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

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.

Ants score again

Frustrated by not getting the shot of the Bombus terrestris queen chewing the comfrey flowers the other day, I check the comfrey quite often. I am surprised - OK, not much I must say - by an ant (Lasius niger) walking on the flowers and entering them using the convenient hole made by the bumblebee queen. So, ants are also secondary nectar robbers!

Ant peeks into the hole

Getting in

and she is in sucking the lovely sweet nectar...

Flower engineers

Yesterday I was following a queen Bombus terrestris, as usual, with the faint hope one will nest in my garden when she landed on a comfrey patch. This was the first time I have seen this species on comfrey, their deep flowers are mostly visited by Bombus pascuorum, B. hypnorum and B. pratorum*, in addition to Anthophora plumipes. The heavy queen landed on top of the flowers, and I was able to approach without disturbing her. She noisily bit the top of a flower and then inserted her tongue on the resulting hole. She then moved to another flower and repeated this behaviour quite a few times before leaving the patch. I was too fascinated watching and I didn't take any photos, fortunately, forensic evidence was left behind and I could take a shot of the actual holes easily spotted in the flowers (above). This queen was engaging in 'nectar robbing', a not uncommon behaviour amongst Bombus terrestris, by which individuals can feed from flowers where nectar is not accessible via the corolla opening. Flowers that are 'robbed' might have a complex floral morphology adapted to attract specific pollinators - for example, nectar can be located in a deep spur; or flowers might have lips that need force opening for entering, or the corolla could be just too deep to reach the nectar inside. Nectar robbing - described by Darwin - is another of the aspects of the foraging repertoire of B. terrestris, which, despite its short-tongue it is an impressively resourceful species. Research has shown how queens learn by experience where exactly to perforate the flowers to reach the nectar optimally, and how they switch to use previously made holes to feed when the proportion of perforated flowers reaches a threshold (secondary nectar robbing, of which honeybees and other bumblebees also benefit).

 Surprisingly, and despite its negative name, nectar robbing is not necessarily detrimental to the plant. The studies comparing seed set on 'robbed' vs. normally visited flowers in bumblebees show that either there is no effect on seed set or an increased seed set when 'robbed', indicating that nectar robbers are indeed involved in pollination. In addition, nectar robbing can happen at the margins of the plant distribution, where the usual long-tongued pollinators might be absent and the 'robbing' could be the only chance for seed set in self-incompatible species. Although to avoid this 'robber-like pollinators' has been proposed, I myself thin that a more suitable name for hole-making bumblebees could be 'floral engineers'.

A B. pratorum feeding on comfrey. Despite its short tongue it feeds via the corolla opening, I guess it must be able to reach the nectar due to its small size, maybe able to stick its head into the corolla itself, or it might be just collecting pollen. B. pratorum has also been reported as a primary nectar robber.

Bombus pascuorum feeding on comfrey flowers through the corolla opening

A close up of a Bombus terrestris queen showing her powerful set of mandibles

More information
Olesen, J.M. (1996) From Naïveté to Experience: Bumblebee Queens (Bombus terrestris) Foraging on Corydalis cava (Fumariaceae) Journal of the Kansas Entomological Society, 69, No. 4, Supplement: Special Publication Number 2: Proceedings of the Eickwort Memorial Symposium (Oct., 1996), pp. 274 -286. http://www.jstor.org/stable/25085724

Goulson, D., Stout, J., Hawson, S., & Allen, J. (1998). Floral display size in comfrey, Symphytum officinale L. (Boraginaceae): relationships with visitation by three bumblebee species and subsequent seed set Oecologia, 113 (4), 502-508 DOI: 10.1007/s004420050402

Maloof, J., & Inouye, D. (2000). ARE NECTAR ROBBERS CHEATERS OR MUTUALISTS? Ecology, 81 (10), 2651-2661 DOI: 10.2307/177331

Ants in my plants

The Black garden ant (Lasius niger) is a very successful ant species. Part of their success lays in their adaptability. They are opportunistic in their choice of food: they feed on dead animals, nectar and pollen, seeds and honeydew. They are farmers, tending for aphids and scale insects and collecting the honeydew they secrete in exchange for protecting them from parasites. At the moment, ants are very active, feeding on plum and cherry nectar. They can actually enter the flower where the nectar is hidden. Although ants are known to pollinate a few plants species, they are mostly 'nectar thieves'. The reason the ants do not usually pollinate flowers are twofold. First, they are so small in relation to the size of the flower that they do not seem to touch the anthers or stigmas when entering the flower, and pollen does not adhere well to the ants surface. Second, individual ants  do not travel between different flowers in a single bout of feeding, but just get enough to travel back to the nest from a single flower - notice the distended abdomen of the ants leaving the plum flower on the photo above. They seem to feed on nectar on any cup shaped flowers lacking specific mechanical obstructions to them entering the flower (such as labiatae, the pea family). In fact, aspects of flower shape might be adaptations by plants to exclude these generalistic nectar thieves.

