Why are bees in decline?

Bees – Apis mellifera – play a critically important role in human society. They sustain food production – the Food and Agriculture Organization of the United Nations (FAO) estimates that of the 100 crop species providing 90% of food worldwide, 71 are pollinated by bees – and biodiversity by providing essential pollination for a wide range of crops and wild plants.

As well as their crucial enabling role as pollinators, bees – particularly honeybees – contribute to human wealth and wellbeing directly through the production of honey and other food and feed supplies such as pollen, wax for food processing, propolis in food technology, and royal jelly as a dietary supplement and ingredient in food.

But honey bees are in trouble. In recent years, beekeepers have reported unusual weakening of numbers and colony losses, particularly in North America and western Europe.

No single cause of declining bee numbers has been identified. However, several contributing factors have been put forward. These fall broadly into three groups:

Biological stressors
Chemical stressors
  • Insecticides, including pyrethroids and systemic neonicotinoids such as imidacloprid, thiamethoxam and clothianidin. Insects can be poisoned by spray, dust and through consumption of pollen and nectar.
  • Fungicides such as chlorothalonil, which is used widely in the US on peanuts, potatoes, tomatoes and other crops.
  • Antibiotics including oxytetracycline, which are used to combat diseases such as American Foulbrood.
  • Industrial pollutants can also have effects on the behaviour and health of bees. For example, manganese – a heavy metal commonly used in the production of steel and batteries – can accumulate in the nectar of plants where it is consumed by bees. One study suggests that manganese consumption has a negative impact on bees’ foraging abilities.
Environmental pressure
  • Changes in agricultural methods – particularly the prevalence of monocultures, the practice of growing genetically similar, or identical, plants over a large area, year after year – have destroyed bees’ habitats and floral diversity, thus reducing the food supply.
  • Beekeeping practices such as long-distance transporting for seasonal pollination. 
  • Climate change is also affecting the availability of food and suitable habitats. Global warming is thought to be reducing the range of some species, and extreme events and seasons may be contributing to high mortality rates in hives.

Bees in decline

Biological stressors
MORE
Chemical stressors
  • Insecticides, including pyrethroids and systemic neonicotinoids such as imidacloprid, thiamethoxam and clothianidin. Insects can be poisoned by spray, dust and through consumption of pollen and nectar.
  • Fungicides such as chlorothalonil, which is used widely in the US on peanuts, potatoes, tomatoes and other crops.
  • Antibiotics including oxytetracycline, which are used to combat diseases such as American Foulbrood.
  • Industrial pollutants can also have effects on the behaviour and health of bees. For example, manganese – a heavy metal commonly used in the production of steel and batteries – can accumulate in the nectar of plants where it is consumed by bees. One study suggests that manganese consumption has a negative impact on bees’ foraging abilities.
MORE
Environmental pressure
  • Changes in agricultural methods – particularly the prevalence of monocultures, the practice of growing genetically similar, or identical, plants over a large area, year after year – have destroyed bees’ habitats and floral diversity, thus reducing the food supply.
  • Beekeeping practices such as long-distance transporting for seasonal pollination. 
  • Climate change is also affecting the availability of food and suitable habitats. Global warming is thought to be reducing the range of some species, and extreme events and seasons may be contributing to high mortality rates in hives.
MORE

Stressors Page

There is growing evidence that the weakening or death of bee colonies is caused not just by one-off sudden attacks by a single stressor, but by the combined effects of a number of stressors. Moreover, stressors acting together can have far more damaging effects on bee numbers than when they act alone.

Such interactions can occur principally between:

Biological agents. For example, Varroa destructor, the most common bee parasite, can act as a vector for a number of viruses. It can also affect the bee’s immune system and make it susceptible to other infections. Another parasite, the Nosema fungus, has also been shown to interact with viruses in stressed colonies.

Chemical agents. Synergistic effects have been observed between fungicides and insecticides used to control Varroa. There is also evidence that antibiotics used in bee hives may make bees more susceptible to insecticides.

Biological and chemical agents. In a US study, bees exposed to the neonicotinoid pesticide imidacloprid sufered higher levels of the Nosema parasite than bees that were not exposed to the pesticide. Another study showed that exposure to neonicotinoids promotes replication of viruses such as deformed wing virus and black queen cell virus.

All of these interactions can be additionally influenced by environmental factors such as beekeeping and agricultural practices, and changes in weather conditions.

Overviews of studies looking at the interactions between multiple stressors in bees can be found in Interaction between pesticides and other factors in effects on bees (H. Thompson, 2012) and The impact of co-exposure by colonies to pesticides and infectious agents (ANSES, 2015)

Bees under attack

Interactive infographic

From mites and beetles to  pesticides and antibiotics – not forgetting environmental pressures such as climate variation and changes in agricultural practices – honeybees face an array of increasingly formidable threats to their existence.

Where have these threats come from, and why now? What can humans do to ease the strain on our pollinators? Scroll through our interactive infographic to find out more about the plight of the honeybee.

Find out more