Pollinators: decline in numbers

There is evidence that populations of bees and other pollinators are less healthy and abundant than they have been. If action is not taken, pollinator declines will have serious implications for biodiversity, for food production and the ornamental garden.

A bumblebee (Bombus jonellus) looking for food on an aster. Credit: RHS/Entomology.

Quick facts

Common names Almost any insect that visits flowers including bumblebees; solitary bees; wasps; hoverflies and other flies; moths and butterflies; some beetles
Scientific names names  Bombus spp; various solitary bee species; Syrphidae; Diptera; Lepidoptera; Coleoptera
Main causes Loss of habitat for nesting and foraging, climate change

What sorts of pollinators are there?

Almost any insect that visits flowers can carry out pollination, and therefore classed as a ‘pollinator. Very conservative estimates suggest we have over 1500 pollinating insect species in Britain, thought the true figure is likely to be much higher. This includes bees, social and solitary wasps, adult parasitoid wasps, hoverflies and other flies, butterflies and moths and some beetles. Groups often seen on flowers are bees, flies, social, solitary and parasitoid wasps and beetles. Occasionally other insects such as lacewings and some true bugs may also be found on flowers.

Bees

Bees can be categorised into three broad groups;

  • Bumblebees (Bombus spp.) are social bees. There are about 24 bumblebee species in Britain about 12 are commonly seen in gardens. At peak strength in midsummer, a bumblebee nest may contain between 50 and 400 bees, the nests last one season and usually queens overwinter.
  • There more than 260 species of solitary bee in Britain, some of which are rare species confined to certain types of habitat. Common types of garden solitary bees include some of the AndrenaOsmiaMegachileLasioglossum and Nomada species. Each female of these bees constructs and provisions a nest on her own and they have annual lifecycles. Many females of the same species often nest in close proximity to one another.
  • The honeybee (Apis mellifera) is a social bee that forms large colonies that overwinter. It can be kept in hives and is the source of honey and beeswax. A strong honeybee colony may contain about 60,000 bees.

Social, solitary and parasitoid wasps

There are seven species of social wasps (including the hornet), and over 250 species of solitary wasp in Britain, whilst they are primarily predatory feeding their grubs on other insects the adults of many species will also visit flowers. It is estimated that there are more than 6,000 species of parasitoid wasp in Britain, most of these lay eggs inside other insects where the larvae feed eventually killing the host (parasites do not usually kill a host, hence the name parasitoid). Many parasitoid wasps will visit flowers.

Flies

There are more than 6,500 species of true fly (Diptera) in Britain many will visit flowers as adults. The often colourful hoverflies (Syrphidae) are one of the more familiar groups of fly that visit flowers, there are approximately 270 species found in Britain. The adults of many species mimic bees or wasps but none possess a sting. Many species visit flowers and they are important pollinators. The larva of hoverflies have different habits depending on the species, many such as the marmalade hoverfly, Episyrphus balteatus are predatory on aphids  others have larvae that feed on decomposing organic matter such as the rat tailed maggots, Eristalis species (adults are known as drone flies), others such as the largest British hoverfly the hornet mimic Volucella zonaria are semi-parasitic in wasp and bee nests. A handful of species feed on living plant material including the bulb flies Merodon equestris, Eumerus strigatus and E. funeralis.

Butterflies and moths

There are more than 2,500 species of Lepidoptera in Britain, less than 60 of these are butterflies. Some butterflies and many moths will visit garden flowers. For more information visit Moths in your garden and Butterflies in your garden.

Beetles

There are approximately 4,500 species of beetle in Britain, they have a very wide range of habits as adults and larvae, some are often found on flowers as adults. These include including soldier beetles, which are predatory as larvae, the thick legged flower beetle which feeds in plants stems as a larvae, the colourful rose chafer which feeds on decomposing organic matter as a larva and longhorn beetles most of which are found in dead wood as larvae.

Additional information about other pollinator groups can be found in the supporting document for the National Pollinator Strategy.

What is the problem?

It is generally accepted that pollinators are in decline. However, whilst the distribution of some species of pollinator has become more restricted the extent of the declines in overall pollinator abundance are largely unquantified. The UK PoMS (Pollinator Monitoring Scheme) aims to better understand how insect pollinator populations are changing across Great Britain.

What we know:

  • Some bumblebee and solitary bee species are doing well and have increased their distribution in Britain. Others have shown marked declines in distribution over the last 30 years
  • Bumblebees and solitary bees that are able to collect nectar and pollen from a wide range of plants, including garden flowers, are thought to be maintaining their numbers and distribution
  • It is species that are more selective in their flower-visiting habits, or have special requirements for nest sites, that have declined and now have a more restricted distribution
  • Many species of moth and butterfly are in decline although this is thought to be largely due to habitat loss due to changes in land use. Less is known about the distribution and abundance of other pollinators such as hoverflies.
  • Many garden plants and agricultural/horticultural crops need bees to bring about pollination by transferring pollen from the flowers’ anthers to the stigmas. These include most tree and soft fruits, and many vegetables including runner beans, broad beans, tomatoes, marrows and courgettes
  • Managed honeybee populations are not under threat and the number of maintained hives has increased in recent years. There are however some problems that have made it more difficult for beekeepers to maintain their hives in good condition. In Europe (including the UK), however,  colony collapse - that has been observed in north America - has not yet occurred

Why are pollinators in decline?