 Black garden ants find Paeony buds secretions irresistible, and it is hard to spot a paeony bud without its handful of feeding ants.

 The ants are now also "milking" scale insects. They encourage the scale insects to excrete the honeydew by 'tickling' them with their antenna.

More information:

C.M. HERRERA, J. HERRERA and X. ESPADALER (1984) Nectar thievery by ants from Southern Spanish insect-pollinated flowers. Insectes Sociaux, 31: 142-154. Here.

Darwin on bumblebees: the quick and clever workmen

We could be tempted to think about insects as little 'robots', behaving blindly and rigidly responding to external cues following an internal instinctive program. However, in the last few decades, research on insect learning has taken off providing an astonishing amount of data on the behavioural plasticity and learning abilities of these animals. Much of the research has taken place using bumblebees as models as they can be easily kept in the laboratory and many individuals can be produced and used in experiments. Bumblebees, as social bees, have a rich behavioural repertoire. After emerging from their natal nests, queens explore their environment to find a suitable nesting site, feed themselves with nectar and pollen, build their nests, make cells, lay eggs and provision their offspring with more nectar and pollen. Foraging involves taking decisions as to which species of plant and which individual flowers to visit, the available flowers changing continuously as the season progresses or from site to site. In addition, not all nectar and pollen is easy to collect: in many nectar-rich flowers, nectar is located deep inside the flower (e.g. Honeysuckle, Nasturtiums) and pollen from different flowers has to be gathered in various ways. Recent studies illustrate the flexibility of individual bee learning, but also the less known aspect of social learning: bees not only learn on their own (from trial and error) but also from each other - and even from other species. Despite the flurry of recent research, many of the facts had been observed and hypothesized by Charles Darwin over a century earlier. Darwin never ceases to amaze me: he was a meticulous, keen observer, paying attention and being enthused by the littlest things; never ceasing to test hypothesis, clearly illuminated by his encyclopedic knowledge in all things related to plants and animals. He gathered observational and experimental data painstakingly, often in his own garden and sometimes even enlisting his own children.
He wrote on social learning:
"One day I saw for the first time several large humble-bees visiting my rows of the tall scarlet Kidney Bean; they were not sucking at the mouth of the flower, but cutting holes through the calyx, and thus extracting the nectar. I watched this with some attention, for though it is a common thing in many kinds of flowers to see humble-bees sucking through a hole already made, I have not very often seen them in the act of cutting. As these humble-bees had to cut a hole in almost every flower, it was clear that this was the first day on which they had visited my Kidney Beans. I had previously watched every day for some weeks, and often several times daily, the hive-bees, and had seen them always sucking at the mouth of the flower. And here comes the curious point: the very next day after the humble-bees had cut the holes, every single hive bee, without exception, [...] sucked through the cut hole; and so they continued to do for many following days. Now how did the hive-bees find out that the holes had been made? [...]  I am strongly inclined to believe that the hive-bees saw the humble-bees at work, and well understanding what they were at, rationally took immediate advantage of the shorter path thus made to the nectar." (Darwin, 1857)
And on the use of experience and memory:
"I observed also bees flying in a straight line from one clump of a yellow-flowered Oenothera to every other clump of the same plant in the garden, without turning an inch from their course to plants of Eschscholtzia and others with yellow flowers which lay only a foot or two on either side. In these cases the bees knew the position of each plant in the garden perfectly well, as we may infer by the directness of their flight; so that they were guided by experience and memory." (Darwin 1876)

A fragment of a description of social learning in bees by Darwin in his work "The effects of cross and self fertilisation in the vegetable kingdom" (1876)

A recently emerged Bombus lapidarius queen feeding on Agapanthus

A Bumblebee Bombus terrestris, worker gathering pollen from a poppy

Links
All of Darwin's work, books, articles and letters are now freely available and fully searchable online in The Complete Works of Darwin Online.
Social learning in insects, a recent summary