What is causing pollinator decline?

Almost any insect that visits flowers can carry out and is, therefore, classed as a ‘pollinator. Conservative estimates suggest we have over 1500 pollinating insect species in Britain, though the true figure is likely to be much higher. Insects often seen on flowers are bees, flies, wasps and beetles. Occasionally other insects such as lacewings and some true bugs may also be found on flowers.

It is generally accepted that pollinators are in decline. However, the extent of the decline is 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 can collect nectar and pollen from a wide range of plants, including garden flowers, which 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 crops need pollinating insects to bring about pollination. These include most tree and soft fruits, and many vegetables including runner beans, broad beans, tomatoes, marrows, and courgettes.
• Managed honeybee populations in maintained hives have increased in recent years. There are, however, some problems across Europe, including colony collapse and varroa mites.

Why are pollinators in decline?
There is no one simple answer, but the problems can be summarised into several broad areas. 

Habitat loss and land use change: The main problem affecting most pollinators is the loss of suitable habitat. Including forage and nesting or breeding sites. Another impact of growing urban environments is the levels of light pollution – and the impact this has on flying insects.

Forage: The amount and quality of flowering resources have declined. Modern farming practices have reduced the number of flowers on farmed land. Where suitable forage remains, its availability is often fragmented, making it more difficult for bee populations to expand and colonise new areas.

Nest and breeding sites: Bumblebees and solitary bees have specific requirements for nest sites and other pollinators such as hoverflies, butterflies and moths have specific habitat requirements for their larvae. The loss and fragmentation of suitable habitats reduce nesting and breeding opportunities.

Climate change: Climate change is affecting the geographic ranges which are suitable for many species, this in combination with habitat loss means that many species are in decline. Ranges of some bumblebees, butterflies and moths are known to be moving northwards, a lack of suitable habitat in the new ranges puts populations of these species at risk.

Pesticides: Pesticides, especially insecticides, are often blamed directly for bee and other pollinator losses. To support pollinators and other wildlife in gardens their use should be avoided. Pesticides, including weedkillers, remove potential food plants and prey species for those pollinators that have herbivorous or predatory larvae. Whilst most pesticides can affect a wide range of organisms particular concern has been raised about some neonicotinoid insecticides (especially clothianidin, imidacloprid and thiamethoxam). These systemic insecticides are 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.

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

Varroa destructor: This 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. 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 around 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.

Invasive Species: Some species, such as Carpenter bees, that are not naturally found in the British Isles can become established here without posing any problems. Others can be disruptive – or worse -to native species; for example, the Asian Hornet, which recently arrived in the UK could devastate British bee species if it took hold and the small hive beetle could damage Honeybee and Bumblebee colonies if it arrived here.

These individual causes are bad enough for bees, and there is evidence to show that they work in combination, weakening bees and other pollinating insects. A hungry bee exposed to pesticides and poor weather and afflicted with pests and diseases is unlikely to survive for long. However, the interactions between these effects are difficult to untangle. Scientists are continuing to carry out much-needed research into the impacts of all these factors together on bee populations.

If you are interested in learning more about how you can help, please contact us at info@pollinatorpioneers.org.uk.