Bees: decline in numbers

What sorts of bees are there?

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 23 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 265 species of solitary bee in Britain, some of which are rare species confined to restricted habitats. Common types of garden solitary bees include some of the Andrena, Osmia, Megachile, Lasioglossum and Nomada species. Solitary bee nests are even smaller and with these non-social bees, each female constructs and provisions her nest on her own

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A honeybee (Apis mellifera) collecting pollen.

A bundle of hollow stems collected from a border can make a useful nest site for some species of solitary bees.

A female solitary bee on a goat willow catkin

A female solitary bee on a goat willow catkin

What is the problem?

There are several problems:

• The strength and health of honeybee colonies has declined, making it more difficult for beekeepers to maintain their hives in good condition
• Some bumblebee and solitary bee species are doing well and have increased their distribution in Britain. Others have shown marked declines 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 generally 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 in numbers and now have a more restricted distribution
• 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
• Plants that are not pollinated will not set fruits or produce seeds

Why are bees in decline?

There is no one simple answer and the problems facing the honeybee are different to those affecting bumblebees and solitary bees.

Honeybees

Several factors have been identified as probable contributory causes of honeybee decline.

• Varroa mite: This is a parasitic mite that sucks bee blood (haemolymph) from the bodies of honeybee larvae, pupae and adult bees. Varroa destructor, to give it its full scientific name, evolved as a parasite of a South East Asian honeybee, Apis cerana, on which it causes little harm. 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 until now, Australia is the only major beekeeping country not yet infested. Varroa destructor was first detected in Britain in 1992 and now infests bee hives throughout Britain and Ireland. Unless beekeepers take steps to control Varroa, infested colonies collapse and die within two or three years. Varroa has gained resistance to the pesticide strips (Bayvarol, Apistan) used to control the mite. The current alternative treatments are less effective and often provide a lower level of control
• 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
• Pesticides: Pesticides, especially insecticides, are often blamed for bee 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 (clothianidin, imidacloprid and thiamethoxam). These systemic insecticides are used by farmers and gardeners to control a wide range of pests. Attention has been focused on this group of insecticides because they came into use during the period when bee decline became apparent and these chemicals can get into sap, nectar and pollen. Several bee poisoning incidents with these neonicotinoids have occurred abroad as a result of incorrect application by farmers and some research has shown harmful 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 two years, including the withdrawal of all products containing imidacloprid and thiamethoxam available to amateur gardeners. This withdrawal (in effect a ban) comes into force on 30 September 2013, but there is to be a period of grace to use up these materials by 30 November 2013. After this time it will be illegal to use them. It remains legal to use other neonicotinoid-based products that are not affected by the withdrawal. Further research will be carried out to assess the withdrawn neonicotinoids effects on bees and it is possible that in two years time they will return to the market.  

Bumblebees and solitary bees

Bumblebees and solitary bees are not attacked by Varroa mite. The main problems affecting them are loss of suitable habitat.This affects them in two ways.

• Forage: some bumblebees and solitary bees collect nectar and pollen from a restricted range of plants. These are usually wild flowers, so garden plants are of no benefit to them. Traditionally managed flower-rich meadows, are now a rare feature of the British landscape and this appears to 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
• Nest sites: some bumblebees and solitary bees have specific requirements for nest sites. The loss and fragmentation of suitable habitats reduces nesting opportunities

What can be done?

Gardeners can help bees in a number of ways;

• Become a beekeeper: Details of county beekeepers associations and training courses can be seen on The British Beekeepers website
• Provide nest sites 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.
• Provide nest sites for bumblebees: Bumblebee nest boxes can be purchased but they are often ignored by queen bumblebees. They prefer to find their own nest sites down tunnels dug by mice or in grass tussocks.  The tree bumblebee, Bombus hypnorum, has recently colonised Britain and will often use bird nest boxes  
• Grow flowers that bees like: Honeybees are active from late winter to autumn, so try and have bee-friendly plants in flower for as much of that time as possible. Use pesticides sparingly. Those based on fatty acids or plant oils and extracts pose little danger to bees but will not control all pests. Avoid spraying open flowers and if possible do spraying in the evening when bees are less active