Menacing tenants

By Athayde Tonhasca

In an apple orchard somewhere in the American state of Pennsylvania, an adult Japanese horn-faced bee (Osmia cornifrons) has just emerged from its nest and makes its way into the big wide world. The apple grower has high hopes for that bee; in fact, he bought many of them when they were still inside their cocoons. The Japanese horn-faced bee was introduced from Japan in the 1970s, and since then it has been widely used in the Eastern United States to improve the pollination of apples and other fruit trees such as peaches, pears and cherries.

A female Japanese horn-faced bee © Chelsey Ritner, Exotic Bee ID.

In their natural habitats, the Japanese horn-faced bee and similar species such as the red mason bee (O. bicornis) nest inside natural cavities such as hollowed reeds, tree holes and cracks in stones. Females use a range of materials, especially mud and pebbles, to build individual nest cells in which they lay an egg. When bees are done, they seal off the nest entrance with mud – so they are known as mason bees. Fruit growers offer bees nesting alternatives such as drilled blocks of wood or bunches of cardboard tubes tightly packed together.

Two types of mason bee nests used in orchards: cardboard tubes (a) and wood blocks (b). Pictures by N. Joshi © Kline et al., 2023.

The future seemed promising for that Japanese horn-faced bee in Pennsylvania. But opportunists were on standby, ready to pounce when an unsuspecting bee leaves its nest. In the blink of an eye, a gang of hypopi (singular hypopus) jumps on the bee, holding on for dear life as their ride flies away.

Hypopi, also known as hypopodes, are a special nymphal stage found in some mites. In this case, the hairy-footed pollen mite (Chaetodactylus krombeini). Hypopi have no head or mouthparts, but are armed with special structures for hanging on; either powerful claws or a sucker plate to glue themselves to their host. These adaptations greatly facilitate phoresis, which is when an organism attaches itself to another for the purpose of transportation. Phoresis is typically found in small and poorly mobile organisms such as nematodes and mites. But curiously, the hypopus stage is usually facultative for mites; it occurs only when conditions deteriorate (food scarcity, overcrowding, dry climate, etc.), so that skedaddling increases the likelihood of survival.

A hypopus, the stage adapted for phoresis © Reynolds et al., 2014.

The departing bee has no chance of avoiding the lurking hitchhikers who react instantaneously to the slightest touch to their dorsal setae (bristles) or to air movement caused by a passing body. And the feats of some of these mites defy credulity; the tiny Histiostoma laboratorium (formally known as H. genetica), a scourge of vinegar fly (Drosophila melanogaster) laboratory colonies, lurches into the air to grab fruit flies flying above them (Hall, 1959. J. Kansas Entomological Society 32: 45-46). Some species that have hummingbirds as hosts rush to the birds’ nostrils at a rate of 12 body-lengths per second, which is a speed similar to a cheetah’s (Colwell, 1985)

Hypopi attached to their host © D.E. Walter, Invasive Mite Identification, Colorado State University and USDA/APHIS/PPQ Center for Plant Health Science and Technology.

After being mobbed by hypopi, the bee carries on with its life. If it’s a female, she will mate and start a nest of her own. When her brood cells are ready, her unwanted companions come out of their lethargic state, jump off and resume their development, maturing and reproducing very quickly, all the while feeding on the pollen and nectar gathered by the bee. When their numbers reach certain levels, they may feed on the bee’s eggs and larvae (details are sketchy). In a few months the mites may reach thousands and overrun the brood cell, leaving space for nothing else.

Hairy-footed pollen mites inside a mason bee nest cell © Pavel Klimov, Wikimedia Commons.

Such massive numbers of kleptoparasites (organisms that steal food from another one) spell serious trouble for Japanese horn-faced bees; their eggs and larvae die or develop poorly for lack of food or direct attack from mites. Some adult bees may not even have a chance to start a new family; they may be so burdened by mites that they cannot fly. They fall to the ground and become easy pickings for ants and other predators.

A mason bee loaded with pollen mites © GeeBee60, Wikimedia Commons.

Several mason bee species are susceptible to the hairy-footed pollen mite, but managed Japanese horn-faced bees have been hit particularly hard, with losses reaching up to 50% of the population. It’s not difficult to understand why. The same way slum housing conditions make people more vulnerable to all sorts of diseases, jam-packed nests increase the chances of mites passing from one bee to another. And the hairy-footed pollen mite does not even depend on phoresis: adults can walk from one nest to another nearby, getting inside through holes in the sealing mud made by parasitic wasps. To make the situation worse, this mite can turn into a dormant stage that survives several years inside an empty nest, rousing back to activity as soon as new tenants arrive.

The effects of the hairy-footed pollen mite on the Japanese horn-faced bee are a reminder of the unintended consequences of well-intentioned actions. Bee houses or bee ‘hotels’ have been promoted as enhancers of wild bee populations, but there’s no indication of such effects. They do however increase the risk of pathogens and parasites: not only mites, but a range of fungi, parasitic flies and wasps bedevil mason bees (Groulx & Forrest, 2017).

