Bumblebee watching

I am doing 30 Days Wild with the Wildlife Trusts. My vague plan is to have an invertebrate emphasis, to make an effort to see more of them given the dismal, windy, cool spring we are having, and to revive the almost dormant blog. Bumblebees appear to be having a particularly bad year. The first workers have been few and small, and even if many of their favourite plants are blooming, I only see the single worker about every now and then. I decided bumblebee watching would be the first wild event of 30 Days Wild. After all, Bumblebees fly during cold, cloudy days.
 Over the years, I have introduced plenty of native plants in the garden, which attract an assortment of solitary bees, bumblebees and other insects. Primroses flower early in the spring and are visited by the Hairy Footed flower bee, Anthophora plumipes, and occasionally by Bee Flies. Later,  Welsh Poppies buzz with bumblebees collecting pollen. At this time of the year Hedge Woundwort (above, a bee's eye view of a flower spike) and White Dead-Nettles are in bloom, and they are followed by long-tongued solitary bees and bumblebees.
This afternoon I managed to see just two bumblebee species, the Carder Bee, Bombus pascuorum and the Early Bumblebee, Bombus pratorum. B. pascuorum is a long tongued bumblebee and likes flowers from the mint family. It visited sage and hedge woundwort, while B. pratorum has a very small tongue and likes flat flowers. It visited Alliums and White dead-nettles. I find that B. pratorum, despite its tiny tongue, sometimes steals nectar through holes at the base of flowers, or is small enough to get inside the flower and reach the nectar.

This Bombus pratorum was visiting White Dead Nettles in the front garden.

A Bombus pascuorum on the sage hedge.

Tree bumblebee threesome

This mass of bumblebees landed heavily in front of us. A Queen Tree bumblebee, Bombus hypnorum, and two males, the one ar the rear mating with her, the other one trying too. Quite impressive she just managed to fly with the load!
 In most bumblebee species, queens mate just once, but Tree Bumblebees are an exception, and queens often mate with more than one male. In experiments by Brown and colleagues, 72 queens that had mated the previous day were offered the chance of mating again. Sixteen of them (22%) mated again. The shorter the mating on the first instance the higher the chance the queens would choose to remate. The average copulation duration was about half an hour.
 Paxton and coworkers (2001) analysed queens, workers and males of 14 Tree Bumblebee colonies using very variable genetic markers. They wanted to find out how many males had fathered the colonies workers, and therefore, the level of relatedness between workers. They found that workers of single colonies are the offspring of up to six males, although most of them (60%) were full siblings, os normally one male sires most of the queen's offspring. A further, very interesting finding was the presence of a few number of 'alien' workers in some nests, which although unrelated to the queen were related amongst them. Their explanation for this was that a nest usurpation, a phenomenon that occurs in bumblebees, had taken place. In the spring, after emergence from overwintering, there is high competition for nest sites amongst queens. In tree bumblebees nest boxes and under eaves are favoured, and dead queens are often found just outside occupied nests. Why would this nest usurpation take place and what are the consequences? Usurper queens take over a ready made, established nest with some larvae or pupae - so they are already ahead of the competition with no effort. From then on, the usurper queen rears the already present larvae or pupae, which, when workers will help her rear her own offspring (in the same way that cuckoo bumblebees take over colonies of other species). Without genetic analysis, the success of not of the usurpation events couldn't be assessed, who was the dead queen, the usurper of the initial queen that had established the nest? The fact that the 'alien' workers were small, suggests that they were produced early in the life of the colony, when the queen had to feed the larvae herself. The fact that they were related amongst them rules out that they are 'lost' workers that  have entered the wrong nest. The alien workers were presumably early produced offspring of an usurped queen and had helped rear the offspring of the usurper, so they were effectively 'slaves'.

More information
Paxton RJ, Thorén PA, Estoup A, & Tengö J (2001). Queen-worker conflict over male production and the sex ratio in a facultatively polyandrous bumblebee, Bombus hypnorum: the consequences of nest usurpation. Molecular ecology, 10 (10), 2489-98 PMID: 11742549

Brown, M. J. F., Baer, B., Schmid-Hempel, R., & Schmid-Hempel, P. (2002). Dynamics of multiple-mating in the bumble bee Bombus hypnorum. Insectes sociaux, 49(4), 315-319. DOI: 10.1007/PL00012654

Tree bumblebees mating

For the last few weeks, Tree bumblebee males have been travelling full speed around the garden marking leaves and stems with pheromones to attract queens. The males and new queens have been produced by bumblebee nests as they are reaching the end of their life cycle. Males leave their natal nest and then hang around the entrance of nests possibly hoping for a queen to emerge. Workers are most likely found dead than alive once the males and queens fly from the nest as their role is completed.
I watched a Tree bumblebee nest in my local wildlife garden, as common in this species, they chose high places, and nest boxes and under eaves are the most easily spotted nest sites. A cloud of males hovered around the entrance.

