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Wool Carder Bees – an accidental introduction

Wool Carder Bees – an accidental introduction

I’m observing a patch of lamb’s-ear in the park of Hohenheim University. A male wool carder bee is hovering over it. When another male comes in, he makes clear that this is HIS patch. He does the same with bees from other species. Even much bigger bumblebees: he bumps into them and doesn’t let them alone until they give up and fly away. The flowers he’s patrolling are only for females of his species. He’s ruthless. No exceptions. He defends not only the pollen and nectar but chose the patch of lamb’s-ear because these plants are hairy. Very hairy. Female wool carder bees “shave” these hairs and use them to build the nest. The male hovers over plants he knows females like to improve his chances to mate. And while I’m thinking about this, fascinated by this little fellow – he bumps into me! Ruthless, as I said.

Wool carder bees are among my favourite bees. Though the list is very long… But anyway, they’re special in many ways. It’s this pronounced territorial behaviour, but also the nesting material they chose. Their larvae grow up in soft brood cells, like in a cotton pad. It’s one of the few solitary bee species in which the males are larger than the females. They don’t look like a typical bee at the first sight: though they grow up in a cushion of plant hairs, they’re yellow and black and not that hairy themselves. I’m trying to attract them to my roof terrace with plants they like, until now without success. I did an excursion this summer to wild meadows where betony was supposed to grow, another plant they like. Too late, unfortunately. But you may imagine that I wouldn’t write about them in a series about invasive bees if everything was just fine.

The mostly distributed unmanaged bee in the world

Anthidium manicatum, which is their scientific name, are quite common in gardens or parks throughout Europe. Their natural distribution covers the whole continent – with exception of the far North. Bees, in the end, mostly prefer it warm. In the 1960s, wool carder bees were accidentally introduced in North America. In contrast to buff-tailed bumblebees we discussed last week, this wasn’t intended at all. Wool carder bees aren’t known for being good at crop pollination. They prefer a range of wildflowers from the mint/deadnettle and the figwort family. These often unite both food and nest resources: the females provide their nests with the pollen of these plants and, in addition, they’re often hairy, providing for the brood cell cotton balls. So they aren’t interesting for farmers or any business offering pollination services.

But there’s a trait that eases their accidental introduction: they nest in cavities. Naturally, these would be crevices in rocks, fissures in wood or cavities in walls. As both wood and rocks are shipped all over the world, this is a way for wool carder bees to get some travel done and discover new habitat. By now, it’s the most widely distributed non-managed bee. After arriving in North America via New York, it’s making its way West. This is a problem: most native Anthidium species occur in the Western states. The rapid range expansion of the introduced wool-carders could replace the native species. I didn’t find indications for this, but the fast increase of their habitat range gives some reason for concern.

Studying the impact of an alien pollinator

The species didn’t stay in the US, it’s now also in Canada, Argentina, Paraguay, Uruguay and Brazil. Outside the Americas, it’s also on the Canary Islands and in New Zealand. Both places with unique fauna and flora. The aggressive behaviour of the males towards other bees makes them quite “dominant” over the resources they defend. When they attack other bees, they sometimes hurt them. I didn’t notice much of this bump the over-motivated male in the Hohenheim garden gave me. But the males have spines at the end of their abdomen. With those, they can injure other bees. However, there’s not much information about the real harm wool carders really do to native bee fauna. A study in New Zealand, concludes that it’s not a major threat to native flora and fauna.

This is interesting to discuss further: One observation drawing to this conclusion is that 80% of the plants that A. manicatum visited in this study were exotic. The authors say that there shouldn’t be enough proper food sources for this bee in New Zealand’s native ecosystems. In addition, on native plants, they observed mainly native Lasioglossum species.  These were attacked less often than the also introduced honey bees or bumblebees. And the attacks to the native bees were much shorter. Apparently, this happens also in Europe, it seems that the small furrow bees (like Lasioglossum) aren’t attacked as often. But, the authors themselves note that more research is necessary, especially on the potential distribution of weeds, to give a more definite answer.

Do wool carder bees do any harm?

For me, this shows the crux of science. Also in assessing risks and dangers, there’s no “one fits it all”. According to this study, there is not a major risk to New Zealand’s native ecosystems. But: what if these ecosystems are already disrupted by some factor? Let’s say nutrient input from adjacent areas or pollution, climate change or an invasive plant already competing with native flowers? There’s no yes or no answer to the question if wool carder bees do any harm. It will always depend on the conditions. This often is difficult to communicate. Some people take these ambiguities as a reason not to listen to scientists. Like those who deny global warming or flat-earthers.

In this case, one conclusion could be “introduced pollinators don’t do harm”. This wouldn’t be adequate, as last week’s example shows. But it also doesn’t mean that wool carder bees are “evil” because they attack other bees. Or innocent observers like me. The “truth” is a difficult thing. As my father told me from an early age: the world isn’t black or white, the reality is in between the extremes.

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