Celebrating the bees on the World Bee Day – Getting to know them better!

This blog was written by 3rd-year PhD students Elena Zioga and Irene Bottero and first appeared on the TCD EcoEvo Blog.

The 20th of May is declared as the ‘World bee day’ and its purpose is to acknowledge the importance of bee pollinators in our ecosystem. Animal pollinators play an important role in the reproduction of many plant species (90% benefit from animal pollination), including food crops (crops pollinated by animals make up 35% of global food production), ensuring the abundance and good quality of fruits, nuts, and seeds, which are crucial for human nutrition. Beyond food, pollinators also contribute directly to medicines, biofuels, fibers (e.g. cotton and linen), and construction materials.

Among all pollinators, bees are considered the dominant pollinators in many habitats across the world, as they depend on flowers to fuel all stages of their life cycle. There are over 20,000 bee species worldwide and they are found in all types of climates, from forests in Europe to deserts in Africa – even in the Arctic Circle. Bees belong to the great insect order of Hymenoptera – that also involves wasps, sawflies and ants – and to the suborder Apocrita, subclade Aculeata. We can say that bees are hunting wasps that changed their habits, shifting from a predatory or carnivorous diet to a herbivorous one based on nectar and pollen. This also had consequences on the physical aspect of bees, which evolved a hairy body to be more functional at holding pollen. In Ireland there are overall 99 bee species, including one honeybee species, 21 bumblebee species and 77 solitary bee species.

The honeybee (Apis mellifera L.) is the main species commercially exploited by humans for its various products (e.g. honey, pollen, royal jelly, propolis and wax). Honeybees are social insects, forming colonies with large numbers of individuals. Each colony consists of a single queen, hundreds of male drones and 20,000 to 80,000 female worker bees. The queen honeybee is the largest bee in the colony and the only one capable of laying fertilized eggs (between 1,500 to 2,000 eggs per day). Drones have no sting and they do not collect pollen or food. Their main purpose is to leave the colony and mate with new queens; if the colony becomes short of food, they are the first to be kicked out! The workers are all females and they are the smallest bees of the colony. They produce wax and they use it to build the honeycomb cells where the eggs are then laid by the queen. They are also responsible for feeding the newborn larvae which are initially fed with royal jelly, and later with honey and pollen. Their responsibilities in the hive change depending on their age: the young worker bees clean the hive and feed the larvae. Then, they start building comb cells and as they become older, they progress into cleaning and guard duties. Their last job is foraging for nectar, pollen, other plant exudates and water. Nectar is collected in their “honey stomach” while pollen is collected in their “pollen baskets”. The forager bees leave the hive each morning to source the best nectar within a 5 km radius. On return to the hive, foragers perform the ‘waggle dance’ to communicate the source of food, distance, and direction. Only female workers may sting, sacrificing themselves for the rest of the colony as they die afterwards.

A forager honeybee, Apis mellifera, covered with pollen (Photo by Irene Bottero)

Bumblebees are large, hairy bees and you can tell when you see one from its loud buzzing sound. All species of bumblebee live in colonies, but their colonies are much smaller than those of honeybees and do not survive over winter. The bumblebee colony will only consist of around 50-150 individuals. Bumblebees will only sting to defend themselves and their colony, but unlike honeybees, they can sting more than once. Bumblebees do not store honey to survive the winter. The little food they do store is saved to feed the larvae and the egg-producing queen. Bumblebees feed on nectar and collect pollen to feed their young. The bumblebee colony will die off at the end of summer and only the new queens will find somewhere to hibernate during the winter, usually underground, and emerge to find new nesting ground ready to start a new colony in spring. They like to nest underground in disused nests of small mammals, or just above the ground, in undisturbed areas with tall grasses and plenty of leaf litter. Bumblebees are very important pollinators to many plants, from herbaceous wildflowers, to shrubs and trees. They can pollinate plants that other pollinators cannot due to their longer proboscis (tongue) and they don’t mind going out on overcast days. They also perform the so called ‘buzz pollination’ by sonicating the male plant parts (stamens) to release pollen which is firmly held by the anthers.