 Today I found the mating pair above on hogweed, the first I find for this species. It is hard to believe that the Tree Bumblebee was not found in N England 10 years ago, now it is so common and widespread. Here is the distribution map for this species from BWARS B. hypnorum project.
More info on B. hypnorum in BugBlog here.

Ants and cherry laurel nectaries

 On passing by a hedge of Cherry Laurel, Prunus laurocerasus, we notice ants. Lots of Black Garden Ants, Lasius niger, walking all over the leaves. We creep closer and see little round marks on the base of the leaves that the ants are paying special attention to. They are nectaries, nectar producing organs in plants, which are most commonly found at the base of flowers. In about 1% of plants (including several species of the genus Prunus, including cherries) these organs are also found on leaves and are called 'extrafloral nectaries'. Extrafloral nectaries are part of a mutualistic relationship between plants and ants, in which both parts benefit. The plants give away the sweet nectar which feeds the ant, and in turn the ant defends the plant. By visiting the plant to obtain nectar ants come across herbivorous insects, particularly caterpillars, that might feed on the leaves, and prey upon them.
 Other nectar-loving insects take advantage from this easy to reach nectar production, and often, bees and bumblebees are found feeding in nectaries. Bees and bumblebees behave in this case as 'nectar robbers' as they do not benefit the plant at all (unlike when they feed on the floral nectaries). In this case, the ant is the one benefiting the plant. If you like the bright green, glossy leaves of the cherry laurel to stay that way, you might want to leave these ants well alone.

Ant feeding on a nectary. There is another pale, round nectary just above the ant.

Male Bombus pratorum visiting the cherry laurel

A morning beewatching

A warm morning of sunny spells, bees come out en force. A Cuckoo bumblebee, possibly a male Bombus vestalis (above), a cleptoparasite of Bombus terrestris fed on the Erysimum Bowles Mauve. Bombus pratorum workers and males are feeding on many flowers, Centaurea montana, Chives and Herb Robert, surprisingly also Foxgloves and also nectar robbing the Comfrey.

A couple of queen bumblebees, Tree (above) and Buff Tailed bumblebees feed together on the patch of Red Valerian for a long time.

On the Oxeye Daisies, a Lasioglossum sp.

The first leaf-cutter bee, Megachile willughbiella, of the year feeding on Erysimum. I also saw it on the geraniums.

 Wall Rocked receives a visit of a very worn Andrena

The new Osmia caerulescens are now everywhere. The males have been checking the bee post. In the next shot, a male stretches its tongue, amazingly long. No surprise its preference for Sage and Lamium maculatum in the garden.

This female sunbathing next to the Sage shows why they are called Blue Mason Bees

The summer bees overlap on early June with the last of the spring bees. An Anthophora plumipes female was briefly feeding also in the amazingly popular Erysimum. Finally, Red Mason Bees, Osmia rufa were abundant. Overall 11 species, not counting honeybees. A wonderful morning bee watching!

On the odd life cycle of bumblebees

I stopped by the comfrey patch this morning. The comfrey has now been in blossom for a good month. A Small White butterfly was feeding on comfrey, but I couldn't snap her. A ginger queen Bombus pascuorum fed on the blossom. A queen wasp got comfortable on a leaf and basked in the sun, as did some hoverflies. While I watched a couple of Anthophora plumipes males patrolling and feeding, a tiny worker of the tree bumblebee Bombus hypnorum, the first of the year, turned up (above). As I compared the sizes of these two bees – the bumblebee worker was smaller than A. plumipes - I reflected on the different life cycles of these two bee species. A. plumipes, a solitary bee, has males and females, while bumblebees, in addition of males and females (queens), have a worker caste. In bumblebees there are no males for most of the year: A male has a relatively short - if hectic - life. Upon emergence in the summer, he will find a queen and mate with her. The queen will store his sperm and use it to fertilise her eggs the following spring, when she will emerge from overwintering, build a nest and lay fertilised eggs, which will hatch into worker larvae. She will be busy collecting nectar and pollen from early flowering trees and plants to rear these larvae, and once the workers emerge, they will take over from the queen in collecting more nectar and pollen for subsequent eggs, although they won't reproduce themselves.  In the summer, the last batch of eggs of the queen will produce new queens and males, which the workers will help rear. Once they leave the nest, the founder queen and workers will die and the cycle starts again. Wasps, ants, bees, and bumblebees (hymenopterans) share a weird way to determine the sex of the offspring: fertilised eggs become females and unfertilised eggs become males: a system called haplodiploidy.

