Jun 092017
Drone tree bee on the perennial cornflower

One of the volunteers at Amisfield this week alerted me a large number of bees buzzing round one of the bird boxes high in a birch tree.  The nest box had been taken over by a colony of tree bees (Bombus hypnorum). 

The tree bee is a newcomer to Amisfield, and to the UK.  It was first recorded in the south of England in 2001 after spreading westwards across Europe from the forests of Siberia and western Asia.  In the last decade or so its range has expanded extremely rapidly so that it is now common across the whole of England and Wales.  It first reached Scotland in 2013 and I saw the first ones in my Haddington garden in 2014.  They are easily recognised, as you can see in the photos, by their bright ginger thorax at the front, a black band in the middle and a white tail.  As its name suggests, the tree bee often nests in hollow trees (other bumblebees more usually nest in holes in the ground) and bird boxes look, to a bee at least, like the ideal tree hollow. 

Tree Bees (Bombus hypnorum) dancing around the entrance of the bird box

Tree Bees (Bombus hypnorum) dancing around the entrance of the bird box

This nest box was used by tree sparrows a couple of years back, but the entrance hole has obviously been enlarged by a woodpecker such that it would no longer provide suitable protection for small birds.  But why were 20-or-more bees dancing around the entrance to their nest, making little attempt either to enter or to go foraging for nectar and pollen as bees are supposed to do?  If they were defending the nest against intruders then you might expect them to be facing outwards, but the white tails in the photo here make it quite clear that they are all facing towards the entrance hole.

A bit of research revealed that these are actually males doing what is called a ‘nest surveillance’ flight.  They are watching the nest from outside, waiting and hoping for a virgin queen to emerge.  If a queen does emerge then she will immediately be attacked by a male and the couple will fall to the ground to mate.  Worker bees emerging from the nest also have to run the gauntlet of these eager males in order to leave on their foraging flights.   

Female worker tree bee visiting the extra-floral nectaries of an  unopened flower bud of perennial cornflower, Centaurea montana

Female worker tree bee visiting the extra-floral nectaries of an unopened flower bud of perennial cornflower, Centaurea montana

Tree bees are good pollinators and I wanted to photograph one on a flower.  I searched the flowers in Amisfield’s long herbaceous borders and saw buff-tailed bumblebees, red-tailed bumblebees, common carder bees, early bumblebees and possibly a white-tailed bumblebee.  I saw solitary bees, hoverflies imitating bees and wasps visiting flowers just like bees.  But it was not until I had walked the full length of both of the long borders twice that I eventually spotted the first tree bee!  To my surprise, this worker bee was ignoring the flowers completely, but was visiting the unopened buds of the perennial cornflower, Centaurea montana, systematically flying from one bud to another.  The perennial cornflower, which also goes by the rather delightful names “great blue-bottle”, or “mountain bluet”, is a common garden plant originating from the mountains of Europe and is closely related to the native cornflower (Centaurea cyanus).

As bumblebees go, the workers of the tree bee are relatively small and they have a rather short tongue.  This means that they are unable to reach the nectar in flowers where the nectaries are protected at the base of a long floral tube, but instead, concentrate on visiting flowers in which the nectar is more accessible such as cotoneaster, raspberries, currants and fruit trees.  The photo at the top of this blog is a male tree bee I saw later on a flower of the perennial cornflower; drones are a good bit larger than the workers and consequently have a longer tongue.  But the perennial cornflower also produces extra-floral nectaries. 

Common garden black ant, Lasius niger, gleaning sugar from the  extra-floral nectaries on a flower bud of perennial cornflower

Common garden black ant, Lasius niger, gleaning sugar from the extra-floral nectaries on a flower bud of perennial cornflower

Extra-floral nectaries are glands that secrete sweet sugary ‘nectar’ on the surface of leaves, or stems.  In cornflowers they are on the surface of the scales that form the flower bud.  It is generally assumed that some plants evolved extra-floral nectaries in order to attract ants.  Ants are voracious predators that eat many different kinds of small insects, but they also have a sweet tooth.  The theory is that if lots of ants are attracted to swarm over a plant looking for sugary nectar then they will also clean the plant of insect pests so that both the plant and the ant will benefit.  Ants do not make good pollinators because they cannot fly (not the workers, at least) so the plants will obstruct ants from gaining access to the nectar in flowers, but encourage them with accessible extra-floral nectaries.  A quick inspection of the perennial cornflowers in my garden this morning revealed numerous ants visiting the flower buds and showing particular interest in the nectaries near the tips of the bud scales.

Wasp licking sugar from the extra-floral nectaries  on a flower bud of perennial cornflower

Wasp licking sugar from the extra-floral nectaries
on a flower bud of perennial cornflower

Plants with extra-floral nectaries can be important in attracting more than ants, including, it would seem, tree bees.  So it has been recommended that native cornflowers (Centaurea cyanus) should be included in wild flower seed mixes particularly because their extra-floral nectaries attract a wide range of predatory and parasitic insects like ladybird beetles, lacewings  and hoverflies that play a crucial role in controlling the populations of pest species.  Interestingly, the wasp that I photographed when looking for the tree bees at Amisfield was also feeding on the perennial cornflower buds.  Wasps have a sweet tooth which may makes them a nuisance at the barbecue, but in the garden they are aggressive predators of many garden pests and do a lot of good.

Unfortunately, attracting ants may not be an entirely good thing.  One of the perennial cornflower plants in my garden is covered in aphids.  Aphids also secrete honeydew, a sugary ‘nectar’, that is attractive to the common garden black ant.  The ants farm these aphids, ‘milking’ them for the nectar, protecting them from other predators and even moving them around from one plant to another.  It is a complicated world.

Colin Legg
June 2017


Behaviour of tree bees: Hill, C.  Introducing the ‘Tree Bumblebee’ Bombus hypnorum.  Available from: https://bumblebeeconservation.org/images/uploads/Tree_bee_article_2015.pdf

Role of plants with extra-floral nectaries in controlling pests:  Kopta T., Pokluda R., Psota V., 2012  Attractiveness of flowering plants for natural enemies. Hort. Sci. (Prague), 39: 89–96.

Stettmer, C. 1993  [Flower-visiting beneficial insects on extrafloral nectaries of the cornflower Centaurea cyanus (Asteraceae).] Mitteilungen der schweizischen entomologischen Gesellschaft 66: 1–8. (in German but cited in various sources).

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