Dubious helpers

By Athayde Tonhasca

Eikyu Matsuyama, an apple grower from Aomori (the northernmost prefecture on Japan’s main island), noticed mason bees building nests inside holes in utility poles and wooden walls near his orchard. As this species of bee (Osmia cornifrons) was a keen visitor of apple flowers, Mr Matsuyama pondered whether he could attract more bees to his orchard by supplying them with pieces of reed as extra nesting sites. This was the 1930s, when apple pollination in Japan was mostly done by hand – a labour-intensive, costly operation. Sometimes the military was recruited to help pollinate apple flowers during the two-week bloom period. Aomori was – and still is – the leading apple-producing region in Japan, accounting for nearly half of the country’s harvest. So any extra help from bees would be much appreciated by the growers.

An apple blossom in the Aomori prefecture with Mount Iwaki in the background © 岩浪陸, Wikimedia Commons.

Mr. Matsuyama’s experiment was a huge success; soon the numbers of mame-ko bachi (the bee’s Japanese name) expanded dramatically, with a corresponding improvement in apple production. Other growers quickly followed suit, and Mr Matsuyama went on to lecture about mason bees at several Japanese universities (Mader et al., 2010). 

In the 1960s, apple growers in the Aomori Prefecture began to use the European honey bee (Apis mellifera) as an alternative pollinator, while researchers improved propagation and management methods for the mame-ko bachi, named the Japanese hornfaced bee in English-speaking countries. Nowadays Aomori growers of apple, pears, peaches and plums are likely to use commercially available Japanese hornfaced bees instead of honey bees because the former is considerably more efficient. It visits about 15 flowers per minute (4050/day) and carries ~267,000 grains of pollen on its scopa (pollen-carrying bristles), while the honey bee visits about 6 flowers per minute (720/day) and transports ~100,000 grains of pollen. Equally important, the Japanese hornfaced bee, like other species in the group, is a bit sloppy in transferring pollen to her nest; about 10% of it remains on her body (Matsumoto et al., 2009). This residual pollen, viable for up to 12 days, has a good chance of ending up on a receptive apple flower and pollinating it.

A Japanese hornfaced bee leaving its nest © Beatriz Moisset, Wikimedia Commons.

The Aomori Prefecture achievements with the Japanese hornfaced bee didn’t go unnoticed. In the 1960s, the American Agricultural Research Service (Department of Agriculture) introduced this species to America to improve pollination of fruit crops such as apple and blueberry. Since then, the Japanese hornfaced bee has spread through the eastern and mid-west United States. And in 2002, another Asian mason bee was found doing its bit for pollination in America: the taurus mason bee (Osmia taurus). Nobody knows how or when this species entered the country: it probably was introduced accidentally at the time of Japanese hornfaced bee importation. These two species look alike superficially, so an unintentional introduction is plausible.

Female Japanese hornfaced bee (L) © Chelsey Ritner, and taurus mason bee © Chelsey Ritner, Exotic Bee ID.

Bees are overwhelmingly valued by the public, so the addition of two new species to the local fauna, even if inadvertently in one case, must be a good thing. Indeed, introduced bees may increase the local pollinating force. But these newcomers may also be harmful to native species and their habitat, most likely because of competition. This happens when two of more species need a common resource – food, water, a place to live – that is in short supply.

Assessing competition: Diet analyses from carbon (δ13C) and nitrogen (δ15N) isotopes indicate a considerable overlap of food taken by the invasive American mink (Neovison vison) and the native, critically endangered European mink (Mustela lutreola) in Spain; these results suggest a substantial competitive pressure imposed by the American mink on the European mink © García et al., 2020.

In the case of bees, the most likely problems from introduced species is competition for food (nectar and pollen) or nesting sites. These interlopers may cause less evident but no less serious problems; they may transmit novel pests and diseases to the native fauna, or they may alter the habitat by pollinating invasive plants and therefore helping them spread. These concerns are more than speculation; the European honey bee and the buff-tailed bumble bee (Bombus terrestris) are notorious for causing trouble in places where they have arrived. We know less about the other 80 or so bees introduced accidentally or deliberately outside their native range, but problems have been documented (e.g., Russo et al., 2020).

The yellow star-thistle (Centaurea solstitialis), a Mediterranean native, has been spreading through 41 of the 48 contiguous U.S. states thanks in part to pollination by the non-native European honey bee © J.smith, Wikimedia Commons.

The Japanese hornfaced bee and the taurus mason bee found America to be very much to their liking; their populations have increased dramatically since their arrival – reaching growth rates of three to five times per year, when conditions are favourable. This is not good news for the blue orchard bee (Osmia lignaria), a reliable orchard pollinator and a native species that has much in common with the intruders: they all emerge more or less at the same time, take pollen from similar plants, and have similar nesting habits. This overlapping of resources inevitably raises the possibility that the blue orchard bee would get the short end of the stick in these species interactions.  

