‘Solitary Bees’

A presentation by Brigit Strawbridge, 31st March, 2022. 35 attendees.

Brigit’s opening slide (above*) showed a solitary bee heavily loaded with orange pollen, but look carefully and you will notice there are no pollen baskets! More about form and function later. It is not just bees that act as pollinators. There are in excess of 200,000 species including bats, beetles, wasps, moths and butterflies, hover flies, birds and so on. They have all evolved alongside flowering plants, mostly with mutual benefits to plant and pollinator.

Some pollinators.

Putting things into perspective, 75% of our global crops need pollinators to increase yield and quality, and the current estimate is that 87% of all flowering plants require pollinators. It is not surprising that insects are a major group of pollinators as they evolved first and have been associated with flowering plants for longer. The number and diversity of insects greatly affects the food chain and can be regarded as an indicator of ecosystem health.

The social bees

Starting with the type of bees we are familiar with, the honeybee has overlapping generations, a high degree of cooperation within the colony and effective communication. There is a caste system and the single queen can live for more than one season. Our 24 species of bumble bee in the UK have solitary fertile females overwintering who go on to head a social colony for one season.

Where do the solitary bees fit in?

The life cycle of solitary bees has no overlapping generations and no caste system. The overwintered fertile females can be regarded as ‘single mothers’ and there are no social traits within these species.
In the UK there are approximately 250 types of solitary bee and, world-wide, an estimated 20,000 bee species, with a gradation in sociality from totally solitary to highly social.

UK examples of solitary bees are red mason bee, ivy bee, leaf cutter bees and blood bees, the latter so called as they have blood red hairs on the thorax and abdomen. In terms of size, bees range from Wallace’s giant bee, Megachile pluto, at 63.5mm long to the world’s smallest bee, Perdita minima, at around 2mm long.

How do solitary bees feed?

Like honeybees, the solitary bees feed on sugar-rich nectar and protein-rich pollen. Sometimes solitary bees also feed on floral oils, and the methods of pollen collection differ from honeybees. The branched hairs of bees are an adaptation enabling pollen collection by electrostatic attraction, the charge building up as the bee flies through the air to forage. Honeybees pass the pollen to the pollen baskets on the back legs, moistened with small quantities of nectar to make the pollen load sticky. Solitary bees collect dry pollen loads.

Solitary bees push the pollen onto a pollen brush or scopa, usually located on the hind legs or beneath the abdomen. Some species additionally use the sides of the body to collect pollen and can accumulate impressive quantities in one foraging session. Other species lack the pollen brush and collect pollen in their crop.

An additional pollen gathering techniques used by solitary bees and bumble bees is ‘buzz’ pollination. The bee vibrates its body to dislodge pollen that then adheres to the branched hairs. This messy way of foraging is beneficial to both bee and plant. There is even a bee called the head banger bee that does exactly what its name describes to dislodge pollen!

Where do solitary bees nest?

Opportunistic bees will nest is cavities almost anywhere. Holes in the lime mortar between stones in old walls, hollow stems, holes in the ground, holes in rotten wood. These are just some of the cavities favoured by the opportunists. Other solitary bees, like the ashy mining bee, will make holes in soil, sometimes to great depths, to secure a safe haven for their eggs. Where suitable conditions exist, aggregates of bees will be found, but each tunnel will only house the eggs from one female. They will also live in Bee Hotels!

One type of solitary bee you will often see in gardens is the leaf cutter bee. They will cut neat ‘bites’ from rose leaves, fly with the pieces to the nest, line the nest cavity with the leaves then block up the entrance once the eggs are laid and provisioned.

Life styles

The basic life cycle is the same for all bees (egg, larva, pupa, adult). Whereas honeybees and bumble bees provision the nest with some form of honey comb, the solitary bees often provision each individual egg with pollen and some nectar, sealing the cavity before laying another egg. When the larva hatches out it feeds on the nectar and pollen in its own store, then pupates. The female eggs are laid first at the back of the cavity and the male eggs are laid last at the outer end of a cavity so that the males emerge first.

Mason bees use mud to line and seal their egg cavities. Leaf cutter bees use sections of leaves and some bees use flower petals for the same purpose. The female wool carder bees prefer to line the nest with soft fibres from hairy plants such as Lamb’s-ear, Great Mullein and Yarrow. These cavity nesting solitary bees form an important pollinating work force as they can pollinate more flowers during any one visit than honeybees or bumble bees.

Brood parasites

There are over 70 Cuckoo bee species that exploit the nests of solitary bees. They are often brightly coloured and adopt a variety of strategies to ensure the success of their progeny over their victims.

Some will detect a nest that is currently being provisioned and will lay their own egg embedded in the cell wall while the victim is away foraging. This egg goes on to hatch and devour the victim’s egg and develops using the victim’s provisions, eventually emerging fully developed. Other cuckoo bees lay eggs in already sealed cells, with the same end result, by injecting eggs into the cell or cutting a hole and sealing it up afterwards. Yet other cuckoo bees favour a more confrontational approach, entering the nest when the owner is at home, replacing eggs and resealing the cells.

Climate change

Many of you will have noticed that flowers are coming out earlier. This will cause problems for any bee species that have specific plants that they rely on for forage as the flowering time and bee emergence time get out of sync.

Brigit’s Hints and Tips

• Dandelions are brilliant flowers, used by many solitary bee species in early spring when there are few other flowers around.
• Vipers bugloss produces nectar freely and is visited by many kinds of bee. Can be cultivated on light sandy soils and is used in many wild flower seed mixtures.
• Marjoram, wild or cultivated, can produce abundant nectar with a high sugar content.
• Borage can replenish its nectar every 2 minutes! Easily propagated in most soils. Visited by all kinds of solitary bees as well as honeybees.
• A pond could be your next project. Ponds attract all kinds of wildlife, including solitary bees.

What you can do to address the problem of bee decline

• Stop using pesticides.
• Grow nectar rich plants all year round.
• Do you have a lawn? Try leaving patches to create wild flower meadows.
• Join community wildlife initiatives.
• Join Buglife or the Bumblebee Conservation Trust.
• Plant flowers in blocks.
• Fill the early and late gaps.
• Avoid double headed flowers and non-native invasive plants.

Some suitable reading material

The Bees in Your Backyard. Joseph Wilson and Olivia Messinger Carril.

The Solitary Bees, Biology, Evolution, Conservation. Bryan Danforth, Robert Minckley, John Neff.

Field Guide to the Bees of Great Britain and Ireland. Steven Falk.

Plants for Bees. Kirk and Howes.

Dancing with Bees, A Journey Back to Nature. Brigit Strawbridge Howard.

Video of Red mason bees hatching from cocoons. Courtesy of Brigit.

How to manage and maintain Bee Hotels. Bumblebee Conservation Trust.

Our thanks to Brigit for making time to come to East Devon from Cornwall to deliver this talk and show us some of her wonderful images of solitary bees.