Bees in the demolition business

By Athayde Tonhasca

The Capayán ruins in Argentina’s La Rioja province are the remnants of a settlement from the colonial period in the late 17th century. This historic site has resisted the ravages of time and weather – the region is very dry – but is threatened by a seemingly harmless resident: the bee Centris muralis.

Ruinas de Capayán, La Rioja, Argentina © World Orgs

This solitary bee is endemic to Argentina, and like other species of the genus Centris, is common in deserts and arid areas, and active when other bees hide from the heat. Some Centris bees are known as ‘digger bees’ because they build their nests in the soil, sometimes in barren patches with no vegetation. So the soft, dry adobe walls in the Capayánruins turned out to be excellent nesting sites for C. muralis – muralis, by the way, meaning ‘of walls’ in Latin.

Centris muralis bee © Vivallo, 2013. Zootaxa 3683: 501-537

The bee’s choice of residence doesn’t bode well for the Capayán archaeological site. Initially, a C. muralis female makes small holes in a wall, which are used by other females to get in and construct their own nests, each comprising several brood cells. With time, the number of cells reach densities of around 700/m2, which is way too high for the fragile structure. Ad in the erosive power of rain that seeps into the holes created by bees, and the walls slowly crumble away; some parts collapse. One particular wall lost over 700 kg of building material and its thickness was reduced by 7 cm thanks to the relentless work of C. muralis.

A damaged adobe wall from the Capayán ruins (a), and detail of a deteriorated area (b) © Rolón & Cilla, 2012. International Biodeterioration & Biodegradation 66: 33-38

Moving a long way north in the Americas, some home owners have their sights on carpenter bees (genus Xylocopa), which excavate holes in wood to build their nests. Any weathered wood will do, including man-made structures such as eaves, rafters, fascia boards, siding walls, decks and outdoor furniture. The effect of their excavations is mostly cosmetic, but significant structural damage can occur when the same area of wood is infested year after year. Carpenter bee holes also increase rot and decay. Persistent concentrations of carpenter bees may require home owners to control their numbers.

An eastern carpenter bee (Xylocopa virginica). Carpenter bees’ most obvious difference from bumble bees is the shining black abdomen ©, Wikimedia Commons. R: galleries of cells excavated by carpenter bees. Note the pollen-nectar loaves in several of the cells © Stephen Buchmann, US Forest Service

On this side of the Atlantic, the Davies’ colletes (Colletes daviesanus) and the red mason bee (Osmia bicornis)nest in holes and crevices, including in walls and mortar joints, sometimes in large numbers. These gatherings have prompted many a home owner to contract someone to get rid of bees that supposedly threaten building structures.

A Davies’ colletes © James K. Lindsey, Ecology of Commanster

Lack of information and misinformation are opportunities for easy money; remedies and fixes can always be bought, even when they are not the most appropriate or none are needed. So it is helpful – and cheaper – to know that these bees are hardly worth the bother. The red mason bee nests only in pre-existing cavities, while the Davies’ colletes can tunnel into soft mortar, especially in old walls. However, their excavating is limited to soft materials such as sandstone and lime mortars already weakened by age and weathering – which may need repairing anyway. A watchful owner can discourage extensive, repetitive burrowing by raking out the affected mortar and repairing the joints (repointing) with new mortar made with cement. Insecticides are not a good idea: they have limited effectiveness and may stain masonry. These are recommendations from The Society for the Protection of Ancient Buildings and English Heritage, which one would expect to take seriously any possible risk to historically important edifices.

Also, we can’t overlook the environmental roles of these bees. Centris muralis is an important pollinator in arid and semiarid habitats, where not many insects find an easy living. Carpenter bees are skilled at buzz pollination, i.e., extracting pollen by vibrating their flight muscles, so they are excellent pollinators of tomatoes, aubergines, and other vegetables and flowers with hidden pollen. The Davies’ colletes is a widespread pollinator of wild flowers, especially daisy-related species (family Asteraceae), while the red mason bee pollinates a wide range of plants, including several fruit trees.