  Most sexual organisms have two sets of chromosomes: a set they inherit from their mother and another that comes from their father, but as bumblebees and other hymenopterans do not have a dad they just have a single set of chromosomes, the set coming from their mother.

How strange is that? If you think about it, the system means that bumblebee males don’t have dads, they also cannot have sons, only daughters, although they can have grandsons and have granddads. In addition, this arrangement causes strange relationships between the family members. We are equally related to our parents than to our kids – on average – but due to haplodiploidy, bumblebee sisters are more strongly related than they are to their mums. This is because full sisters have received an identical set of chromosomes from their dad, in addition to the set they receive from their mum, which is a mixture from the sets the queen received from her parents.

 This unbalanced genetic relationship between mother-daughters and sisters is thought to underlay the evolution of the worker caste, which do not reproduce, but help their mother to rear their own sisters.

The solitary bee Anthophora plumipes

A wasp? No, the exquisite wasp mimic hoverfly Myothropa florea

This is the real wasp. A queen common wasp 

Hoverfly Syrphus ribesii 

Another basking fly

A queen Bombus pascuorum

Twelve bugs of Christmas

Today we woke up to a mild, sunny day and I was pleasantly surprised by the diversity of active bugs about. I made a count of species and when I got to twelve I was reminded of the traditional carol and made up a buggy version:

12 ladybirds walking | 11 winter-gnats dancing |10 honeybees buzzing |9 bluebottles basking | 8 leafhoppers leaping | 7 woodlice hiding | 6 hoverflies flying | 5 snails sleeping | 4 spiders weaving | 3 harlequins | 2 drone flies | and a bumblebee on ivy

The honeybees in the local wildlife garden were coming out tripping over each other and ladybirds (7 spots mostly but also harlequins and 22 spots) were awaken by the mild temperature. I counted 5 spider species outside (Tegenaria, Zygiella, Pholcus, Steatoda, Linyphia). The main surprise was the bumblebee worker, extremely fast and active and with pollen baskets full with pollen.

Here is a slideshow of bugs seen and photographed today

Winter bees on Mahonia

A lovely, still sunny day, I walk to a clump of Mahonia on a corner of an avenue. I have posted about this bug magnet in previous years (here, here and here) At this time of the year, Mahonia (Oregon Grape) is on bloom, its sweet scent and profuse, sunny-bright yellow flower spikes attract all late nectar lovers. Today, the bushes were busy, Bluebottles, Drone flies, honeybees and three (!) bumblebee species were feeding on it. A Bombus pratorum queen about to land, with a dronefly on the foreground on the top photo. This is one of the garden plants that are likely to have contributed to favour winter colonies in bumblebees. Bugs love this bush, and so do I. 

Bombus terrestris queen. I have seen a worker this week too.

Bombus hypnorum

Honeybee

Dronefly and bluebottle

Late bumblebee collecting pollen

It is not unusual to see bumblebees in Autumn. Queens are still about, storing up some reserves before hibernation. What I found unusual is a worker of a Bombus pascuorum collecting pollen in my patch of Lamium maculatum today. The bumblebee visited each flower inserted its head deep into it and as exiting, wiped its furry back with its rear legs collecting the pollen on its corbicula or pollen basket. I understand that bumblebees collect pollen for their larvae, so there must be still be B. pascuorum active nests this late in the year.

A tree bumblebee queen

 This tree bumblebee, Bombus hypnorum, queen has been visiting the garden in the last couple of days, with her fresh, striking tawny/black/white pattern very apparent. The last time I saw this species this year was in July, when males patrolled in the garden and queen bees could be seen searching for nest (or hibernation sites). August, three years in a row, yielded no sightings. Yesterday's queen visited Erysimum "Bowles's Mauve". This perennial wallflower, which often appears in BugBlog shots, and I highly recommend in any wildlife garden, has a very long flowering season and attracts a range of solitary bees and bumblebees, hoverflies and butterflies.

