Where do all the bees go?
A sure sign that spring is in the air is the return of bees to your garden but have you ever wondered where the bees have been during the colder months? The answer is more complicated than you might think because there are around 250 species of bee in the UK alone, and the only thing they have in common is that they don't fly south for the winter. This huge group of species act as essential pollinators for many flowers and crops, so it is important that we understand what resources they need to survive.
Honeybees huddle together to keep warm
Honeybees (Apis melifera) are a semi-domesticated species which forms large colonies with up to 60,000 workers. Although usually found in artificial hives managed extensively by beekeepers, they can sometimes form wild colonies in tree cavities and rock crevices. Most of the honeybees that you see today are strains that have been imported from Europe.
Like all other species of bee and most insects, honeybees are ectothermic (taking heat predominately from the environment) for most of the year. However, during the winter honeybee colonies survive by working together to maintain temperature collectively. Clustering together each bee shivers its flight muscles to generate heat. Bees on the outside of the cluster act as insulation for those in the middle where the temperature is maintained at about 35oC. If the body temperature of the bees drops below 8oC they are unable to move their flight muscles and die.
During the summer each worker honeybee will only live 1-2 months. However, honeybees that hatch later in the year are adapted to survive longer, up to 6 months, to get the colony through the winter while no brood is being produced. In the autumn when the temperature drops below 10oC the bees return to the hive only venturing out when temperatures are warmer to dispose of body waste and keep the hive clean.
The population is much smaller during the winter and survives on honey stored from earlier in the year. If the honey supplies run out the colony will starve. The winter cluster of bees stays centrally in the hive and moves gradually upwards through the honey stores. This pattern of minimal movement means that clusters can fail to find honey stores in other parts of the hive and colonies have been known to starve to death surrounded by honey. Losses of honeybee hives over the winter have also been linked to the level of parasitic varroa mites (Varroa destructor) within the hive reducing the average lifespan of the worker bees.
Bumblebees hibernate underground
You may not have noticed but there are many different types of bumblebee across the UK and the queens, males and workers within a species all look different too.
Winter life is lonelier for a bumblebee queen. Although they form colonies in the spring they overwinter alone. After mating in the autumn each bumblebee queen will dig a small home to spend the winter. There has been a lot of interest in bees and their habits lately with concerns about population declines and the loss of valuable pollination services bees provide. However, exactly what a bumblebee looks for in its winter home is still a mystery. The last scientific research was published in 1969 by David Alford when he stumbled upon an area used for hibernation by loads of bees and recorded the characteristics of each queen's home.
The first bumblebees you see in the spring are these queens looking for somewhere to nest. The queens are easily recognised as they are a lot bigger than the workers that appear after colonies have been formed. Most bumblebee species don't make their own nest. They look for holes in the ground vacated by rodents or tree cavities.
The queen lays her first batch of eggs and forages for food. Once this first brood has developed they become workers and help with the foraging allowing the queen to become a full-time egg layer.
At the end of the colony life, new queens and males are produced and go out into the world to mate with bees from other colonies and start the whole cycle again. Typically, the colony, including the original queen and all the males will die leaving only the newly mated queens to start again next spring. However, with climate change and warmer winters some colonies are now surviving the winter. The buff tailed bumblebee, Bombus terrestris, has been recorded foraging on winter flowering plants. The Bee and Wasp Recording Society are collecting data to work out how widespread this phenomenon is and if it is happening in cities because gardeners are providing nectar and pollen from exotic winter flowers.
Solitary bees don't emerge from their brood chambers
A lot of people don't even realise that solitary bees exist; many of the species at least superficially resemble honeybees, bumblebees or wasps. In the UK we have more than 200 species. They make their nests in tunnels in the ground, mortar or dead wood or within hollow stems. Every female makes her own nest cells - they don't work together as a colony like bumblebees and honeybees. In the spring the adults emerge and females lay eggs individually, provide a pollen food packet and seal up the chambers.
You can put up a bee-house in your garden to support these non-aggressive bees which often lack suitable nest sites. The winter strategies of this large group of bees also vary considerably. Some overwinter as adults while others stay in larval form and pupate in the spring. Common solitary bees you might see include the red mason bees (Osmia rufa) and leaf cutter bees. Mason bees form a cocoon and pupate in the autumn. The fully formed adults remain in their cocoon until the spring. Once emerged each adult will only live a few weeks, just long enough to reproduce and start the cycle again.
Why are bees important?
They may be small but insects make an enormous contribution to the UK economy (over £400 million per annum). Bees are incredibly important pollinators. Many wildflowers can't reproduce at all without insect pollination and the yields of many important crops from beans to apples are dependent on bees. Although honeybees are important to providing pollination services to agriculture, research has shown that crop harvests are even larger when there are several species of bee visiting crops. This makes it important that we understand the habitat requirements and threats to all of our diverse bee species.
Bees are threatened by a variety of factors including disease, pollution and pesticides, invasive species and climate change. The interaction of these threats with land use change (the most important threat to biodiversity) puts a huge strain on already reduced populations. Land use change across the UK as a result of agricultural intensification has reduced the amount of habitat suitable for bees. Neat and tidy farms with few wild spaces have reduced wildflowers to forage on and less sites where bees can find space to nest or overwinter.
As a result of these changes in farming practice, two species of bumblebee and many species of solitary bee have been driven to extinction in the UK. Agri-environmental schemes are now in place that encourage farmers to create more bee friendly habitats but more research is needed to fully understand the importance of different flowering plants, nest sites and overwintering requirements to make these schemes effective.
How can you help bees?
Urban habitats such as parks and gardens are becoming increasingly important for bee species as natural habitats are being lost. Providing plenty of flowers producing nectar and pollen to keep colonies well provisioned is important throughout the year. Bird nest boxes are now sometimes being used by the tree bumblebee (Bombus hypnorum) and solitary bees will take up home in artificial nests increasing the range of bees that you see in your garden.
You can record bees that you see with the Bumblebee Conservation Trust BeeWatch and specific data is being collected on winter foraging bees by the Bees Wasps and Ants Recording Society.