I took a few photos of a large Bombus terrestris bumblebee (queen I think) visiting foxglove flowers in St. James Park, London on a fine day last week. When I looked closely at the images I noticed a few ants within individual foxglove flowers.
Lasius niger worker ants – and other species presumably – often tend aphids on foxglove flowers (Digitalis purpurea) and may forage for nectar on the flowers (1). But ants are not always welcome visitors to the flowers; they are detrimental to their fitness for a number of reasons (2). Firstly, they are usually too small to be much good as pollinators and secondly, their aggressive behaviour may put off more useful pollinators! They are ‘nectar thieves’ taking nectar without providing any mutual benefit for the plant, and also potentially diminishing the appeal of the flower to hard-working pollinators such as bees, which might stay away. Some plants try to keep ants away by using physical or chemical barriers, or offering them an alternative source of nectar, via extrafloral nectaries – EFNs (3). Foxgloves also have guard hairs to deter smaller bees from entering the flower (4), but it seems that ants can easily pass through these.
Does the presence of the ants affect the behaviour of the bees? Bumblebees can apparently detect whether another bee has visited a particular flower recently and thereby avoid wasting time by visiting a depleted nectar source. Do they do the same with ants? Ants do leave scent markings – e.g. pheromone trails marking a source of food – and laboratory experiments have shown that bumblebees potentially could avoid ant-visited flowers (1), if they put their minds too it! But they do not seem to use this ability much in the wild. Perhaps the ants are not taking very much nectar, so it is still worthwhile for the bee to visit the flower. Also, the bumblebees are in and out so quickly, and there are so many individual flowers clustered together on a foxglove inflorescence that it is not worth taking the time to sniff out whether ants had been in or not!
Would the bumblebee avoid a flower with an ant sitting in it? Perhaps big bumblebees like this one are not bothered by ants? Or do the ants get out of their way? Anyway, my observation was that the bumblebees were in and out of the flowers so quickly it was very hard to tell whether they avoided flowers containing individual ants. Perhaps they just steam rollered over them! Someone way know the answer to this?
- Ballantyne, G., & Willmer, P. (2012). Floral visitors and ant scent marks: noticed but not used?. Ecological Entomology, 37(5), 402-409.
- Willmer, P.G., Nuttman, C.V., Raine, N.E., Stone, G.N., Pattrick, J.G., Henson, K. et al. (2009) Floral volatiles controlling ant behaviour. Functional Ecology, 23, 888–900.