance. The then constructs a thin partition from to seal the cell, repeating the process of provisioning, oviposition, and partitioning to create 5-12 cells per nest, at a rate of 1-2 cells per day; the nest is finally capped with a thicker plug once complete. Although some related megachilid bees, like leafcutter bees, use chewed leaves for partitions, true mason bees (genus Osmia) rely primarily on sourced from nearby moist . Mason bees provide no beyond provisioning the cells, reflecting their solitary nature with no formation of colonies or hives. Once the nest is sealed, the female abandons it to seek additional nesting sites, potentially filling 2-3 tunnels in her lifetime before dying by mid-summer; offspring development proceeds independently within the sealed cells.

Development stages

Mason bees, like other bees in the genus Osmia, undergo complete , progressing through four distinct developmental stages: , , , and . This univoltine life cycle typically completes one generation per year, with most development occurring during the warmer months and a prolonged period for overwintering. The egg stage begins when a female mason bee lays a single small, white on a provision of and within a sealed brood cell in the nest. Eggs are cylindrical and measure about 1-2 mm in length, with a thin, flexible membrane. They hatch after approximately 1 week, influenced by ambient temperatures around 20-25°C. In the larval stage, the newly hatched —resembling a small white grub—begins feeding immediately on the stored pollen-nectar loaf provided by the . Larvae pass through 5 over 3-5 weeks, growing rapidly while defecating and molting between each ; the first often occurs within the remnants of the . By the fifth , typically in early summer, the mature larva spins a silken cocoon within the brood cell for protection during subsequent stages. The pupal stage involves inside the cocoon, where the transforms into an bee. This process begins after a brief prepupal of 2-6 weeks, followed by active pupation lasting about 1 month, resulting in a fully formed by late summer. The is initially pale and gradually darkens as wings, legs, and other structures develop. Adult occurs in early spring, typically when daytime temperatures consistently reach 10-15°C, with males emerging first to await females. Adults chew through the cocoon and any preceding partitions or nesting material to exit, often in mid-April for species like . The entire post-laying development from egg to spans 10-11 months. Overwintering occurs as fully formed adults within the protective cocoons inside the nest, enduring cold temperatures in a state of metabolic that can last 6-10 months depending on and latitude. This ensures survival until spring conditions trigger emergence.

Foraging and diet

Adult mason bees primarily feed on from flowers for their energy needs, while females also collect both for their own consumption and to provision larvae. This dual-purpose supports their short adult lifespan of 4-6 weeks, during which they focus on reproduction. Mason bees exhibit a broad diet, utilizing and from a variety of , though they show a particular affinity for species such as apples, cherries, and pears. Mason bee larvae rely on a mass-provisioned diet consisting of a mixture of and , commonly referred to as a pollen-nectar paste, which the female prepares and deposits in nest cells before laying an egg. This paste is rich in proteins derived from , essential for the larvae's rapid growth and development through multiple instars over 3-5 weeks. The high protein content supports metabolic processes and tissue formation, enabling the larvae to spin cocoons and pupate efficiently. Foraging activities of mason bees are confined to a range of approximately 0.5-1 km from their nest sites, allowing them to exploit nearby floral resources while minimizing energy expenditure. They preferentially visit open-faced flowers, such as those of fruit blossoms, which facilitate easy access to and due to their short mouthparts and style. This preference aligns with their role in early-spring of crops, where such flowers predominate. Mason bees demonstrate high foraging efficiency, with females capable of visiting 2,000 or more flowers per day under optimal conditions, far exceeding that of honey bees. Their activity is diurnal, peaking in the morning as temperatures rise, which coincides with the opening of many temperate flowers and enhances their access to fresh resources. This temporal pattern is regulated by their , synchronizing emergence and locomotion to environmental cues like light and warmth. Specialized structures like the on their abdomens enable efficient collection during these bouts.