This graph shows the number of days per month I've seen this bumblebee in the last three years. Workers are active during May and June, most of the July activity is males marking and patrolling their flight paths and queens. The rest of the sightings in the autumn and early spring are queens feeding or nest searching.

Mating buff-tailed bumblebees

Next to a busy road, by the base of a tree, surrounded by concrete and oblivious to the noisy traffic, this pair of White-tailed bumblebees were mating this morning. The queen lying motionless, a bit sideways, the male tickling her rhythmically with his legs. I have been taking some videos with my camera lately to record some unusual, or more common bug behaviour. I hope this works as it is the first time I upload a video to BugBlog.

* * *

I wrote the piece above almost 5 months ago, on the 19th of July, when I saw the bumblebees mating. I tried, and miserably failed to upload a video to YouTube. Here it is, finally posted. Thank you Crystal Ernst! More to come.

Buzzing bees in poppies

There were lots of poppies around today, and they buzzed with a high pitch produced by the bees and bumblebees inside, often more than one, gathering their almost black pollen. Poppies are unusual flowers, bright red, bowl shaped, with black centers and radial symmetry, and they do not produce nectar, just lots and lots of edible, protein-rich pollen. Poppies open at dawn, and before they do, their anthers start to release pollen. By the following day, fully fertilised and depleted of pollen, the flower loses its petals. Although the red and black colour combination of poppies - like some tulips and other Mediterranean flowers - is through to have evolved to be fertilised by beetles, bees and bumblebees take advantage of the pollen bonanza offered by these short-lived flowers. Bumblebees do not need much time to learn how to collect nectar, even from complex flowers, but pollen collecting is much trickier. The powdery pollen needs to be released from the anthers, brushed from the bees hairy body, mixed with nectar to make it sticky, packed into pellets and fixed to their corbicula, the specialized area on their legs adapted to carry the pollen. Nigel Raine and Lars Chittka, from Queen Mary University of London, carried out some experiments in a greenhouse to investigate how bumblebees learn to collect pollen. They used freshly collected poppy flowers from a field nearby before each bumblebee foraging trip, and tested six Bombus terrestris bumblebees that had never collected pollen before.

The figure above shows how pollen from the wild collected poppies drastically decreased along the morning - as the wild bees outside collected it - by 9:00 am there wasn't much left. The researchers then computed the rate at which their bumblebees collected pollen from the poppies, related to the available pollen at each foraging bout, by removing and counting one of the pollen pellets brough to the colony after each foraging trip by each bee.
 Bumblebee pollen collection behaviour markedly changed with experience:

During their first few visits, all bees were surprisingly clumsy, one bee even failed to collect any pollen during its first foraging bout despite making 56 flower visits. In the early stages of their foraging career, bees were observed to collect pollen loads that fell apart, or were so large that they fell from the bee’s corbiculae (pollen baskets) before reaching the nest. As each bee gained foraging experience, the frequency of such events rapidly declined. Bees also changed how they used ‘buzzing’, a technique of holding the anthers in their mandibles while vibrating their flight muscles, to facilitate pollen collection. While naïve bees typically buzzed either all or no flowers, skilled foragers would selectively ‘buzz’ flowers containing less pollen.

The following graph shows how foraging efficiency increases with foraging trip, indicating that bees learn to be more adept at collecting pollen. Despite this, the bees seemed to forget most of what they had learn overnight, as efficiency was much lower in the first trip of the second day than in the last trip of the previous day. :

A honeybee and a bumblebee share an Opium poppy

References
Raine, N., & Chittka, L. (2006). Pollen foraging: learning a complex motor skill by bumblebees (Bombus terrestris) Naturwissenschaften, 94 (6), 459-464. DOI: 10.1007/s00114-006-0184-0
Dafni, A.; Bernhardt, P., Shmida, A., Ivri, Y. and Greenbaum, S. (1990). Red bowl-shaped flowers: convergence for beetle pollination in the Mediterranean region. Israel Journal of Botany, 39, no1-2, pp. 81-92, 81-92 Other: 0021-213X

