Giant Resin Bees – continuously gaining ground

Giant Resin Bees – continuously gaining ground

I first saw Giant Resin Bees two years ago. The first glance of them was coming back after a field day to the institute in Bologna. And then, looking closer, I was first quite excited about this huge bee. Italy has more than 900 bee species, and I feel quite ignorant very often seeing them in the field. But my excitement turned into concern very soon, realizing that it was an alien bee. However, they fit perfectly into this series of introduced bees. In which, I realize this only now, I’m writing only about bees I’ve seen myself. When I first saw it, I had never heard of Megachile sculpturalis (their scientific name) before. Which doesn’t mean anything, but this bee was what got me more conscious about invasive bees also in Europe.

The first sighting out of its native range in East Asia (Japan, China, and Korean peninsula) wasn’t in Europe, though. It was observed in North Carolina already in 1994 and has then expanded until reaching Canada in 2002. Like the wool carder bees last week, this species nests in cavities and uses holes in wood and stems. They also like “bee hotels”. Giant Resin bees arrived in Europe in 2008, when they were first observed in France, near Marseille. They spread quite fast throughout France, like a recent paper shows. And, again, they didn’t limit themselves to a single country. They soon arrived in Switzerland, then in Italy. It took some years though to arrive North of the Alps: the first record in Germany was in August 2015.

Giant Resin Bees – spreading over Europe

Since these first discoveries, M. sculpturalis had quite a success story in Europe. In France, it arrived already in the Pyrenees, near the Spanish border. The last sighting recorded in the cited paper was in 2016. Therefore, it’s quite probable that they already crossed the border. In Italy, it has reached Tuscany, as Laura Bortolotti (from the CREA-AA in Bologna) told me a few days ago. This means that it passed the Apennines, which, like the Alps or Pyrenees, are a geographical barrier for the distribution of a bee species. The first sighting in Germany was very near the Swiss border. Now it’s already in Freiburg, which is the northernmost finding until now. In the East, it arrived until Hungary.

Why I tell you all these geographical references has a simple reason. It shows the enormous invasive potential of Giant Resin Bees. This confirms the suggestion of Quaranta and colleagues: they suggested that the species arrived via wood transports from France. And as it’s a polylectic species (i.e. visiting a variety of plants, without specialization), and also accepts trapnests (“bee hotels”) for nesting, they can establish quite quickly once arrived in a new area. Actually, artificial nests seem to be their favourite places, as the study in France suggests. Only 28% of the nests were recorded in old trees. On the other hand, 67% were in bee hotels of some kind. The remaining 5% of the nests were in a wooden beam. This could be a bias (bee hotels are easier to find than natural nests and people observe them closely). But it could also mean that the trend to offer trapnests is helping this invasive bee out. We will see.

We have two human factors that seem to influence the spread of Giant Resin Bees. There’s a third one. Apparently these bees like the flowers of the Japanese pagoda tree. As Paul Westrich reports on his site, many nests he observed were provisioned with the pollen of this tree. It’s often planted in gardens and parks and flowers in late summer when M. sculpturalis is active. In Italy, however, also native pollen sources were recorded. There’s no evidence until now that there’s competition for pollen resources with other bee species – at least I didn’t find one. But if they really prefer introduced plants for foraging, a way to limit their distribution – and help native bees – would be to plant more native flowers. And the spread of exotic plants as a consequence of alien bee species should be monitored, as well.

And again – any impact on the native bee fauna?

A possible impact of this bee species, however, may be the competition for nesting spaces. In North America, they occupy nests of the carpenter bee Xylocopa virginica. And they seem to be aggressive towards other bee species. In France, this competition was also observed in nests of Mason Bees and Carpenter Bees. Bortolotti and Quaranta from the Institute in Bologna, who are following this bee in Italy, didn’t make direct observations of this behaviour. What they saw, however, was aggressive behaviour between females of the Giant Resin Bees themselves. They never saw them enter nests of other species, but they eagerly did with nests of their conspecifics.

Giant Resin Bees, invasive bees, pollinators

Look at those mandibles! Giant Resin Bees competing for nest sites. Photos: L. Bortolotti

However, they found some dead Carpenter Bees around the trap nests. Bortolotti couldn’t tell me if this was because of aggressive behaviour from the Asian species or for other reasons. To her, it didn’t seem likely. So the impact of this species remains unclear. But definitely needs monitoring to protect European bee species. We don’t know anything about any diseases these intruders could transmit as well… Bortolotti drew my attention also to another aspect: she observed that Giant Resin bees may compete with smaller species. Not for nest sites, but for nesting material, like plant hairs. Here’s where Wool Carder Bees come in. This time not as invasive species, but as native pollinators. In this aspect, the aggressivity of the males defending resources may be a good thing. It would be interesting to observe such an interaction between a male wool carder and a female M. sculpturalis… If anybody did, please tell me!


invasive species, pollination, Xylocopa, Anthidium

Xylocopa females were found dead in front of the nests. There may be some competition going on for nesting material with Wool Carder Bees. Photos: L. Bortolotti

A recently introduced bee in Bologna…

While I was researching for this post, I found a recent paper from Bortolotti and colleagues, reporting about another Asian bee recently arrived in Europe: Megachile disjunctiformis. It’s the first record out of its native range. This year it nested also in the institute’s garden, but I didn’t see it. Also, the two introduced species seem to compete for nest sites. But this is limited by the size of the second Asian species: it has a similar size to Wool Carder Bees. Therefore, it may occupy holes too small for the Giant Resin Bee. But, you may already notice, this puts another threat to this species and others of a similar size. We don’t know yet if they would supersede already existing nests of Mason Bees, for instance.

The flying season of this species is in summer (June to August in Japan) and they plug their nests with a mixture of mud and resin. Bee hotels may attract them, as they nest in cavities like many other invasive bees. This recent introduction, however, may be a chance to understand better the dynamics of invasion. Until now, it moved from the site of the first record in Castel Maggiore to Bologna. It’s under supervision from the very beginning… Maybe this helps to understand better also if and how invasive bees adapt to new environments.


invasive bees, pollination, pollinators

M. disjunctiformis are smaller than Giant Resin Bees. They may compete with Wool Carder Bees for nest sites. Photos: L. Bortolotti.

The flying period of this species apparently is the same than in its native range: females flew in June to August. However, some males were observed in September in 2017. This raises some questions: how many generations does this species have every year? In most bee species, males fly before females – but is it the same also in this one? One male seemed newly emerged, indicating a second generation – but does this correspond to the biology of this bee? Or is it a change in the new habitat that could make it even more invasive? We still don’t know.

I’m also wondering if we have to expect more and more invasive bees with global warming. The range of bees adapted to cooler climates is already shrinking and the competition with alien bee species could add up. I think there’s still a lot to understand in this area. But anyway, before it’s too late, we should begin to control much more strictly and thoroughly the introduction of alien (bee) species. Pollinators are already in a difficult situation, we shouldn’t be adding to this even more.


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