what's causing the bee decline?
there are numerous issus that contribute to the bee decline, but the most important one is industrial agriculture and the use of pesticides, followed by climate change and diseases and parasites.
Industrial agriculture and pesticides
intensification of agriculture
Intensification of agriculture causes the loss and fragmentation of habitats for pollinators, such as agroforestry systems, grasslands, old fields, shrublands, forests, and hedgerows. This is thought to be the major cause of wild pollinator declines.
Monocultures and other agricultural habits
Monoculture is the cultivation of one crop in an area. Industrial monocultures and the lack of plant biodiversity within and around croplands, limit the amount of food that pollinators have access to. Practices such as tillage, irrigation, and the removal of woody vegetation, also destroy nesting sites of pollinators. A parallel decline in plant diversity at the local scale with the decline in bees and other pollinators has been shown both in the UK and the Netherlands.
pesticides​
Large-scale herbicide application drastically reduces noncrop plant diversity and abundance, which limits food availability for bees at any given moment. The chemical destruction of habitats through the massive application of herbicides can have long term consequences, particularly on the distribution of pollinators in agro-environments.
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Neonicotinoids are a systemic agricultural pesticide that does not stay outiside of a plant when applied, but enters the plant's vascular system, incorporating itself into all parts of the crop's tissue. They pass readily into reproductive tissues and interfere with the mutualist, as well as antagonist (insects).
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They pose as a significant threat, as honeybees exposed to neonicotinoid-laced nectar have exibited signs of decline in foraging success, reduced offspirng production and even direct death. Bee-collected pollen can contain high levels of multiple pesticide residues. Pollen is the main protein source for honeybees, and it plays a crucial role in bee nutrition and colony health.
Climate Change
Climate change is affecting pollination by disrupting the synchronised timing of flowers blooming and the timing at which bees pollinate. Flowers are blooming earlier in the growing season due to rising temperatures, before many bees have a chance at pollinating the plants, causing what is known as a 'season creep' Thus, when bees finally begin pollination there is limited nectar available and competition for these valuable resources becomes more intense.
Diseases and Parasites
Varroa mites
Varroa mites are external, obligate parasites of worker and drone honey bees. Although Varroa mites can feed and live on adult honey bees, they mainly feed and reproduce on larvae and pupae in the developing brood, causing malformation and weakening of honey bees as well as transmitting numerous viruses.
Heavy Varroa mite infestations can cause scattered brood, crippled and crawling honey bees, impaired flight performance, a lower rate of return to the colony after foraging, a reduced lifespan and a significantly reduced weight of worker bees. Colony symptoms (parasitic mite syndrome), include an abnormal brood pattern, sunken and chewed cappings and larvae slumped in the bottom or side of the cell. This ultimately causes a reduction in the honey bee population, supersedure of queen bees and eventual colony breakdown and death.
Other diseases
While varroa mite is the leading biosecurity threat, honey bees may be affected by a range of pests and diseases including Nosema Ceranae, Tropilaelaps mite, Braula fly, American foulbrook, European foulbrood, Small hive beetle and Stonebrood.