An eastern carpenter bee carrying pollinia from milkweed (Asclepias sp.) © Beatriz Moisset, Wikiwand. R: a red mason bee dusted with pollen © bemma, Wikimedia Commons

The perceived threats from these building dwellers are a good example of conservation conflicts: clashes between nature and human interests. But like any other conflict, there is ample room for compromise, which starts by understanding and assessing the problem. Centris muralis is causing damage in an unusual setting, and perhaps no mitigation is possible. But the risks posed by the other digger bees are not great or can be alleviated. Judicious evaluations, a degree of tolerance and a gentle touch would prevent us from compounding the stress inflicted upon pollinators by our over-dominance of the planet. 

Now showing

“What’s blooming in the Battleby living wall just now?”, I hear you ask. Rather a lot it appears. The watering system has performed flawlessly, the waifs, strays and cuttings that Jim Carruthers stealthily introduced are taking nicely, and those plants that patiently awaited their turn in the spotlight are finally enjoying their moment.

Nevertheless, there are a few surprises.  For one we have fox and cubs.  Now that might have caused you to spill your tea and choke on your biscuit, but worry not I’m being mischievous.  The fox and cubs in question are Pilosella aurantiaca a rather lovely deep-orange flowering plant. It earns its unusual common reference from the fact that the open flowers sit alongside the buds – and this combination of colour and ‘family’ presence explains the quirky name.

It is ideal for the Battleby green wall as it has shallow roots, well-suited to the little pockets on the wall. It also likes being in full sun, so once more the Battleby structure scores highly on the desirability chart.

Herbs, we know, are fantastic for pollinators. Marjoram, thyme, mint, sage, rosemary … the list of beneficial herbs goes on. One herb in the Battleby wall which is thriving is chive. To be useful for pollinators you need to let them flower, and what a glorious sight their pinky, purple pom-pom flowers are. A huge draw for bumblebees they add a welcome contrast in the predominantly green living wall.

There is something near mesmeric about watching chives flower heads swaying in the wind, and they are a big draw for insects. They are of course a member of the garlic family.

Also providing a splash of potentially culinary colour are wild strawberries. This diminutive plant adapts well to the shallow pockets on the green wall, the lovely little white flowers are now being replaced by small shiny strawberries and what was a fairly dense green plant is transformed into one of the sparklier elements on show.

Around the base of the living wall a clump or two of viper’s bugloss has taken hold and it is another bee magnet. The bright blue flowers are bright and help explain the plant’s ‘blue devil’ nickname. It isn’t just bees that are drawn to viper’s bugloss, hang around and you are bound to see a variety of flies.

The popularity of another startlingly cheerful blue flowers seen around Battleby, borage (Borago officinalis), is down to the fact that they are laden with nectar, and have the rather appealing characteristic for bees of replenishing their supply of nectar very quickly. It’s popular with gardeners too as a companion plant, placed near tomatoes and strawberries it is said to boost pollination. 

Finally the roof of the Battleby Living wall has seen one of the planks of wood which form the fascia curl dramatically. This has exposed a cosy gap behind the fascia and an opportunistic queen wasp was quick to spot this potential nesting site.  It’s a good 8 feet or so up off the ground, but best to be aware if you are enjoying the sights (and sounds) of our Living Wall.

Tidings of Comfrey and joy

At this time of year if I want to photograph bees I often head for nearby clumps of comfrey.  Along the banks of the River Tay they are easy to spot, and a magnet for insects.  These stands of blue, and occasionally pink, white or purple, flowers are seemingly irresistible for bumblebees. 

Comfrey is one of those plants that enjoys a long association with people too.  The Romans and Greeks were known to have many uses for the hairy leaves which they believed could both stem bleeding and heal broken bones.  The word comfrey is said to have its roots in the Latin for ‘growing together’.

Today, however, it is likely to be gardeners who would sing the plant’s praises.  As a fertiliser it enjoys rave reviews. For those who enjoy composting it is a great resource and apparently lots of fun. Although from what I read it requires patience, as it performs best in a fairly slow composting fashion. 

