Pollinators: decline in numbers
There is evidence that bees and other pollinator populations are less healthy and abundant than they have been. If action is not taken, pollinator declines will have serious implications for food production and the ornamental garden, since many plants rely on bees and other insects, such as hoverflies to transfer pollen from one flower to another in order to set fruits and seeds.
What sorts of pollinators are there?
Almost any insect that visits flowers can carry out pollination, over 1500 insect species are thought to carry out pollination services in the UK. This includes bees, hoverflies and other flies, butterflies and moths and some beetles.
Bees can be categorised into three broad groups;
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
Bumblebees (Bombus spp.) are also social bees but their nests die out in late summer or early autumn. There are about 24 bumblebee species in Britain but only about 12 are commonly seen in gardens. At peak strength in midsummer, a bumblebee nest may contain up to 200 bees
There are about 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 Andrena, Osmia, Megachile, Lasioglossum 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.
Hoverflies and other flies Hoverflies are a family of true flies (Diptera) there are approximately 270 species found in the UK and there are more than 6 000 species of fly. Any true fly that visits flowers can carry out pollination, but it is the colourful hoverflies that are often the most noticeable. The adults of many species mimic bees or wasps but none posses a sting. Many species visit flowers and they are thought to be important pollinators. The larva of hoverflies have different habits depending on the species, some are predatory on aphids such as the marmalade hoverfly, Episyrphus balteatus, others have larvae that feed on decomposing organic matter such as the rat tailed maggots, Eristalis species. Three species can be pest problems as larvae, the bulb flies Merodon equestris, Eumerus strigatus and E. funeralis.
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 and there are a number of problems facing honeybees the extent of the declines in overall pollinator abundance are largely unquantified.
What is known:
The strength and health of honeybee colonies has declined, making it more difficult for beekeepers to maintain their hives in good condition. In Europe (including the UK), however, extensive colony collapse - that has been observed in north America - has not yet occurredSome 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 distributionIt 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 distributionMany 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 courgettesPlants that are not pollinated will not set fruits or produce seeds.
Why are pollinators in decline?
There is no one simple answer and the problems facing the honeybee are different to those affecting bumblebees, solitary bees and other pollinators.
Several factors have been identified as probable contributory causes specifically of honeybee decline.
Varroa destructor: This is a parasitic mite that sucks bee blood (haemolymph) from the bodies of honeybee larvae, pupae and adult bees. Varroa destructor evolved as a parasite of a South East Asian honeybee, Apis cerana, and appears to cause little harm to its natural 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 not yet infested. Varroawas first detected in Britain in 1992 and now infests bee hives throughout Britain and Ireland. Unless beekeepers take steps to control Varroa, infested colonies usually die out within two or three years. Many population of Varroahave gained resistance to the pesticide strips used to control the it. The current alternative treatments are less effective.
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. The mite can transmit some viruses within the colony and increase the rate of infection compared to hives that are mite-free. 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. This affects their efficacy as pollinators and nectar gatherers. Adult honeybees that develop in late summer will normally overwinter in the hive and survive until the spring. Those weakened by viruses die prematurely. A colony that appears strong in late summer can die out over winter or is reduced by the spring to a greatly weakened and non-viable colony.
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 will 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. Hives that have insufficient honey must be fed with sugar solution to top up their stores. For the reasons above feral (unmanaged) honeybee colonies usually die out after only a year or two.
Bumblebees, solitary bees and other pollinators
Bumblebees and solitary bees are not attacked by Varroa mite. The main problems affecting them and other pollinators are thought to be the loss of suitable habitat. This affects them in two ways.
Forage: specialist pollinators including some bumblebees and solitary bees collect nectar and pollen from a restricted range of plants. These are often wild flowers, so many garden plants are of no benefit to them. Traditionally managed flower-rich meadows, are now a rare feature of the British landscape and this may be a contributory factor in the decline of some bumblebee and solitary bee species. Where suitable habitat remains, it is often fragmented, making it more difficult for bee populations to expand and colonise new areas. Those bumble bee and solitary bees that feed on a wide range of plants can do well in gardens.
