It’s complicated

By Athayde Tonhasca

Himalayan balsam (Impatiens glandulifera) was brought to the British Isles in 1839 as an addition to Kew Gardens’ collection of ornamental plants. As usually happens with introduced species, Himalayan balsam escaped into the wild, causing consternation ever since. It has spread throughout damp woodlands and along rivers, flourishing in thick stands up to 2 metres high that overshadow the local vegetation. This plant does well in a variety of climatic conditions and soil types, and has a tremendous capacity to spread.

So nobody likes Himalayan balsam. Nobody but pollinators.

Himalayan balsam © MurielBendel, Wikipedia Creative Commons
Himalayan balsam © MurielBendel, Wikipedia Creative Commons

This invasive is a nectar factory. Each flower produces about 0.5 mg of sugar per hour, a rate far higher than any European plant; flowers of most species yield less than 0.1 mg/h. And because the plant flowers late in the season, nectar it available at a time when other sources start to become scarce. So naturally, bumble bees, honey bees and wasps go for it with gusto. And there is something in store for hoverflies as well; they feed on the copious amounts of pollen produced by these flowers. Predictably, the number of bumble bees and other insects increase in areas invaded by Himalayan balsam. 

This abundance of food could have undesirable side effects. Many bees get the proteins, carbohydrates, lipids and amino acids they need from a variety of pollen sources. But thanks to the plentiful and readily available pollen from Himalayan balsam, bees stick to this easy option: in some situations, up to 90% of the pollen collected by honey bees comes from this plant, with unknown consequences to bees’ development and health. The profusion of pollen and nectar could also indirectly harm other plants: if native species receive fewer visitors, their pollination could be compromised. But the evidence for such outcomes is contradictory. Some studies suggest that Himalayan balsam reduces flower visitation and seed production of native plants; others have demonstrated no differences, or a ‘magnet effect’: Himalayan balsam attracted pollinators to itself and to plants nearby. 

A marmalade hoverfly (Episyrphus balteatus) and a common carder bee (Bombus pascuorum), two Himalayan balsam beneficiaries © Charles James Sharp (L) and André Karwath, Wikipedia Creative Commons
A marmalade hoverfly (Episyrphus balteatus) and a common carder bee (Bombus pascuorum), two Himalayan balsam beneficiaries © Charles James Sharp (L) and André Karwath, Wikipedia Creative Commons

Alien species are a hot and controversial topic among conservationists. Some highlight the damage caused by introduced species to the native fauna and flora, habitats, the economy and even human health. But other conservationists point out that alien species may have neutral or positive impacts: that is, they are alien but not necessarily invasive. The invasiveness of Himalayan balsam has been well documented, but there are mitigating factors in its favour: in some situations, this plant had no effect on local species composition, or at worst it only replaced a few ruderal species (plants that colonise areas that have been disturbed). And its presence may check the spread of harmful alternatives such as the giant hogweed (Heracleum mantegazzianum).

Assessing the impact of alien species is important because a great deal of money and resources have been spent on controlling or eradicating them, quite often unsuccessfully. It is usually assumed that invasive plants are bad for pollinators, but there isn’t much evidence to support this assumption. Like many aspects of species’ ecology, data are scarce, results are often contradictory, and generalisations are risky. In summary: it’s complicated.

Come in, she said, I’ll give ya shelter from the storm (Bob Dylan)

By Athayde Tonhasca

Life in the British uplands can be harsh, even for species adapted to cold temperatures and scarce resources. In these habitats, a mountain avens flower (Dryas octopetala) can be a safe berth for a fly or occasional bee. Insects get more than pollen and nectar from this plant: they also get warmth: the temperature on a mountain avens flower can be up to 30C higher than the surrounding air. 

Warm and cosy mountain avens flowers © Pantona, Wikipedia Creative Commons

Mountain avens flowers get warm because they follow the sun throughout the day, a phenomenon known as heliotropism or solar tracking. Moreover, flowers of mountain avens and some species from continental Europe such as alpine buttercup (Ranunculus adoneus) – from which we learned most of what we know about heliotropism – are usually bowl-shaped, so sunlight is reflected towards their centre. Heliotropism and flower form allow the plant to function like a satellite-tracking antenna, maximizing light interception.

 A mountain avens flower and a tracking radar © Robert Flogaus-Faust (L) and Daderot, Wikipedia Creative Commons   

But how do these flowers rotate to keep up with the sun’s position? 

We don’t know for sure, but can assume auxins are behind it. This group of hormones are involved in just about every aspect of plant growth and development, including phototropism (growing towards light).

Auxins and phototropism. Left: Auxin (pink dots) are evenly distributed in the plant’s tip. Centre: The repositioning of the sun causes the auxins to move to the opposite side of the plant. Right: The concentration of auxin stimulates cells to grow or elongate © MacKhayman, Wikipedia Creative Commons

Mountain avens’ heliotropism may be similar to what happens with the common sunflower (Helianthus annuus). In the morning, the stem and upper leaves of a young sunflower plant face east. As the day progresses, auxins move from the western to the eastern side of the plant. Auxins promote water absorption and tissue elongation, so the plant slowly bends westwards. The auxin gradient is reverted at night, and the plant is reoriented eastward. However, this cyclical movement stops when the plant flowers. So contrary to what some people think, sunflowers’ flowers do not follow the sun; they are always facing east (although wind or rain can change their position).

Solar tracking of a sunflower plant © Kutschera & Briggs, 2015. Phototropic solar tracking in sunflower plants: An integrative perspective. Annals of botany. 117. 10.1093/aob/mcv141

Heliotropism is an asset for plants with short growing seasons. The temperature of an alpine buttercup’s gynoecium (the female reproductive organs) can be 5.5°C higher than the flower’s surroundings. Heat accelerates pollen germination and the growth of pollen tubes; it also leads to heavier seeds and higher germination rates. So plants have greater reproductive success.

Heliotropic flowers’ absorption of solar irradiance encourages insects to visit and stick around, basking and foraging. The extra warmth increases their metabolism, and boosts their flight capability. Frequent and long-lasting insect visits are important for many upland plants, which cannot self-fertilise and rely mostly on flies for pollination. These insects lack pollen-carrying structures and, generally speaking, are much less hairy than bees. So the longer a fly frolics on a flower, the greater the chances it will get some pollen grains stuck to it. With luck, some pollen will be carried to another flower, and pollination will happen. A warm welcome pays off for plants and insects alike.

The hoverflies Melanostoma mellinum and Scaeva selenitica, upland flower visitors © James K. Lindsey (L) and Sandy Rae, Wikipedia Creative Commons

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.