Commercial bumblebee colonies are used for pollination in greenhouses with tomatoes, peppers, squash, strawberries, blueberries, cranberries, and many other crops.

The bumblebee Bombus lapidarius feeding on oilseed rape flower (Photo by Irene Bottero)

Solitary bees make up the largest percent of the Irish bee fauna. Solitary bees do not form a colony. Instead, they create nests in hollow reeds or twigs, holes in wood, or, most commonly, in tunnels in the ground. The female solitary bee typically creates a cell that she lays an egg into and places some food (nectar and pollen mix) for the larva when it hatches, then seals the cell off. A nest may consist of numerous cells. The adult solitary bee does not provide care for the brood once the egg is laid and usually dies after making one or more nests. Solitary bees take one whole year to pass through a complete life cycle and may only survive as adults for a few weeks. This isn’t long enough for them to raise their offspring, so the young bees must fend for themselves. The males usually emerge first and are ready for mating when the females emerge.

Andrena spp. bee with pollen on its legs. (Photo by Irene Bottero)

Bees are a large group of insects with various morphological characteristics and diverse nesting and food preferences. Here are some cool facts about those magnificent creatures!

To bee or not to bee

Some groups of insects can be very similar to bees, like flies (hoverflies) or wasps. Flies can adopt a mimic “bee pattern” to trick the predators, that will avoid them for fear of painful stings! Some Irish examples of this mimicry are represented by Eristalis tenax that looks like a honey bee and Volucella bombylans similar to a bumble bee. In other groups the similarity is less evident, but still present, like in Syrphus ribesi and in many Cheilosia species.

Comparison between an Eristalix tenax (on the left) and a honey bee (on the right, photo by Irene Bottero)
Comparison between a Volucella bombylans (on the left) and a bubmble bee (on the right, photo by Irene Bottero)

To distinguish bees from flies it could be useful pay attention to their eyes (much larger in flies) and to their wings (4 in bees and only 2 in flies). Antennae and mouth also have different aspects.

In wasps, the differences are more related to the shape of the body, and to the presence of branched hairs on bees. Some genus of bees are very similar to wasps and distinguishing them might be very tricky and might require the use of microscopes!

Comparison between a wasp (on the left) and a honey bee (on the right)

Same species but many forms!

Within the same species we can find big differences between the individuals, depending on their gender, age, and social role.

Many bees show differences between females and males – sexual dimorphism. In some cases, the differences can be particularly emphasised, and the two genders can appear as belonging to different species (i.e. Bombus lapidarius, Osmia bicolor, Andrena and Lasioglossum species).

Bombus lapidarius male and female. The male is identified by yellow hair, a characteristic that is absent in the female

The first important difference is the sting, that only can be found in females, because of its origin; the sting in fact evolved from the ovipositor system. It can be often be retracted inside the abdomen and thus not being visible at a first sight – so be careful petting a bee!

Another important difference between the genders are the hairy pollen baskets or brushes, only evident in females, on the hind legs or under the abdomen (except the cleptoparasite species that don’t collect pollen themselves, but parasite nests of other bee species).

The size and the colours also differ between gender. Usually males are smaller and less colourful than females. Moreover, their antennae are longer (13 segments instead of 12).

Some tricks can also help to quickly identify the male from the female. In honey bees for example, the eyes of the drones are much bigger than the eyes of workers (females), and if a bumble bee has yellow hair on its head is for sure a male (yes, you can pet it!!).

Another huge difference is at chromosome level: males only have half of the normal number of chromosomes because they originate from unfertilized eggs. This phenomenon, called haplodiploidy, is very rare in nature and it seems to be one of the drivers of the social behaviour in eusocial bees. In fact, the males have no fathers (even though they have grandfathers – weird!) and they share only their mother’s genetic material; the females (workers) share the 75% of their genes (25% from mother and 50% from the father – we only share 50% of our genetic material with our siblings!)