Racing male tree bumblebees

In the last two weeks, the garden has been overtaken by frenzied male bumblebees. They follow a set circuit, round and round, racing from bush to bush and then having a little bumble in each. If you wait for a bit on a particular spot on the route, you are likely to see another bumblebee a few minutes later passing by in the same direction, doing exactly the same. Most of the males I have been able to identify doing this are Bombus hypnorum. Today, a male B. hypnorum - which can be distinguished from the female by his white moustache - got trapped inside the conservatory and I had his portrait taken (above). Many male bumblebees have recently emerged from their nests, never to return, and their mission is to find queens and mate with as many as possible. In many bumblebee species, males' strategy consists on tracing a route, sometimes hundreds of meters long, often circular, depending on the species, and marking certain places along the route with pheromones produced by scent glands in their jaws. Males join already set routes and therefore many males, some of them probably their siblings, go round the same routes every day, stopping to feed occasionally. Queens encountering a route are attracted by the pheromone and are then intercepted by males. The discovery and first description of these male bumblebee flight paths - from Bombus hortorum males - dates back to Charles Darwin, from observations he carried out at Down House. Although he didn't realise pheromones were involved, he noticed bumblebee routes and them stopping and bumbling at places he called "buzzing places", and marveled at the fact that the same or very similar routes were used year after year:

I then followed their route for about a hundred and fifty yards until they came to a tall ash, and all along this line they buzzed at various fixed spots. At the far end, near a pollard oak, the track divided into two as shown in the plan. On some days all the bees flew in the direction I have described, but on others some arrived from the opposite direction. From observations made on favourable days, I think that the majority of individuals must fly in a wide circle. They stop every now and then to suck at flowers. I confirmed that whilst in flight they move at about ten miles an hour, but they lose a considerable amount of time at the buzzing places. The routes remain the same for a considerable time, and the buzzing places are fixed within an inch. I was able to prove this by stationing five or six of my children each close to a buzzing place, and telling the one farthest away to shout out " here is a bee " as soon as one was buzzing around. The others followed this up, so that the same cry of " here is a bee " was passed on from child to child without interruption until the bees reached the buzzing place where I myself was standing.

  This sketch of the grounds of Down House shows the part of the male bumblebee flight route studied by Darwin with the help of his children. What fun must have been to have him as a dad!

References

Freeman, R.B. (1968). Charles Darwin on the routes of male bumblebees. BULLETIN OF THE BRITISH MUSEUM (NATURAL HISTORY) HISTORICAL SERIES , 3 (6), 177-189.


Stiles, Edmund W. (1976). Comparison of Male Bumblebee Flight Paths: temperate and tropical. Journal of the Kansas Entomological Society., 49 (2), 266-274.

Tree bumblebee with foxglove

It is the 10th anniversary of the first recorded British Tree Bumblebee, a continental European species which crossed the channel on 2001, esblished successfully and has spread since to much of England. It's natural colonisation has been closely followed by Stuart Roberts, from the Bees, Wasps & Ants Recording Society (BWARS), who is collating a survey of this spread and has published an information sheet. The bumblebee is very easy to identify, with a tawny thorax and a black abdomen and a white tail. If you have any records you'd like to submit, check the website for more information. In my area, the tree bumblebee is doing really well and it is one of the commonest species at the moment. You can see them foraging on Ceanothus, cotoneaster, brambles and snowberry flowers. Today is the first time I have encountered them feeding on Foxgloves. This individual visited several spikes in succession, given me the chance to get a shot.

Robbing and Buzzing

I took this shot of a male Bombus pratorum nectar robbing a few days ago: the bee's short tongue is inserted into a hole previously pierced at the base of the flower, where it can easily reach the nectaries. The other flower on the right has also these "drinking holes", which once present, are used by short or long tongued bees alike. The first B. pratorum workers of the year, which are very small, do appear to be able to feed through the corolla opening, and often visit intact comfrey flowers. A possibility is that the small workers are actually just collecting pollen - not nectar - using a  "buzzing" technique. This consists on holding onto the flower and then vibrating its wings - and making a characteristic high pitch sound. At certain vibratory frequencies, the pollen becomes dislodged from anthers and streams out of the flower, where is gathered by the bee.

B. pratorum worker collecting comfrey pollen, note the white pollen baskets

You might have seen bumblebees buzzing on poppies, as that way they dislogde much pollen, which covers their hairy bodies and that they then collect into their pollen baskets. These buzzing visits by bumblebees are common in several species, including comfrey, borage and kiwifruit and some of these flowers are actually pollinated this way. Not only B. pratorum, but other bumblebee species gather pollen in this way. Despite being relatively common, this behaviour is easily overlooked, for example, if you spot the bees casually, or from some distance, or there is background noise, but you can see the difference in sound between a bee collecting nectar (the first bee in the video, a B. pascuorum) and bee collecting pollen (a B. pratorum) in Comfrey in this YouTube video by pixiebaggins:

