Last year was not a good year for bumblebees.   Despite the warmth and sunny days of late Spring and Summer, they did not fare well.  Overall, numbers dropped by about a fifth.  We know this thanks to the efforts of the Bumblebee Conservation Trust.  The trust runs a nationwide bee walk scheme each year.  This involves hundreds of trained volunteers, walking across a thousand plus fixed routes, counting the bumblebees that they see.  This enables the Trust to calculate the number of bumblebees observed for each kilometre walked. This gives a measure of the health of bumblebee populations. Last year’s walks indicated ‘historic lows’ for 15 of our native species of bumblebee.  Red tailed bumblebees were particularly affected. After a relatively mild winter, many queen bumblebees emerged early from hibernation.  They then met with the rain and cold of early Spring. Though a bumblebee can cope to some degree with cold - by shivering to generate heat, the rain left them unable to forage in search of pollen and nectar.  Queen bumblebees have to establish a colony singlehandedly.  They may lay eggs, but if they remain within the nest to keep the larvae warm then they starve.  If they leave to find food then the larvae die in the cold.   So the weather is critical to the establishment and success of a colony.  The first brood of eggs need to be warmed by the queen, but she needs to be able to leave to feed herself and collect food for the growing larvae.  Last year, the establishment of colonies and rearing of the first generation of worker bumblebees was limited.  Indeed, it was estimated that the worker population in June fell by perhaps 50% for some species. [caption id="attachment_43455" align="aligncenter" width="675"] Coming into land[/caption] Sadly, the disappointing numbers for 2024 is part of a long term decline in bumblebee numbers, with some species become extinct (either locally or nationally).  This decline is associated with  Changing climate.  With global warming, we are experiencing warmer, wetter winters and hotter, drier summers, coupled with extreme weather events.  Bumblebee nests can be flooded by heavy rain. Habitat fragmentation.  Many woods and wild flower meadows have been lost over the decades, so that bumblebees (and other animals) find it difficult to move around in the disconnected landscape. Biological corridors have been lost.  Local populations can become inbred, which leads to a loss of genetic variation.  Flower-rich meadows which offered pollen and nectar are now a rarity.  Even brownfield sites can offer food for bumblebees - from the ‘weeds’ growing in them. The expansion of agriculture, towns and motorways. The countryside has been gradually devoured by the expansion of agriculture, towns and transport routes. These have contributed to the loss of natural ecosystems, like the meadows mentioned above, and also hedgerows. Modern agriculture also makes considerable use of many pesticides and herbicides.  The neonicotinoids [neonics] were used extensively as seed dressings for crops, such as oil seed rape and maize.  Even when exposed to very low concentrations of these chemicals, bees and bumblebees suffered. More recently it has been demonstrated that sulfoxaflor, another pesticide affects bumblebees.  It can reduce the number of workers and queens produced by a colony by 50%. [caption id="attachment_43460" align="aligncenter" width="675"] buff tailed bumblebee[/caption] As ever, local bumblebee populations can be helped by ensuring that gardens have a wide diversity of nectar-rich flowers, plus “No mow May” also has a positive impact.  

Last year was not a good year for bumblebees, according to the Bumblebee Conservation Trust.  The trust has been collecting data on bumblebee numbers since 2008, through the BeeWalk.  The BeeWalk project began as as a small scale initiative at Weybourne, Norfolk.  This involved counting the bumblebees seen on a monthly walk along a set route from March to October.  The BeeWalk was opened up first to members of the BCT and then to the wider public - a citizen science project.   Today, it is the Trust’s longest running (and largest) project.  For each site in the BeeWalk scheme, the bumblebee species seen are recorded, and whether the bee is a queen, worker or male.  Also recorded is information about the site (habitat type, land use, temperature and other relevant information).  The fact that the BeeWalk has been running for some years and is now widespread (across England, Wales and Scotland) means that it is possible to monitor how bumblebee populations are changing and correlate with any changes in climate and land use. In March last year, bumblebee numbers were relatively OK, having emerged from winter dormancy into warm and sunny conditions. Queen bumblebees need stable weather to establish their colonies.  In April the weather turned wet and cold but numbers remained stable.  But in June, the weather was particularly ‘unseasonable’ and worker bees of many  species were noticeably absent from many counts.  The effect was especially true of those species that normally reach peak numbers in early summer.  White tailed and red tailed bees were down by 60% and 74% respectively. In contrast, the garden bumblebee (Bombus hortorum) showed a smaller decline of just 12%. In July and August, the weather improved as did numbers but across the country overall bumblebee numbers (for all species) were down by 22%.  The red tailed bumblebee has declined year on year since 2015, but last year was the worst.  On a more positive note, some of the rarer species (the shrill carder & brown banded carder) have shown year on year increase. The decline highlights the sensitivity of pollinators to changing weather patterns, and emphasises the need for conservation efforts through: Protecting natural environments Restoring wild flower meadows The use of sustainable land management practices eg avoidance of insecticides (such as the neonicotinoids) Improving habitat connectivity to support bumblebee  movement and survival

