I have been posting on the Tawny Mining Bees, Andrena fulva, recently. I have been watching suitable nesting sites for signs of activity and today I came across many nests located in groups in several grassy areas. It was a bit windy and the female bees often missed their nests when landing. Instead of walking the short distance, they would fly again, carry out what looked like a positioning flight, and landed on top of their nest mound and got inside.
Why do bees nest in this way? They are solitary, so each bee will make her own cells and storage pollen and nectar toward her eggs. Apparently bees are able to detect the smell of conspecific active nests and preferable fly towards them; they also tend to return to their natal sites, and, obviously, successful nesting areas would tend to increase their nest density with time. These are proximate explanations, they tell us how bees actually find the nesting sites. But do bees actually benefit from nesting in an aggregation as opposed to doing it on their own? Aggregated nests must have a strong benefit to counteract the costs associated to the behaviour, such as increased competition or higher diseased transmission. Five hypothesis as to the adaptive value of nesting aggregations have been put forward:
1) Bees might be selecting very specific environmental conditions to locate their nests, for example, soil of a particular consistency or nectar sources nearby. This hypothesis has been investigated and appears to hold for some species, but not for others.
2) Nest sites might act as "information centres", where bees would find from others where the best foraging resources where. This, although possible, has no empirical support.
3) Newly nesting individuals might nest in aggregations because they act as markers of successful nesting sites.
4) Bees might benefit from reusing old nests, so that some of the costs of digging would be offset.
5) Nesting communally might offer some antipredator or antiparasite benefits, maybe by confusing predators, or communal defence. Data in support of this hypothesis is conflicting: parasites can either favour aggregations, by being more effective the less aggregated bees are, or dispersed nesting, when they locate clumped nests more effectively. Some bees gain protection from parasites by nesting communally, for example, as I covered in the recent post on Melecta, individuals of the host species Anthophora attack parasites near their nest, therefore conferring some protection to neighbouring nests.
Whatever the reasons, Andrena fulva nesting aggregations must be one of the easiest to observe in British solitary bees. Just look for their little molehills on the grass. A little red head might be peeking from inside.
Michener, Charles D. (1974). The social behavior of the bees: a comparative study. Harvard University Press. Other: ISBN-13: 978-0674811751
Rosenheim, Jay A. (1990). Density dependent parasitism and evolution of aggregated nesting in the solitary Hymenoptera Annals of the Entomological Society of America, 83 (3), 277-286