The key to its composting credentials is as a rich source of potassium, making it suitable to nourish garden flowers and fruits, including tomatoes, which as every gardener will testify take a bit of feeding.

Although it begins flowering in May it is not unusual for profuse comfrey leaves to be around until the first frosts. Fast forward a couple of months and your ears will be reverberating to many a Christmas Carol, don’t make the mistake of thinking the lyrics in ‘God Rest Ye Merry, Gentlemen’ are saying ‘comfrey and joy’, it is I am assured ‘comfort and joy’. Though no doubt you will be anticipating some comfort and joy if your composting endeavours have gone well.

Others view comfrey, a member of the borage family, as a useful addition in the battle to control slugs. A patch of comfrey is seen by some as a ‘distraction plant’ to lure slugs away from more prized species. But spare a thought for the poor slug, there are many types in the UK and the number of species which are considered pests is in single figures, yet this gastropod is almost universally vilified 

Many of you will be familiar with the UK Pollinator Monitoring Scheme’s FIT Count (Flower-Insect Timed Count) and if you cannot find any of the 14 target flowers listed above you are free to choose another target that is attracting insects at your location. Comfrey is one of the flowers listed in that list of alternatives, just be sure to note that you have used this flower species if submitting a result. The sharp eyed might even not parasitic wasps and lacewings hanging around comfrey.

Distinguishing between common comfrey (symphytum officinale), hybrid comfrey and Russian comfrey is a task for experts. Fortunately pollinators are not always that discerning, any bushy stand of comfrey it seems will satisfy their needs.

For some pollinators the bell shaped flowers are a boon. This hardy perennial thrives in damp areas and for bumblebees with long tongues its lengthy flowering period (roughly May to August) is extremely helpful.  However, other bees are known to access it’s flowers. Dave Goulson’s writings on comfrey are always approving, and he notes “…one of the very best plants for bees. Visited by long and short-tongued species, the latter often robbing from holes bitten in the tops of the flowers.” 

Comfrey it seems is providing comfort and joy after all.

Further reading

Athayde’s blog on nectar robbing

Colourful parcels of poison 

By Athayde Tonhasca

For over 430 million years, plants and animals – particularly insects – have been entangled in an evolutionary tug of war. Through the inexorable process of natural selection, plants produce chemicals that deter or harm herbivores. These in turn develop the ability to overcome the plants’ defences, which in turn puts pressure on the plants to come up with ever more efficient chemicals. And so this arms race goes on.

As a consequence of these adaptations and counter-adaptations, plants have accumulated an arsenal of more than 300,000 catalogued products, known as secondary metabolites, which defend them against enemies. Among these chemicals, cyanogenic glucosides – CNglcs for short – are particularly efficient. When a plant-eating insect rips through plant tissue, CNglcs react with other chemicals to produce hydrogen cyanide, also known as prussic acid. This substance is highly poisonous to most animals (nearly twenty of Agatha Christie’s characters were dispatched with hydrogen cyanide or the related potassium cyanide). 

Cassava (Manihot esculenta) is a major source of calories for many people living in the tropics, particularly sub-Saharan Africa. The grated roots must be thoroughly washed to remove their cyanogenic glucosides © Thamizhpparithi Maari, Wikipedia Creative Commons

However, some millipedes, centipedes and insects have become immune to CNglcs. And in an ingenious twist of plant-herbivore coevolution, a few creatures have turned these poisons into phagostimulants (substances that induce eating).

Why would this happen? Through several behavioural and physiological adaptations, these herbivores have developed the ability to sequester CNglcs from their food plants, that is, ingest and store them in their bodies, without triggering the chemical reactions that create poisonous hydrogen cyanide. By accumulating CNglcs, these herbivores then put the toxic chemicals to work in their own defences against predators.

No insect illustrates this process of chemical manipulation better than burnet moths (Zygaena spp.), which sequester cyanogenic glucosides from the common bird’s-foot trefoil (Lotus corniculatus) and related food plant species (family Fabaceae).