Nest and breeding sites: some 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.
Pesticides: Pesticides, especially insecticides, are often blamed for bee and other pollinator losses. The instructions on the packaging usually state “Dangerous to bees”. This is because if the chemical is sprayed directly on to bees they are likely to be harmed. All pesticides are intensively researched before approval is given for their sale and use. This includes the pesticides’ effects on the environment and some beneficial insects. The effects on honeybees are assessed both inside the hives and on bees while they are foraging for nectar and pollen. When used according to the manufacturer’s instructions, and by not spraying open flowers, the risk to bees can be reduced.
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 pests. 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 several bee poisoning incidents with these neonicotinoids have occurred abroad as a result of incorrect application by farmers and some research has shown harmful, often sub-lethal effects on the foraging ability of honeybees and the colony size of bumblebees. However, other research showed no clear evidence of harm being caused to bees when the chemicals are applied correctly. Due to the potential impact of these neonicotinoids, in April 2013 the European Commission restricted their use for at least two years, including the withdrawal of all products containing imidacloprid and thiamethoxam available to home gardeners. This withdrawal (in effect a ban) came into force on 30 September 2013. It is now illegal to use them.
In April 2018 it was decided that the ban would become permanent and extended to include plants that are either not pollinated by bees such as cereal crops, which are wind pollinated, or ones that are harvested before they flower such as sugar beet in outside spaces. This withdrawal of approval will become effective before the end of 2018. Imidacloprid and thiamethoxam may still be used by professional growers on plants grown in a fully enclosed greenhouse environment with stringent conditions that prohibit treated plants being put outside until they have finished flowering, nor planted in the soil until nine months have elapsed after growing medium treatment. Two other neoniotinoids, both considered of lower bee toxicity, remain available to professional growers; acetamiprid and thiacloprid. Only acetamiprid is available to home gardeners. It remains legal to the neonicotinoid-based products that are not affected by the withdrawal.
Pesticides, including weed killers can also remove potential food plants and prey species for those pollinators that have herbivorous or predatory larvae.
Remember: plants in flower should not be sprayed with insecticide due to the danger to pollinating insects.
What can I do?
Gardeners can and do help pollinators in a number of ways;
Due to the rich diversity of plants growing in Britain’s gardens, these outside spaces, and the gardeners who tend them, are critical to support a wide range of pollinators. There are often more pollinators in our nation’s gardens than in surrounding agricultural land.
There is though always space for more pollinators in our gardens. Here’s how you can do more to support pollinators in your garden, outside space or community.
Avoid using pesticides wherever possible, never spray open flowers and always read and follow label instructions. Preventing and reducing pests and diseases by good cultivation, cultivar selection, garden hygiene and using biological control should always be the first line of defence. If pesticides are used consider using short persistent organic products. Accepting the presence of some pests can provide larval food for pollinators, for example aphids are food for some hoverfly larvae.
Provide nest sites for bees; either make your own or you can purchase nests for solitary bees. Some will nest in hollow stems, such as bamboo canes or herbaceous plant stems. Hole diameters in the range 2-8mm (up to 1/3in) are required. Cardboard nest tubes can be bought in garden centres. Holes 2-8mm (up to 1/3in) diameter can be drilled in fence posts or logs. Place these nest sites in sunny positions. Some solitary bees nest in the ground, either in bare soil or short turf. They will find their own nest sites, so tolerate the small mounds of soil deposited by the female bees when they excavate their nest tunnels. Purpose built bumblebee nest boxes often go unused. Instead bumblebees often prefer to nest in holes in the ground, for example under hedges with plants along the base. One species, the tree bumblebee often uses bird nest boxes.
Provide water for pollinators. Bees and other pollinators sometimes need to drink; having a shallow margin of a pond or a shallow dish filled with stones or marbles filled with water will provide a safe source of water.