A female blue orchard bee © Chelsey Ritner, Exotic Bee ID.

LeCroy et al. (2020) set out to assess just that. They examined trap catches of the two introduced bees, the blue orchard bee and other five native mason bees from four eastern American states from 2003 to 2017 (5,901 records). Their results: all native species declined from 76 to 91% since 2003, while the exotic species were doing just fine; populations of Japanese hornfaced bee were stable, while taurus mason bee numbers increased by 800% since 2003. This latecomer represented 22% of captures in the years 2003–2009, raising to over 43% of all captures from 2010 to 2017, making it the most common mason bee species in the region.

We don’t know the consequences of such dramatic population shifts. The two exotic species may take up the pollination work from the bees displaced by them, although a complete, seamless substitution is improbable: the dynamics between plants and pollinators are bound to become different, although we can’t say how. Native and alien bees may adapt and co-exist; in the worst case scenario, native species may become threatened. Many Americans would not be happy to watch their native bees pushed aside by intruders, no matter how benign or useful they may be. But there’s little they can do about it. Once alien species become established, it’s extremely difficult and expensive to get rid of them. 

Here in the UK, gardeners, allotment holders, schools and assorted nature lovers have shown increased interest in buying mason bees – and many suppliers, domestic and from abroad, are ready to oblige. But it’s impossible to tell whether these newcomers are healthy, or whether they will take food and shelter away from the local bee fauna. The case of the Japanese hornfaced bee and the taurus mason bee in the US is a cautionary tale about the uncertainties of species introductions: you are never sure how the story will end.

Pandora trying unsuccessfully to make amends for releasing humanity’s evils. Art by Frederick S. Church (1842–1924), Wikimedia Commons.

Home is where the heart is

By Athayde Tonhasca

Most of the 20,000 or so known species of bee build their nests in the ground, excavating tunnels and constructing chambers where they lay their eggs. But one group of species, the mason bees from the genus Osmia, took another path regarding housing. Most of them occupy or expand naturally occurring cavities – such as crevices under or between stones, cracks in a wall, holes in dead wood, hollow stems and tree bark – to transform them into cosy, safe environments in which to raise their young. 

Mason bees are solitary, i.e., each female builds and keeps a nest on her own. But they often nest close to each other, and in large numbers. They release scents to mark their nest entrances, which allow each bee to find her home among many others nearby (the genus Osmia was named after the Greek for ‘odour’; osmonosology is the branch of medicine dealing with organs of smell and olfactory disorders). 

Mason bees are quite resourceful in converting a cavity into a nest: depending on the species, they can use mud, chewed leaf material, pebbles, petals, pith and resin in diverse combinations to build chambers, walls and a plug to seal off the nest. 

A bee house with a section removed to reveal red mason bee (O. bicornis) nest chambers built with mud. The yellow dust is pollen.

Different species of mason bees have specific building skills, which allow them to occupy a variety of habitats. The mountain mason bee (O. inermis), one of Britain’s rarest species, is essentially a boreo-alpine denizen: it lives on wind-swept, open slopes, with not many accommodation options. That’s not a problem for this bee: it nests in crevices on bare, well-drained rocky surfaces.

A mountain mason bee © Arnstein Staverløkk/Norsk institutt for naturforskning, Wikimedia Commons, and its habitat.

An equally rare close relative, the pinewood mason bee (O. uncinata), is at home in areas of Scots pine (Pinus sylvestris) at lower altitudes. Here, the bee makes use of a material in ample supply: tree bark. Females drill their nests in the bark of living trees and dead stumps of Scots pine.

A female pinewood mason bee at her nest entrance © Müller et al., 2020. Alpine Entomology 4: 157–171, and her habitat © Richard Webb, Wikimedia Commons.

The lodging needs of mason bees can be quite specific: the hairy-horned mason bee (O. pilicornis) nests in fallen dead branches, while the large black-bellied mason bee (O. nigriventris) gnaws out its nest in pieces of larch (Larix decidua) and Scots pine lying on sun-exposed ground. 

Several mason bees and related species habitually nest inside empty snail shells. These structures are conveniently shaped to allow the arrangement of brood cells in a row, and easy closure at the shell aperture. Besides, shells are abundant, resistant and long-lasting housing units. In Britain, the gold-fringed (O. aurulenta), the two-coloured (O. bicolor) and the spined (O. spinulosa) mason bees are helicophiles (snail-lovers), and they go to a lot of effort to move shells to a good spot, build and provision the brood cells, seal the nests and sometimes camouflage them. You can watch a two-coloured mason bee at work here.

Opened nests of O. notata with brood cells side by side (14); O. pinguis with brood cells in a row (15); O. aurulentacovered with leaf pulp (16); Hoplitis fertoni with brood cells side by side (17, photo G. Le Goff); O. rufohirta with a single brood cell (18, photo P. Westrich); O. bicolor with a barrier of pebbles and earth (18, photo A. Krebs) © Müller et al., 2018. Journal of Hymenoptera Research 65: 61-89.

