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.
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.
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.
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.
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.
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.
Scottish pollinators 