A bee hotel: not such a great idea © Colin Smith, Wikimedia Commons.

American fruit growers do their best to keep mites under control by replacing the nesting tubes yearly, sterilising wood blocks, or removing and storing bee cocoons during the winter. If you have a bee house but don’t have the resources, time or inclination to do the same, you should follow Colin Purrington‘s advice: buy a garden gnome instead.

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 pushy squatter on the march

By Athayde Tonhasca

In 2016, French researchers installed dozens of bee houses in twelve of Marseille’s public parks to monitor the local populations of solitary bees. In the following year, to the researchers’ consternation, most of the units had been taken over by the giant resin bee (Megachile sculpturalis) instead of mason bees (Osmia spp.), the usual bee house tenants. 

A female giant resin bee collecting pollen © Paula Sharp, Entomology and Nematology Department, University of Florida

The giant resin bee arrived in France via Marseille in 2008, probably as a stowaway from China, Japan or any of the other eastern Asian countries of its natural range. This bee nests mostly in wood cavities, so a shipment of timber was the likely means of entry. It spread quickly through France, northern Italy, Switzerland and South Germany; then it sneaked into Austria, Slovenia, Hungary and Spain. The European tour was not the giant resin bee’s first adventure outside its native area. It landed in the United States in 1994, probably through a port in the state of North Carolina. From there, it spread to most of the east coast estates. This newcomer is considerably larger than other solitary bees – females range from 22 to 27 mm – so it has been fairly easy to track its spread over Europe and America.

The spread of the giant resin bee in native areas (increasing year of records, from yellow to green) and in invaded areas (from yellow to red). Black dots are records with no available year © Polidori & Sánchez-Fernández, 2020. Global Ecology and Conservation 24 e01365

We may assume that the arrival of a bee can only be a good thing: the giant resin bee collects pollen from dozens of plant species, so it may join the local pollinating force. Unfortunately, it also adds to the roster of troublemakers that could cause havoc to other species and their habitats such as the honey bee (Apis mellifera) and the buff-tailed bumble bee (Bombus terrestris). Once outside their native ranges, these important pollinators are protagonists in well-documented cases of outcompeting other bees, endangering native species, transmitting diseases, and altering the local flora by pollinating aggressive weeds. 

Competition for nesting sites is one of the possible consequences of giant resin bee invasions. This bee nests in wooden structures, but it cannot chew through wood to excavate its own home. So it relies on pre-existing holes such as vacant beetle galleries, or openings in fallen or rotting wood. It will also readily take old nests of carpenter (Xylocopa spp.) and mason (Osmia spp.) bees. But empty cavities are not always available in sufficient number, and in these situations the giant resin bee resorts to brute force; in America, females have been caught pulling Eastern carpenter bees (Xylocopa virginica) out of their nests and making themselves at home. They will then build their own nest chambers with wood fibres, leaf fragments, mud, and resin.

The North American Eastern carpenter bee, a potential target of giant resin bee bullying © Bob Peterson, Wikipedia Creative Commons

In France and other European countries, several bees and wasps have been victims of nest usurpation such as the Mexican grass-carrying wasp (Isodontia mexicana). This wasp, itself an invasive species – and a recent arrival to Britain – captures grasshoppers and crickets and takes them to its nest to feed its larvae. But the wasp’s life plans can be ruined by a house-hunting giant resin bee, who will extract stored grasshoppers from the wasp’s nest and boot her out.

In Europe, Megachile lagopoda (L) and the Mexican grass-carrying wasp can lose their nests to giant resin bees © Gideon Pisanty (L) and pjt56, Wikipedia Creative Commons

Britain is not a home for the giant resin bee, but this is likely to change. It could arrive in a shipment of timber, or just come on its own: the English Channel is not much of a barrier for a bee capable of long distance flights thanks to its unusually large size. 

We can’t predict the consequences of such an invasion for our native bees. In America, the Eastern carpenter bee apparently has not been much affected, possibly because the life cycles of the two species don’t overlap completely. But in continental Europe, there is concern about the fate of several bees. The giant resin bee will take natural cavities or any manmade structure such as brick holes, metal pipes and plastic tubes, so bee houses are perfect for them. The observations in France suggest that the emergence of native bees from bee hotels is negatively correlated with the occurrence of giant resin bees. If that’s the case, bee houses may be not only detrimental to the native fauna, but also aid the spread of an invasive species.

A giant resin bee capping a bee house’s brood cell with resin and mud © Alonso Abugattas, Entomology and Nematology Department, University of Florida

If the giant resin bee becomes established in Britain, our plants may gain another pollinator, and our bees may have to deal with another hassle. Time will tell whether any of these outcomes are significant. As countries become increasingly connected by trade and travel, and the environment changes rapidly and unpredictably thanks to global warming, we will hear more stories such as the giant resin bee’s globetrotting.