Bumblebees (and bees) collect nectar and pollen.  Pollen is a vital food, used in the various stages of a bumblebee’s life. In Spring, newly emerged queens feed on pollen, then it is used to feed its their sister workers. The workers, in turn, take over the feeding of the colony (the larvae and future queens). If not enough pollen is collected, then the colony will not thrive, which can have significant long term effects.  Bumblebees are already facing many threats (from habitat fragmentation, agrochemicals and disease). The collection of pollen is a demanding process, and bumblebees will forage over a wide area.  They start their pollen collecting activities earlier than many insects as they can warm themselves up by ‘shivering’, that is, rapid muscle contractions which generate heat, warming the insects up ready for flight.  Bumblebees can fly in colder conditions and at higher elevations than many other insects. However, research at North Carolina State University has shown that the North American bumblebee (Bombus impatiens) can overheat when exposed to high temperatures (circa 42oC plus).  So,  if a bee is carrying a significant load of pollen and it is a hot day, its muscles have to work harder and the bee is at risk of overheating. A bumblebee loaded with pollen may be 2oC hotter than an unladen bee; it may be reaching its ‘thermal limit’ - a temperature at which its organs are damaged.  Climate change means that many parts of the world are now experiencing extreme weather events, when temperatures can reach into the forties. [caption id="attachment_39978" align="aligncenter" width="675"] Bumblee leaving foxglove[/caption] Increasing temperatures could affect the foraging activities of bumblebees in a significant way - affecting how much pollen is collected and how much pollination takes place.  If pollen collection is reduced then colony development is affected and so population numbers will be affected.  Bumblebees are key pollinators in natural and agricultural systems, and if their numbers decline there will be ecological and agricultural consequences.  

The decline in many insect populations across the globe is worrying, threatening economies and ecosystems.  A German study in 2017 indicated that the mass of flying insects (in various natural areas) had fallen by some 70%+.  The decline in insect populations has been associated with habitat fragmentation, the spread of agriculture and the use of pesticides, with the neonicotinoids being particularly associated with damage to bee and bumblebee populations. Recent work at the University of Konstanz suggests that when bumblebee colonies are exposed to limited resources of nectar and exposure to the herbicide - glyphosate,  then their colonies may fail.  Bumblebee colonies need a good supply of nectar as a ‘fuel’ in order to maintain a constant brood temperature (of approximately 32oC).  Only at this sort of temperature does the eggs & larvae develop quickly from egg to adult, and the colony grow from a single queen to several hundred bees.  If the temperature is not maintained, then the brood develops slowly or not at all.  The loss of wild flowers (and their nectar) plus the use of the herbicide (in agricultural areas) looks to be a problem for the bumblebees. Just as bumblebees are facing problems, so are honey bees.  The bees have faced infections with a variety of viruses, such as the deformed wing virus.  This virus affects wing development so that the wings are 'stubby' and useless, plus they may be deformities of the abdomen and leg paralysis;  the insect cannot function and dies.  The virus is transmitted by the Varroa mite - a parasite (that also feeds on the bees’ tissues).  The virus was originally identified in Japan in 1980’s and is referred to as DWV-A.  However, a new form of the virus (DWV-B) was identified in the Netherlands in 2001 and it is spreading across Europe, and to other continents.  Sadly, this variant of the virus kills bees faster and is more easily transmitted (according to research at the Martin Luther University).

