A lethal bully loose in the garden

By Athayde Tonhasca

It’s a sunny mid-afternoon on a midsummer day in a British garden. A bumble bee wavers lazily over a patch of lamb’s ear (Stachys byzantina), as if considering whether its flowers are worth a visit. Before the bee makes up its mind, out of nowhere a black and yellow projectile collides mightily against it. The stunned bee falters and dips in the air, and is hit again. Struggling to stay aloft, it turns around and flees as fast as its battered wings allow it. If the poor bee could glance back, it would spot the aggressor now turning its attention to an unsuspecting honey bee.

The bumble bee and honey bee had the misfortune of entering wool carder bee (Anthidium manicatum) territory. Males of this species are notoriously aggressive towards perceived threats, either other males or any bee that may have an eye for plants from which female wool carder bees collect pollen, nectar or nesting materials.

A male wool carder bee and details of its menacing abdominal spines. © Bruce Marlin (L), Soebe (R) Wikipedia Creative Commons

Nowadays ecological research is focused on addressing specific questions, often within tight budgets and time frames. But there was a time when ecologists would sit down, watch and take notes. This old-fashioned approach has revealed the ruthless determination of male wool carder bees. Their charge sheet includes knocking down five bees in quick succession, and breaking the wings of bumble bees and honey bees. Victims have been knocked to the ground and mauled by bites and strikes from the attacker’s abdominal spines. Two males have been spotted hovering face to face like stags readying for battle, and when they clashed, the smaller bee fell to the ground, wings outstretched and abdomen vibrating (presumably dying or limping away afterwards). 

Such Rambo-like aggressiveness has a biological cause: polyandry (from the Greek for ‘many husbands’), which is when a female mates with several males in a breeding season. This mating system is uncommon among bees; for most species, females copulate once with a single male. But wool carder bees are polyandrous, just like honey bees. Monogamy is not an option for wool carder bees, and males have to deal with another headache: a physiological quirk known as ‘last male sperm precedence’. This happens when the male copulating last in a sequence of partners has a better chance of fertilising the female. So to assure paternity, the male must fend off any potential competitor and do as much mating as possible with any female in his territory: as often as every six minutes. Like most solitary bees, female wool carder bees lay eggs continuously throughout the breeding season, and they can mate up to 12 times a week.

For the female, it’s not all excessive attention from aggressive lotharios: she benefits from a patch of pollen and nectar free of competitors. She can focus on feeding and building her nest, which celebrated naturalist and entomologist Jean-Henri Fabre considered ‘quite the most elegant specimen of entomological nest building’. She begins by stripping the fuzz from the leaves and stems of lamb’s ear and related plants such as mint, deadnettle and sage (family Lamiaceae). She rolls the material into a ball – watch it – an operation akin to ‘carding’, which is the process of separating wool threads for the production of cloth.

A carding machine and a female wool carder bee collecting nesting material from lamb’s ear. © Wikipedia Creative Commons

She carries this bundle to a pre-selected cavity such as a hole in dead wood, a crevice in the mortar joints of a wall, or a hollowed plant stem. She will build the nest high up, where she’s less likely to stumble into spider webs. Once inside the nest, she shapes the collected fibres into a cell in which she lays an egg and deposits a mass of nectar and pollen to provide for the larva. She builds several cells in a single cavity, then seals up the entrance.

Rendition of wool carder bee life stages inside of a cavity nest. From left to right: pupa, larva, egg, and adult (female). © Samantha Gallagher, University of Florida Featured Creatures

This bee is found in variety of habitats, from gardens to open woodland and coastal sand dunes, where it collects pollen and nectar from a range of plants. It has a Palearctic origin (Europe, Asia and North Africa), but was accidentally introduced to north-western USA is 1963. From there, it has dispersed throughout the country and the Americas all the way to Uruguay. It is spreading in Britain too: once confined to southern England, it has established in Dumfries and Galloway and was recorded in Edinburgh in 2011.

The wool carder bee is a successful coloniser and the most widespread non-managed bee species in the world, which makes one wonder about possible consequences to other bees. Studies in America, where this bee is listed as an invasive species, have shown that the common eastern bumble bee (Bombus impatiens) avoids foraging near the troublemaker, but is not affected otherwise. The territory claimed by a male wool carder bee is no bigger than 1.3 m2, so there are plenty of available spaces; apparently the bumble bee simply looks elsewhere for food. Similarly in New Zealand, where the wool carder bee arrived in 2006, there has been no indication of harm to the native bee fauna. It looks like local residents are good at adapting to this feisty newcomer.