A resourceful bumblebee

Iris flowers are tricky for bees. They are quite unlike other flowers. Basically, each flower is made of a wide landing platform with striking nectar guides, but to be able to collect the nectar, bees have to push through a lip pressing against this platform - on top of this lip are first the stigma and then the anthers. As the bee enters, it often looks like it has to press with some force to lift the lip to form a gullet high enough. As she enters, its back presses against the stigma, depositing any pollen it might carry; then her back rubs against the flower's anthers, collecting some pollen. On her way back, the bee presses the stigma against a groove, so preventing self-fertilisation. This contraption ensures that visited flowers get cross-pollinated. Long-tongued bumblebees, Bombus hortorum and B. pascuorum and fork-tailed flower bees, Anthophora furcata, are able to enter the flower and feed the way the flower intended. Today I saw a buff-tailed bumblebee, Bombus terrestris, landing on the flowers of Iris versicolor, and appearing to have sensed there was nectar in them, tried in various ways to enter a flower, failing. The bumblebee started then biting the flower lip, in an unsuccessful attempt to gain entrance (top, check its jaws wide open). In other posts I have dealt with this piercing technique used to successfully steal nectar from tubular flowers like comfrey and honeysuckle, where this short-tongued bumblebee cannot reach the nectar.

Not this way - it missed the entrance and got on top of the lid

not this way either...

... and probing with its tongue for nectar after biting the flower lip.

Although this bumblebee was unsuccessful this time, the persistence of its behaviour after failing to get nectar first time round attests to the resourceful behaviour of this successful bumblebee.

Winter active bumblebees

We had our first frost yesterday, and it was also a frosty morning today. But coming back home this afternoon, with the light already going weaker, I came across a Tree Bumblebee, Bombus hypnorum feeding on a large Mahonia bush. I have posted before on this bumblebee, a recent natural colonist in the U.K. In the last two decades, reports of winter active bumblebees - mostly Bombus terrestris - have steadily been accumulating, especially in the south of Britain. Queen bumblebees are occasionally active in warm winter days, but the reports referred to queens collecting pollen - a sign they are actually nesting, not hibernating - or workers - indicative of active nests. These observations depart of the usual bumblebee life cycle in northern Europe, where winter hibernation of queens is the usual case. Whereas there are many native flowers in bloom during the winter to support a second generation in the Mediterranean, this is not the case in northern Europe. What resources are active winter bumblebees using? Stelzer and coleagues tested the hypothesis that winter bumblebees are making use of non-native flowers in parks and gardens. To do this, they set up B. terrestris colonies and followed them during two winters. In the second winter they introduced a new technique - micro-tagging - which allows automatic recording each time the tag passes nearby the tag reader set at the nest entrance. They attached the small RFID tags to the thorax of 64 individuals, and positioned a micro-balance at the entrance of the nest so that bumblebees also weighed themselves as they went in and out the nest, so that they can estimate the increase of weight of each bumblebee at the return of each foraging trip. Their results showed that bumblebees in their experimental nests were able to forage for nectar successfully during the winter, with nectar returns per hour foraging comparable or higher to yields obtained during spring and summer.

The researchers also carried out transects in the Royal Botanical Gardens in Kew to ascertain which plants were being visited by Bumblebees during the winter:
Only cultivated plants were in flower and the main plants visited by B. terrestris were Arbutus unedo and Salvia uliginosa in October (38.2% and 42.3% of the total recordings during that month, respectively), Arbutus spp. (A. unedo and A. x andrachnoides) (31.2%) and Mahonia spp. (Mahonia x media ‘Winter Sun’, Mahonia x media ‘Charity’ and Mahonia lomariifolia) (43.7%) in November, Mahonia spp. (69.0%) in December, Salix aegyptiaca in January (48.2%) and Lonicera fragrantissima (24.1%) in February.

The high yields obtained by the experimental bumblebees might be explained by the nectar sources being large bushes with large volumes of nectar per flower and the flowers being in large inflorescences, such as the ones in Mahonia (see photo above) and willows. In addition, there is no competition for nectar and pollen in the winter months by other foraging bees. Only a few honeybees were around, and these are not active below 10oC, while the bumblebees can forage at temperatures close to 0oC. Further work is necessary to assess if this generation is actually successful, that is, if males and queens are produced, but these results show how human impacts - gardening possibly coupled with climate change- have unexpected effects on the life cycles of native organisms.

Stelzer RJ, Chittka L, Carlton M, & Ings TC (2010). Winter active bumblebees (Bombus terrestris) achieve high foraging rates in urban Britain. PloS one, 5 (3) PMID: 20221445