In those eusocial species, differences can be spotted between different castes: queens are usually bigger than workers and males. The workers have poorly developed ovaries and although they can in some cases lay unfertilized eggs, they cannot give birth to female individuals. 

Aging can also affect the appearance of the bees. The sun can bleach bright hairs turning them into greyish/whitish/brownish. Some other individuals can lose some hair and look smaller or darker than normal.

Real or fake bumble bee?

Within our 21 bumble bee species, 6 of them are called cuckoo bumble bees. These are cleptoparasites, exploiting the nest of other bumble bees. The cuckoo bumble bee females usually look like the queens of the host species. They enter the nest of true bumble bees and replace the queen, subjecting the workers, that will feed the new “queen” and her offspring. Despite looking very similar to true bumble bee species at first glance, some hints can help us to distinguish cuckoo bees. Bumble bees are usually very hairy, and their legs are modified to better collect pollen grains. In true bumble bee female, the lower portion of their hind legs (hind tibia) is flat and smooth with long hair on the borders (this is the “pollen basket”). Cuckoo bumble bees do not collect pollen and thus their bodies look different: they are less hairy and the hind tibia are “dull, convex and slightly hairy”. Moreover, their wings might look darker and smoky, compared to ones of the true bumble bees.

Leg differences between true female and male of bumble bee and cuckoo bumble bee from “Identification guide to Ireland’s bumblebees” National Biodiversity Data Centre

Cuckoo bumble bees are not the only cleptopasite bees. The cuckoo behaviour is present throughout the bee groups, even though the way each taxon behaves can differ. Some species wait for the host to leave the nest to place an egg inside an open cell. When the parasite hatches, it feeds on the provisions collected by the host, and the host grubs themselves. Other parasites lay the eggs in sealed cells, breaking the cell wall or penetrating it with their long and sharp ovipositor organs, or make their way through the nest killing the host if it is present.

Cleptoparasites differ from their hosts because have different habits and aspects. They do not have parental care; they develop a stalker attitude and waiting for the best moment to occupy the nest and they visit flowers only for their own need. For these reasons their bodies can lose their pollen-collecting characteristics, ending up in some cases, to be very similar to wasps – e.g. Nomada species.

Nomada spp. – the body is not hairy and is wasp-shaped. Photo by Irene Bottero

Nest sweet nest

When we think about bee nests, we can think about the hives, an anthropogenic solution to manage the honeybees. But in nature there are very different types of nests, and every group of bee has its peculiar home.

Nests can be located in the ground or be aerial. In Ireland, the majority of the solitary bees (such as Andrena, Colletes and Lasioglossus) and the totality of bumble bees are ground nesters. The only exception between bumble bees is represented by Bombus hypnorum, new to Ireland (first spotted in 2017), that nests above the ground in tree holes. Ground nester can be found in different types of dry and sun-exposed surfaces – flat, slopes or vertical – and despite some species can be found in different soils, other just nest in very peculiar ones (like for example clay or sand). The ground nests, that can be isolated or aggregated, consist in tunnels that terminates with cells. Bees found incredible solutions to make their nests waterproof, thanks to waxy or cellophane secretions.

Andrena cineraria emerging from the ground nest
Internal structure of a ground solitary bee nest

The above ground nests (or aerial) have a different structure because they are located inside holes (in trees, walls, bee hotels).

An artificial solution to provide aerial nests for solitary bees. The tubes closed that we see in the bottom suggests that some insects nested inside. Photo by Irene Bottero

These nests have different cells organised in a line, where the males larvae are laid closer to the entrance and the females in the inner part. The cells are filled with provision and they separated by walls that can be constituted by different materials. Sometimes it is possible to recognise the species the nest belongs to thanks to the materials used to make the cells – e.g. Megachile species, as the common name suggests (leaf cutter bee) uses leaves, Osmia bicornis uses mud etc.

