By Athayde Tonhasca
During these unprecedented/strange/challenging times (pick your favourite cliche), many Britons have swapped their holidays abroad for the domestic great outdoors. This shift may help explain a spike in the number of bee swarm sightings along trails and in open spaces. In most cases, people are witnessing the comings and goings of the heather colletes (Colletes succinctus). This bee is usually found on heathlands, drier parts of moorland and coastal dunes – places with abundant heather (Calluna spp.) and heath (Erica spp.), its main sources of pollen.
The heather colletes nests underground in bare or thinly-vegetated south-facing spots; sandy banks quickly warmed by the sun are particularly favoured. Each female digs a burrow, stocks it with pollen and lays an egg on the pollen mass. The larva feeds on the pollen and emerges as an adult the following year.
Like many solitary bees, heather colletes nest close to each other, usually cheek by jowl. These aggregations can be massive: in one case, 60 to 80,000 tightly packed nests along a 100-m stretch of a river bank. Not surprisingly, these concentrations of swarming bees have prompted many a rambler to turn back or give a wide berth to the restless mob of honey bee lookalikes. Such precautions are unnecessary because these bees are harmless. They are not at all aggressive, and their stings are too weak to penetrate human skin. People who stop to admire them may catch a sight of clusters of bees rolling around. These are mating balls, comprising several males jostling furiously to mate with a female, who is hidden in the middle of the melee. As soon as a male succeeds – usually the larger one – the mating ball breaks apart. Females are monandrous, that is, they have one mate at a time, so they are not receptive to other males.
Nest aggregations, which are common among several species of mining bees, are a bit puzzling because of the risks they represent. The abundance of provisions (pollen and nectar) stored by female bees, and so many juicy larvae and pupae in the same place are godsends to predators and parasites.
So why do bees aggregate? It could be that adequate nesting sites are scarce: the ground has to be within certain physical specifications for secure tunnelling – the right type of soil, texture, drainage, slope and temperature – so several local bees may be attracted to the few good spots. Nesting needs help explain why many solitary bees display natal philopatry, which is the tendency to return to the site of their birth. It makes sense for a newly emerged bee to stick around: why take chances somewhere else when its place of birth ticks all the boxes? So the colony keeps growing, sometimes for decades. Aggregation may also result from having food nearby: broods have better chances of success if their mothers had easy access to pollen and nectar.
Whatever the reason, aggregations are quite handy for farmers who take advantage of the crop pollinating skills of some species such as the alkali bee (Nomia melanderi).
On the island of Colonsay, another species appreciates heather colletes nest aggregations: the red-billed chough (Pyrrhocorax pyrrhocorax). This bird feeds mostly on arthropods, and ants, beetles, moths and spiders are its usual prey. The choughs on Colonsay have learned to excavate heather colletes nests to get a nutritious, plentiful meal.
Hungry choughs are not likely to threaten heather colletes populations, considering the small number of birds on the island and their preference for farmland food such as crane flies and dung beetles; bee meals are probably opportunistic.
The heather colletes has experienced a small decline throughout Britain in the last 10 years or so for unknown reasons, but the population is still widespread and abundant in many places. They will probably carry on amazing and sometimes unintentionally startling nature ramblers for years to come.
Scottish pollinators 