There is no one simple answer, the problems facing pollinators and other invertebrates are complex but can be summarised into several broad areas.

All pollinators

Habitat loss and land use change: The main problem affecting most pollinators is thought to be the loss of suitable habitat. Including forage and nesting or breeding sites. 

Forage: The amount and quality of flowering resources have declined. Modern farming practices have reduced the amount of flowers on farmed land and traditionally managed flower-rich meadows, are now rare. This is likely to have affected the specialist pollinators including some bumblebees and solitary bees that collect nectar and pollen from a restricted range of plants more than generalist pollinators. Where suitable forage remains, its availability is often fragmented, making it more difficult for bee populations to expand and colonise new areas. In urban areas bumblebee solitary bees and other pollinators that feed on a wide range of plants can do well in gardens.

Nest and breeding sites: Bumblebees and solitary bees have specific requirements for nest sites. Many other pollinators such as hoverflies, butterflies and moth have specific habitat requirements for their larvae. The loss and fragmentation of suitable habitats reduces nesting and breeding opportunities. Gardens can provide suitable breeding places for many pollinators.

Climate change: It is thought that climate change is changing and will change the suitable geographic ranges of many species, this in combination with habitat loss means that some species will decline.

Pesticides: Pesticides, especially insecticides, are often blamed directly for bee and other pollinator losses. Pesticides, including weed killers can also remove potential food plants and prey species for those pollinators that have herbivorous or predatory larvae. Particular concern has been raised about some neonicotinoid insecticides (especially clothianidin, imidacloprid and thiamethoxam). These systemic insecticides were used by farmers and gardeners to control a wide range of invertebrates. Attention has been focused on this group of insecticides because minute quantities of these systemic chemicals get into sap, nectar and pollen of treated plants. In addition, some research has shown harmful, often sub-lethal effects on the foraging ability of honeybees and the colony size of bumblebees.  Following these concerns approval for most uses of the three active ingredients listed was withdrawn. Imidacloprid and thiamethoxam may still be used by professional growers on plants grown in a fully enclosed greenhouse environment. Two other neonicotinoids, both considered of lower bee toxicity, remain available to professional growers; acetamiprid and thiacloprid. Only acetamiprid is available to home gardeners as it is currently considered less toxic to bees and other pollinators.

Remember: plants in flower should not be sprayed with insecticide due to the danger to pollinating insects.

Honeybees

Whilst managed hives are not in decline several factors are causing concern for beekeepers;

  • Varroa destructorThis is a parasitic mite that sucks bee blood (haemolymph) from the bodies of honeybee larvae, pupae and adults. Varroa destructor is a natural parasite of a South East Asian honeybee, Apis cerana, and appears to cause little harm to this host. When the European honeybee (Apis mellifera) was taken to South East Asia, it picked up the mite with disastrous consequences. The mite has since spread round the world, Australia is the only major beekeeping country without the mite. Varroa was first detected in Britain in 1992 and is now widespread. Unless beekeepers take steps to control Varroa, infested colonies usually die out within three years.

  • Diseases: Honeybees and their larvae are affected by many diseases caused by bacteria, fungi and viruses. Research has shown a link between certain bee viruses, Varroa and colony decline. Varroa can transmit some viruses within the colony. Some mite-transmitted viruses, such as deformed wing virus, acute paralysis virus, slow paralysis virus and cloudy wing virus, weaken honeybee colonies by reducing the longevity of adult honeybees. Adult honeybees that develop in late summer will normally overwinter in the hive and survive until the spring but those weakened by viruses die prematurely. A colony that appears strong in late summer can die out or become greatly weakened over winter

  • Neglect by the beekeeper: Honeybees today need more care and management because of the need to prevent damaging levels of Varroa mites building up. “Leave alone” beekeepers are more likely to lose their bees. In early autumn, it is important to ensure honeybees have enough honey in their hives to keep them going until nectar becomes available again in the spring.

Get involved

There are several projects monitoring the status of pollinators that gardeners can get involved in: 

UK Pollinator monitoring scheme (PoMS): Gathering information on the status of pollinators is vital in efforts to understand what is happening to their populations and to conserve them. Run by the Centre for Ecology & Hydrology (CEH) the pollinator monitoring scheme aims to establish how pollinator populations are changing in Britain. Their 10-minute Flower Insect Timed Count can be carried out anywhere, including parks and gardens.

BeeWalk: Is a project run by Bumblebee Conservation Trust it is a standardised bumblebee-monitoring scheme which involves volunteer ‘BeeWalkers’ walking the same fixed route once a month between March and October, counting and identifying any bumblebees seen.

Garden Butterfly Survey and Big Butterfly Count: Run by Butterfly Conservation these surveys record the butterflies in gardens the wider environment.

Click here for more information on how gardeners can help our declining bees and other pollinators.

National Pollinator Strategies

The UK and Irish Governments have recognised the decline in pollinators launching national pollinators plans:

The strategies set out a number of actions and goals which aim to answer some of the questions surrounding the amount of decline and the causes of pollinators.

As a stakeholder the RHS is a member of the Pollinator Advisory Steering Group of the National Pollinator Strategy (England) which informed the strategy and is committed to support its implementation. Part of the strategy encourages gardeners to choose plants that provide resources for pollinators and endorses the RHS Plants for Pollinators logo.


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