Ecological role

Pollination services

Mason bees are highly effective pollinators due to their unique pollen collection method. Female mason bees gather dry using specialized scopal hairs on the underside of their abdomens, allowing grains to adhere directly to their bodies rather than being packed into leg baskets as in honeybees. This facilitates frequent contact with flower stigmas during foraging, pollinating nearly every flower they visit, in contrast to honey bees which pollinate a smaller proportion. These bees primarily target early-spring blooming plants, including crops such as apples, cherries, almonds, and blueberries, where their activity aligns with peak bloom periods. They also pollinate a range of native wildflowers, contributing to broader services. In terms of efficiency, a single female mason bee can visit and pollinate 2,000 to 3,000 flowers per day, far exceeding the capacity of individual honeybees. In commercial , approximately 250–300 mason bees can provide equivalent to about 90,000 honeybees for an acre of orchard, making them a valuable supplement for crops like apples and cherries. Since the 1970s, mason bees—particularly the blue orchard bee ()—have been employed commercially in North American orchards, often alongside honeybees, to enhance yields. Studies demonstrate they boost fruit set by 20-50%, with notable increases in cherry production reaching up to fivefold in managed settings.

Interactions with other species

Mason bees, belonging to the Osmia, engage in a range of interactions with other species, including predation, , competition for resources, and mutualistic associations that influence their survival and . Predators of mason bees include several bird species that target both adults and larvae. Woodpeckers, starlings, , swallows, and tanagers often excavate nesting blocks or natural cavities to consume bee larvae and pupae, causing significant damage to populations. Spiders prey on adult mason bees during foraging flights, while ants raid unguarded nests to feed on eggs, larvae, and provisions. Parasitic relationships pose a major threat to mason bee brood. Chalcid wasps of the genus Monodontomerus, such as M. obscurus, are common kleptoparasites that lay eggs directly into the pollen provisions of Osmia nests; their larvae consume the stored food and developing bee larvae, often leading to high mortality rates. The fungal pathogen chalkbrood, caused by species of Ascosphaera (e.g., A. aggregata), infects larvae through spores acquired by adults from flowers, resulting in mummified brood that hinders emergence. Mason bees face competition from other cavity-nesting solitary bees for limited nesting sites. Species like leafcutter bees (Megachile spp.) and other Osmia congeners, such as the blue orchard mason bee (O. caerulescens), vie for suitable holes in wood, reeds, or stems, potentially reducing reproductive success in dense aggregations. with honeybees (Apis mellifera) is minimal, as the latter nest in large colonies within hives rather than small cavities, leading to limited resource overlap beyond floral foraging. Mutualistic interactions support mason bee physiology and nesting. Certain plants, including species with hollow stems like common reed () and elderberry (* spp.), provide natural nesting cavities that mason bees readily use, facilitating their solitary lifestyle. Additionally, gut-associated in Osmia species, such as and , aid in the digestion of provisions and contribute to larval nutrition by breaking down complex carbohydrates.

Conservation and management

Threats

Mason bee populations are endangered by a combination of natural and anthropogenic factors that disrupt their solitary nesting habits and resource availability. Habitat loss and degradation, driven by , , and , diminish suitable nesting cavities in dead wood or stems and access to for nest , affecting a substantial portion of North American species. For example, prairie habitat conversion has contributed to declines in Osmia illinoensis. Pesticides represent a major chemical threat to mason bees. Neonicotinoids such as and are highly toxic to s via contact exposure, with showing greater sensitivity than honey bees (sensitivity ratios of 2.39 for and 7.96 for ). Pyrethroids and macrocyclic lactones are also toxic to s, though less so than neonicotinoids. Studies indicate that larval exposure to neonicotinoids in provisions can cause sublethal effects, such as reduced , though direct mortality is not always observed. Climate change intensifies these risks through phenological mismatches, where warmer temperatures delay bee emergence relative to floral blooms, and prolonged droughts that restrict mud sources in arid regions. Experimental warming of nests in led to 70% mortality in Osmia spp. over two years, compared to under 2% in controls, alongside reduced adult body mass and fat reserves. Diseases further compound vulnerabilities, with chalkbrood fungus (Ascosphaera torchioi) infecting larvae by outcompeting them for resources, resulting in chalky mummification and dispersal that can devastate provisions. Nosema ceranae microsporidiosis impairs larval growth and in Osmia bicornis. Their solitary limits , amplifying susceptibility to such pathogens compared to social bees. Additional pressures arise from resource competition from invasive congeners such as O. cornifrons and O. taurus, with O. taurus populations surging 800% since 2003 in the Mid-Atlantic U.S. while six native species declined 76–91% over approximately 15 years. Parasites including Chaetodactylus mites briefly referenced here contribute to nest failures but are detailed elsewhere.