If bumblebees are exposed to heat stress during their development (they go through four stages : egg / larva / pupa / adult or imago) their bodies develop asymmetries.  The wings in particular are affected, so the left and right wing are shaped differently.  This asymmetry can be measured and has been used by a team from the Natural History Museum and Imperial College to investigate how changing climate over time has affected bees. They investigated four species of bumblebee [Bombus hortorum, B. lapidarius, B. pascuorum and B. muscorum] in museum collections that dated back to 1900 CE.  The bees were ‘held’ in collections at various museums  [Natural History Museum (London), National Museums Scotland (Edinburgh), Oxford University Museum of Natural History, Tullie House Museum and Art Gallery Trust (Carlisle) and World Museum (Liverpool)].   [caption id="attachment_38920" align="alignleft" width="300"] Landing[/caption] Using digital images of many bees collected at different times over the last 130 years, they measured the asymmetry of their wings.  The data from these measurements were then correlated with information about annual rainfall and mean annual temperature in the year the bee was collected.  It became clear that wing asymmetry was associated with hotter and wetter years; and that each of the bee species displayed greater asymmetry, hence stress, in the second half of the twentieth century.  As hotter and wetter conditions are predicted to become more frequent with climate change, it is probable that bumblebees will experience greater stress, indeed they may be in for a ‘rough time’ as this century progresses. Apart from investigating wing asymmetry, the team used a leg from some of the historical specimens to analyse the DNA / genetic make-up of the bumblebees (B. lapidarius).   With the DNA data from these bees (dating aback over a century),  the Natural History Museum and the Earlham Institute were able to construct a ‘reference genome’ - a standard against which they can see how bee genomes change over time.  This may ultimately reveal how bees are adapting (or not) to a changing climate / environment.

We know that insects (especially, bumblebees, bees, hover flies) are the world’s top pollinators, and we also know from many reports that many insect species are in decline.  Crops such as tomatoes, blueberries, peppers, cocoa, coffee, almonds and cherries are dependent on these pollinators.  Climate change, increasing temperatures and extreme weather events are affecting plants and animals across the world, and it seems that social insects, like bumblebees, are particularly impacted. Research with bumblebee colonies (at Stockholm University) has indicated that if the colonies are exposed to higher temperatures (than normal) then the workers in the colonies were smaller.  This decrease in body size could affect their foraging behaviour and the collection of pollen,  which would mean less food brought back to the colony and reduced pollination of plants. Studies in the United States looked at some 20,000 bees  (bumblebees, leafcutter bees, mason bees etc) along the Rocky Mountains, a region which is vulnerable to climate change.  It was found that the larger bees (particularly bumblebees) and those that built nests with combs were affected most by increases in temperature.  On the plus side, smaller (soil nesting) bees fared better.  Bumblebees would seem to have a lower heat tolerance.  The loss of bigger bees, which generally can fly and forage further may again mean reduction in long distance pollination (which promotes outbreeding in plant populations). One reason why hot or hotter weather affects bumblebees is that it influences the nectar that the bumblebees collect.  The balance of the various micro-organisms (bacteria and yeasts) in the nectar changes.  Whilst bumblebees are attracted to nectar with some microbes in it, a small change in temperature can speed up the metabolism / growth of the microbes so that they use up more of the sugar - with the result that it is less palatable / less nutritious for the bees.  Experiments conducted at the University of California have shown that bees did not ‘like’ the nectar rich in microbes, nor a sterile one - with no microbes at all. There seems to be a 'happy medium' in terms of the composition of the nectar. There seems to be a growing consensus that climate change, increasing temperatures and extreme events are pushing bumblebees (in particular) beyond their physiological limits. [caption id="attachment_38081" align="aligncenter" width="650"] Bumblebee visiting foxglove[/caption]

Silwood Park is part of Imperial College, a postgraduate campus, located some 25 miles west of central London, near Ascot. It is a centre for research and teaching in ecology and allied disciplines. The campus includes areas of wet woodlands, acid grasslands, traditional orchards and parkland. The veteran and ancient trees support an significant number of rare species of insects, lichens and fungi that depend on decaying wood. Silwood is the heart of the wildlife corridors for the surrounding area.  Read more...

The appearance of bumblebees is one of the hallmarks of Spring, and their buzzing can be heard in gardens throughout the summer months.  They are important pollinators in our gardens, but also for some crop species that use buzz pollination (like tomatoes). Like other bees, indeed insects in general,  bumblebees face a number of threats - one of which is a fungal parasite (Nosema bombi). This fungus reduces the lifespan of worker bees in the bumblebee colonies and also the production of new queens; it has been associated with the decline in population numbers of North American bumblebees. Read more...