Don’t miss spotting solitary bees this spring

April is when nature truly comes into its own. There are signs in February and March, but April is when trees begin to leaf in earnest, dandelions appear everywhere, and bees and butterflies take to the wing. Swallows, swifts, and martins return, and, if you are especially fortunate, you might even hear a cuckoo.

Bees, in particular, are a highlight of this month for me. Queen bumblebees, having spent the winter in hibernation, now take to the air with their energetic buzzing as they search for pollen and nectar to replenish their reserves and begin nesting. However, it is not just bumblebees that deserve attention. Earlier this month, I spotted a colony of solitary, ground-nesting bees along the banks of the River Ilen. These were ashy mining bees, grey and black, and adorably teddy bear-like. Watching them busily excavate tunnels in the soil in which to lay their eggs was a sight to behold.

Solitary bees

We have 21 species of bumblebee in Ireland, and along with one honeybee, these are collectively known as social bees. However, these are outnumbered by solitary bees, of which there are 81 species in Ireland, including the ashy mining bee. Unlike honeybees and bumblebees, each female solitary bee builds and provisions her own nest. They do not form communal hives, and their life cycle spans a full year. Females lay eggs that will not emerge as adults until the following spring. Adult solitary bees live for only a few months, meaning that the parent bees die and leave their offspring to develop and emerge from the nest alone, hence the term ‘solitary bees’.

As there is no communal nest to defend, solitary bees are generally docile and approachable, with a very limited ability to sting. They are also remarkably effective pollinators. Unlike social bees, which collect pollen in compact baskets on their legs, solitary bees carry pollen more loosely on their bodies. As they move between flowers, this pollen is more easily transferred, increasing pollination efficiency. Studies suggest that some species of solitary bees can perform the work of up to 120 honeybees. 

The ashy mining bees I spotted tunnel into earth to create their nests. Mining bees make up around 80% of Ireland’s solitary bee species. However, there are also cavity-nesting bees, which don’t dig their own nests but use pre-existing cavities. Mason bees and leafcutter bees are two examples. Solitary bees are a diverse group, each with unique traits.

Mason bees

Our native gold-fringed mason bee is a striking example of a species with a unique nesting habit. They are the only Irish bee known to nest inside empty snail shells. I have yet to spot this ginger-haired, black bee, but they are found in sandy habitats, and there have been reports from Inchydoney.

They are active from April to August, and, after mating, females carefully select suitable shells in which to nest. Inside each shell, she lays between one and four eggs in separate compartments, known as cells. She constructs dividing walls using chewed leaves mixed with saliva, called leaf mastic, and seals the entrance in the same way when the nest is complete. This behaviour illustrates how mason bees earned their name, as they use materials such as mud, clay, and leaf mastic to partition and seal their nests.

Before sealing the shell, the female provides each egg with a supply of nectar and pollen to sustain the developing larva. Since learning about this remarkable bee, I have taken to examining snail shells on sand dunes such as those at Barley Cove and Inchydoney, looking for signs of sealed entrances. 

Leafcutter bees

Leafcutter bees are equally remarkable, with an extraordinary ability to cut precise circular pieces from leaves or petals to build their nests. Once you learn to recognise the signs, it becomes easy to spot their handiwork on plants.

Using powerful jaws, the female cuts neat sections of vegetation and carries them back to her nest, often appearing to defy gravity as she transports pieces larger than herself. Leafcutter bees typically nest in pre-existing cavities, such as hollow stems, holes in wood, or even man-made bee hotels.

Within the nest, the female lines the cavity with leaf fragments to form a series of cells. As with other solitary bees, she places a mixture of pollen and nectar, lays a single egg, and, in her case, seals it with a piece of leaf. Females can be identified by the dense hairs beneath their bodies, often tinged orange, which they use to carry pollen.

The time is now

One of the most remarkable features of solitary bees is how females control the sex of their offspring. Fertilised eggs, which develop into females, are purposely laid at the back of the nest, while unfertilised eggs, which become males, are placed near the entrance. As a result, males emerge first, ready to mate when females appear.

Much of their activity takes place early in the year. Species such as the ashy mining bee peak in April and May, then disappear again. This makes spring the perfect time to observe them.

Similarly, some butterflies have short flight periods and are not seen later in the summer, including the green hairstreak and the orange tip. So do not miss out. Get outdoors and see what you might discover.