Human management practices

Human management of mason bees, particularly species like the blue orchard bee (Osmia lignaria), involves providing artificial nesting structures, handling cocoons to synchronize emergence with crop bloom, supporting garden habitats, and rearing populations for commercial pollination. Amid severe honey bee colony losses (55.6% of managed colonies lost between April 2024 and April 2025), mason bees are increasingly promoted as non-stinging, native alternatives to enhance pollination resilience. Nesting aids typically consist of bee houses made from drilled wooden blocks, grooved boards, or bundles of paper tubes and reeds with cavities approximately 7.5 mm in diameter and at least 15 cm long. These structures are placed 1-1.5 m above ground in sunny locations facing southeast, protected from wind and rain, often under overhangs or within wire enclosures to deter predators. Cocoon handling begins with harvesting nests in late summer after provisioning, followed by cleaning to remove parasites using screens or solutions. Cleaned cocoons are then refrigerated at 0-5°C for 180-200 days to overwinter, allowing timed release in spring by warming to 10-25°C for emergence protocols aligned with floral availability. In gardens, mason bees benefit from planting early-blooming pollinator-friendly flowers such as those in the rose family (e.g., apples, cherries), willows, and , alongside providing accessible mud sources like moist clay blocks for nest construction. sprays should be avoided during their active period from mid-April to mid-June to protect foraging adults and emerging offspring. Commercially, O. lignaria is reared for , with populations increased 2-6 fold annually through trap-nesting and release at rates of 250-300 females per acre for crops like apples and almonds. Transport regulations, implemented since the , emphasize regional shipping within states or defined U.S. zones (e.g., Western states) to prevent spread, with requiring pest levels below 1% and documentation of origins. Monitoring involves tracking emergence rates via cocoon counts or X-ray inspections, while integrated pest management includes early nest removal, protective screens, and sanitation to control parasites like Monodontomerus spp. at low thresholds.

References

  1. https://extension.usu.edu/pests/files/Bees/blue-orchard-bee.pdf
  2. https://hortsense.cahnrs.wsu.edu/fact-sheet/megachilidae-mason-bees-osmia-spp/
  3. https://extension.oregonstate.edu/catalog/pub/em-9130-nurturing-mason-bees-your-backyard-western-oregon
  4. https://txbeeinspection.tamu.edu/mason-bees/
  5. https://edis.ifas.ufl.edu/publication/IN982
  6. https://www.ars.usda.gov/research/publications/publication/?seqNo115=109261
  7. https://extension.psu.edu/mason-bees-in-the-home-garden/
  8. https://www.fs.usda.gov/wildflowers/pollinators/pollinator-of-the-month/mason_bees.shtml
  9. https://idtools.org/exotic_bee/index.cfm?packageID=1181&entityID=8979
  10. https://www.oed.com/dictionary/mason-bee_n
  11. https://nativebeeology.com/2018/05/21/osmia-powerhouse-pollinators/
  12. https://en.wiktionary.org/wiki/mason_bee
  13. https://www.spanish.academy/blog/bzzz-learn-everything-about-bees-in-spanish/
  14. https://idtools.org/exotic_bee/index.cfm?packageID=1185&entityID=9084
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