While many mason bees look for walls, bark, rocks or shells to find a home, a North American species, O. integra,goes for other more convenient, abundant and easy to work building materials. A female may nest on soft coastal dunes, or inside dried cow pats. Cow dung may seem an unusual choice, but it is soft, has no roots or stones to impede excavation, does not break apart easily once dried, and has good insulation. What’s not to like?

Osmia integra © Laurence Packer, Discover Life, and her home © Karora, Wikimedia Commons.

There are approximately 500 species of Osmia in the world and 12 in Britain. They are excellent pollinators, and some species are being widely used to complement or substitute the efforts of the European honey bee (Apis mellifera) in fruit tree orchards and other crops. These bees’ range of nesting options and their flexibility to adapt to local conditions make them dependable pollination agents.

A sloppy but efficient pollinator

By Athayde Tonhasca

We hear a lot about the pollination services provided by the European honey bee (Apis mellifera), so you may be surprised to know this bee is not that competent at its job. A honey bee moistens the pollen she collects and carries it tightly packed on her corbicula, or pollen baskets, so pollen grains are not easily dislodged when the bee visits another flower. Moreover, honey bees learn quickly to collect nectar with minimal contact with the flower’s anthers, so reducing the chances of pollen transfer. They are also good at flower constancy (the trait of visiting the same type of flower over and over), which is not good for plants that need cross-pollination between different varieties, such as apples. Thus, paradoxically, honey bees’ efficiency as food collectors reduces their efficiency as pollinators. These shortcomings are offset by the huge numbers of bee workers per hive and the fact that they are so amenable to management.

In comparison with the tidy honey bee, the red mason bee (Osmia bicornis) is a messy flower visitor. Females have low flower constancy, flying all over the place, and carry dry pollen loosely attached to their scopa (a mass of hairs under the abdomen). This means that pollen grains have a greater chance of becoming detached from the bees’ bodies and ending up on a flower’s stigma.

A red mason bee with her scopa loaded with pollen © Jeremy Early, Nature Conservation Imaging

What’s more, the red mason is one the most polylectic bees in Europe, that is, it collects pollen from a variety of flowers from unrelated species: 18 plant families altogether, including willows (Salix spp.), maples (Acer spp.), birch (Betula spp.), oaks (Quercus spp.) and several fruit trees in the family Rosaceae such as apples, pears, plums, cherries and peaches. Unsurprisingly, this bee is an excellent orchard pollinator; 500 or so female red masons can pollinate as many trees as 2-4 honey bee colonies. 

Like other Osmia species, the red mason is a cavity-nesting bee; it makes itself at home in preexisting holes and fissures in soil banks or dead wood, abandoned insect burrows, hollow stems, or cracks and holes in walls – which explains the common name, ‘the mason bee’. It may also excavate soft mortar, hence the reason for another common name: ‘the mortar bee.’ The red mason readily occupies man-made structures such as ventilation bricks, the space beneath roof tiles, even inside door locks. So this bee is the most likely tenant of bee houses.

Once a female occupies a cavity, she will construct a series of compartments (brood cells) and stock them with pollen as food for her offspring. She will then close the nest entrance with a mud plug. But she’s not done once the nest is finished: if conditions are right, she may build another six nests before the season is over. The larvae will eat the pollen and emerge as adults the following year to start the cycle again. See red mason bees in action here.

A session of a mason bee nest. Each cell contains one egg and a provision of pollen 

Mason bees tend to nest close to each other in aggregations of 50 to 250 females. And they are diligent pollinators, as demonstrated by these facts and figures:

  • A female bee may construct 16 cells per nest, 1 cell/day.
  • She will fly 300-400 m on average, up to 600 m, in search of flowers.
  • Nineteen foraging trips are needed to collect the pollen and nectar for each cell.
  • Her pollen load weighs 100-250 mg, up to 300 mg.
  • She may visit 75 flowers each trip, up to 25 flowers/min, and she will stock up each cell in about 3.5 h.
  • A cell with an egg that will develop into a female bee may contain 8 million grains of pollen. Fewer for male bees (they need less food): 4.6 million.

This hardworking bee is good news for wild flowers, and also for crop production. The red mason is an effective pollinator of rapeseed oil and a number of crops grown under polytunnels and glasshouses, such as strawberries and raspberries. Other mason bees have been managed as orchard pollinators in Japan and USA for many years; there is growing evidence that the red mason can play a similar role in orchards in Britain and other European countries.

A female red mason bee and sealed nests in a bee house

The red mason bee is common throughout most of the UK from late March to June/July. During this short time as an imago (the adult stage), this bee will contribute to the pollination of countless wild flowers, crops and fruit trees. The red mason bee deserves to share the spotlight with the honey bee.