Six-spot burnet moths (Zygaena filipendulae) taking nectar from a thistle © Lorne Gill/NatureScot

But that’s not all; burnet moth larvae can also produce their own CNglcs from amino acids taken from their host plant and store these chemicals in specialised cavities in their skin. When a larva is threatened by a predator, it contracts its body to expose the CNglcs viscous droplets.

CNglcs droplets on a six-spot burnet larva © Zagrobelny & Møller, 2011. Phytochemistry 72: 1585-1592

As a result of these chemical manipulations, caterpillars and adults are extremely unpalatable. Which explains why burnet moths fly about during the day: few predators would dare to take them. And these enemies are forewarned of the danger by the moths’ black-red contrasting colouration, which is a common pattern among poisonous animals.

But the six-spot burnet moth’s association with poison is even more remarkable. Females release gaseous plumes of hydrogen cyanide to attract males, and refuse to mate with those with a low content of CNglcs. If a male is suitably toxic and accepted, it transfers some of its own chemicals to the female during mating. It is believed that this ‘nuptial gift’ is relocated to her eggs to protect them against predators.

We don’t know much about burnet moth pollination, but in Britain they contribute to the reproduction of the fragrant orchid (Gymnadenia conopsea) and the pyramidal orchid (Anacamptis pyramidalis). 

A fragrant and a pyramidal orchid, both pollinated by burnet moths © Thommybe (L) and JLPC, Wikipedia Creative Commons

When life gives you lemons, you can make lemonade; but when life gives burnet moths cyanide, they make chemical weapons.

A sloppy but efficient pollinator

By Athayde Tonhasca

We hear a lot about the pollination services provided by the European honey bee (Apis mellifera), so you may be surprised to know this bee is not that competent at its job. A honey bee moistens the pollen she collects and carries it tightly packed on her corbicula, or pollen baskets, so pollen grains are not easily dislodged when the bee visits another flower. Moreover, honey bees learn quickly to collect nectar with minimal contact with the flower’s anthers, so reducing the chances of pollen transfer. They are also good at flower constancy (the trait of visiting the same type of flower over and over), which is not good for plants that need cross-pollination between different varieties, such as apples. Thus, paradoxically, honey bees’ efficiency as food collectors reduces their efficiency as pollinators. These shortcomings are offset by the huge numbers of bee workers per hive and the fact that they are so amenable to management.

In comparison with the tidy honey bee, the red mason bee (Osmia bicornis) is a messy flower visitor. Females have low flower constancy, flying all over the place, and carry dry pollen loosely attached to their scopa (a mass of hairs under the abdomen). This means that pollen grains have a greater chance of becoming detached from the bees’ bodies and ending up on a flower’s stigma.

A red mason bee with her scopa loaded with pollen © Jeremy Early, Nature Conservation Imaging

What’s more, the red mason is one the most polylectic bees in Europe, that is, it collects pollen from a variety of flowers from unrelated species: 18 plant families altogether, including willows (Salix spp.), maples (Acer spp.), birch (Betula spp.), oaks (Quercus spp.) and several fruit trees in the family Rosaceae such as apples, pears, plums, cherries and peaches. Unsurprisingly, this bee is an excellent orchard pollinator; 500 or so female red masons can pollinate as many trees as 2-4 honey bee colonies. 

Like other Osmia species, the red mason is a cavity-nesting bee; it makes itself at home in preexisting holes and fissures in soil banks or dead wood, abandoned insect burrows, hollow stems, or cracks and holes in walls – which explains the common name, ‘the mason bee’. It may also excavate soft mortar, hence the reason for another common name: ‘the mortar bee.’ The red mason readily occupies man-made structures such as ventilation bricks, the space beneath roof tiles, even inside door locks. So this bee is the most likely tenant of bee houses.

Once a female occupies a cavity, she will construct a series of compartments (brood cells) and stock them with pollen as food for her offspring. She will then close the nest entrance with a mud plug. But she’s not done once the nest is finished: if conditions are right, she may build another six nests before the season is over. The larvae will eat the pollen and emerge as adults the following year to start the cycle again. See red mason bees in action here.

