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. Varroa 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 usually die out within two or three years. Many population of Varroa have 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.