Internal organisation of an Osmia nest. Eggs (upper part of the photo) and larvae (bottom) lie on pollen provision.

A very particular and weird type of nest is the one belonging to some Osmia species. They use empty snail shells to lay their eggs in, and that can hide them to protect them (e.g. in Ireland, Osmia aurulenta).

Some shells used as nests by different Osmia species

Save the bees!

In Ireland, six species of bumble bees are critically endangered, seven are endangered, sixteen are vulnerable and thirteen are near threatened. In the last 80 years, three species of bees have become extinct, and the distribution and forty-two species of solitary bee has decreased of approximately 50% (six species critically endangered, ten endangered, fourteen vulnerable, twelve near threatened and thirty-eight of least concern).

The decline is driven by many threats including climate change, pesticide application, habitat disruption and introduction of new species (including some parasites species). Since bees play an important role in our ecosystem and they are such amazing creatures, it should be everyone’s responsibility to protect them and prevent their decline. Even small things of our everyday life can have a big impact on bee health, and everyone can contribute protecting them by following the All-Ireland Pollinator Plan guide of simple actions towards that cause. Trying to reduce herbicide, insecticide and fungicide application in our gardens can be a first step, as these substances were proven to have a negative impact on bees. Avoiding mowing the lawn, or at least leaving some patches with flowers, will help to preserve food resources (flowers) that pollinator will use to feed on. Allowing native species to grow in our gardens and parks or planting specific plants with different flowering periods, will sustain populations of pollinators through the seasons, helping them to feed their larvae and establish colonies. Preserving hedgerows is also fundamental since they represent a semi-natural feature particularly important in the agricultural landscape, that provides food and nest resources and enables the movement of the individuals across the landscape. Moreover, preserving our soil and providing bee hotels will help to create a suitable environment for the bees to nest.

The All-Ireland Pollinator Plan provides all the guidelines for easy actions that everyone (farmers, local authorities, schools and every citizen) can adopt to preserve the bee biodiversity.


Steven Falk and Richard Lewigton (2019) Field guide to the bees of Great Britain and Ireland. Bloomsbury.

National biodiversity data centre – https://www.biodiversityireland.ie/

Identification guide to Ireland’s bumblebees – National biodiversity Data Centre – https://www.biodiversityireland.ie/pdf-guide-to-identifying-irish-bumblebees-is-available/

Wikipedia – Haplodiploidy – https://en.wikipedia.org/wiki/Haplodiploidy

All-Ireland Pollinator Plan 2021-25 – https://pollinators.ie/aipp-2021-2025/

Fitzpatrick et al. (2006) – https://www.npws.ie/sites/default/files/publications/pdf/Fitzpatrick_et_al_2006_Bee_Red_List.pdf

Fitzpatrick et al. (2007) – https://pure.qub.ac.uk/en/publications/rarity-and-decline-in-bumblebees-a-test-of-causes-and-correlates-

IUCN Red List – https://www.iucnredlist.org/

About the Authors:

Irene and Elena are PhD students in the Plant-Animal Interactions Research group, in the School of Natural Sciences at Trinity College Dublin, and are supervised by Jane Stout.

Irene Bottero is a 3rd year PhD student in Botany (Trinity College Dublin). She is part of PoshBee project and in her thesis she is evaluating the impact of different habitat types on pollinators, specifically, honeybees, bumblebees, solitary bees, hoverflies, and butterflies.
Elena Zioga is a 3rd year PhD student in Botany (Trinity College Dublin). She is part of PROTECTS project and in her thesis she is evaluating the levels of pesticide residues in pollen and nectar of plants growing in Ireland.

Previous World Bee Day blogs:

2018 World Bee Day – are we preaching to the converted?

2019 World Bee Day 2019

2020 World Bee Day 2020: Trinity’s research round up and Bees: common myths and misunderstandings

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