A session of a mason bee nest. Each cell contains one egg and a provision of pollen 

Mason bees tend to nest close to each other in aggregations of 50 to 250 females. And they are diligent pollinators, as demonstrated by these facts and figures:

  • A female bee may construct 16 cells per nest, 1 cell/day.
  • She will fly 300-400 m on average, up to 600 m, in search of flowers.
  • Nineteen foraging trips are needed to collect the pollen and nectar for each cell.
  • Her pollen load weighs 100-250 mg, up to 300 mg.
  • She may visit 75 flowers each trip, up to 25 flowers/min, and she will stock up each cell in about 3.5 h.
  • A cell with an egg that will develop into a female bee may contain 8 million grains of pollen. Fewer for male bees (they need less food): 4.6 million.

This hardworking bee is good news for wild flowers, and also for crop production. The red mason is an effective pollinator of rapeseed oil and a number of crops grown under polytunnels and glasshouses, such as strawberries and raspberries. Other mason bees have been managed as orchard pollinators in Japan and USA for many years; there is growing evidence that the red mason can play a similar role in orchards in Britain and other European countries.

A female red mason bee and sealed nests in a bee house

The red mason bee is common throughout most of the UK from late March to June/July. During this short time as an imago (the adult stage), this bee will contribute to the pollination of countless wild flowers, crops and fruit trees. The red mason bee deserves to share the spotlight with the honey bee.     

Blaeberry bonus

The other day I enjoyed a trip to Glenfeshie, and it dawned on me that my outings to this favourite spot are changing.  Years ago a trip to Glenfeshie would inevitably have meant scaling Sgor Gaoith. However, lately I’ve become increasingly absorbed in exploring the glen itself. This time my visit was devoted, not entirely, but substantially, to contemplating blaeberry.

Now, blaeberry is one of those words you need to watch in spell-checker. Microsoft Word is determined to tell me that I actually want to say blueberry. I don’t.  But then again the software has my sympathy, for surely no plant owns quite so many names. The list is impressive, and includes bilberry, huckleberry, wimberry, trackleberry, and whorlberry.

Vaccinium myrtillus, to avoid confusion, is widespread in our Caledonian Forests, but not exclusively so. You can also find it on moors, heaths and bogs. When it flowers in May this evergreen shrub is a magnet for bumble bees and other pollinators, and if you want to spy the elusive bilberry or blaeberry bumble bee (Bombus monticola) then this is a plant worth watching.

Bombus monticola (monticola translates from Latin as ‘living amongst mountain’) isn’t the only bumble bee to forage on this straggly deciduous shrub. However, it is thought that they are fairly heavily dependent on this particular plant, probably because in truth there is little else available at this time of year. In Glenfeshie, where the pressure of grazing has been substantially reduced monticola has a fighting chance with blaeberry. 

That said the blaeberry bumble bee will forage on the mountain tops, and for me it was whilst looking for dotterel and ptarmigan that I realised this plucky bee was covering the summits as well as the glens.

The blaeberry plant is, like the bumblebee, as attractive as they come.  The light green leaves have blushing fringe of pink, and the deep pink flowers hang like exotic miniature lanterns. When it fruits the dark blue-black berries are bold against green or reddening leaves.

It is one of our edible wild fruits and is used in jams and jellies as well as having been pressed into service to temper rougher, harsher whisky. The human uses of blaeberry don’t end there and the blue-black berries were once a go to source of dye when clothing needed a bit of colour.  Such practical uses earned it a special place in the hearts and minds of many.

Nan Shepherd in her magnificent The Living Mountain talks about blaeberries in the same glowing terms as the arguably tastier cloudberries.

So what to make of blaeberry? Understated yet beautiful.  Food for the blaeberry bumblebee, butterflies such as the mountain ringlet, and small mammals including the red squirrel. A source of relish and dye for our forefathers.  Important for the chicks of ptarmigan, grouse and capercaillie. And for me? A plant that can easily distract you from the mountain tops.

Futher reading:

The blaeberry bumble bee, the hardy highlander

Bumblebee Conservation Trust – The Bilberry Bumblebee