Pollinators provide important ecosystem services that contribute to the growth of floral biodiversity. The rapid decline of both wild and domesticated pollinators is evident in the decrease of plants that rely on services provided by them. One of the major pollinators is the honey bee (Apis spp) and potential drivers that contribute to the alarming decrease in their population are classified into three categories: pest and pathogens, environmental stressors and lack of genetic diversity and vitality. The interaction between these three categories may further enhance the negative effect of each other. Identification of these stressors is imperative for proper implementation of management strategies to resolve negative impacts of decreasing pollinators on plant diversity, ecosystem stability, crop production, food security and human welfare (Potts et al, 2010).
One of the primary pollinators reared for agricultural production is the honeybee. The high production of 96% of animal –pollinated crops is attributed to the honeybee. The pollination services provided by the honeybee also extend to wild floral plants; however, there is no data to accurately support their contribution. Evidences of regional decline of domestic honeybee are primarily derived from the USA and Europe. In the USA honeybee stock experience a 59% loss of colonies between 1947 and 2005 while Europe experience 25% loss of colonies between 1985 and 2005. Recent preliminary studies conducted by the Bee Informed Partnership with the University of Maryland showed 37.7% reduction of managed honeybee population between October 1, 2018, and April 1, 2019 in the USA.
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The overall loss for the entire year (April 1, 2018, to April 1, 2019) is approximated to 40.7%. This has raised concern for agricultural crops and wild plants depending solely on the pollination services of honeybee and their future availability pollination services. Albeit the regional losses, there has been a global increase in honey bee hives by 45% since 1961 which is insufficient to support the ever increasing agricultural crops with high demand for pollination services. The status of the wild and domestic honey bee population on a local-scale is unknown due the lack of coordinated monitoring programs. Hence, the indirect evidence of decline in bee population is deduced by the correlation of the observed stressors or factors found to have an impact on the decline of managed population. The identification of these potential drivers of bee decline is necessary for guidance in management strategies to enhance the resilience of food security, biodiversity and natural ecosystems (Potts et al, 2010).
The first category of potential stressor for the decline of bee population deals with pest and pathogens of honeybees. Honeybees are social insects that aggregate in colonies which create a favorable environment of warm temperature and high concentration of host that attracts pest /parasite and pathogens. The parasitic mite and pathogen viruses have been shown to be one of the major causes of Colony Collapse Disorder (CCD) in honeybee (Gifford, 2011).
The obligatory ectoparasitic mite Varroa destructor is native in Asia with an affinity for the species A. cerana. This parasite is absent in Australia and New Zealand South Island but recently, they are found in other parts of the world. The infestation of Varroa destructor weakens the colony and thus reduces honey production; however, under severe infestation the bees totally abandon the hive. The reddish brown parasitic mite has a dorsoventral flattened body covered in setae. Majority of the mite life cycle is spent inside the bee colony. The mite clings to the abdomen and directly feeds on the bee by using the chelicerae to pierce the bees’ intersegmental membrane and sucking the haemolymph (blood). During heavy infestation feeding on pupa results in shorten abdomen and distorted wings. Some other damages of mite infestation includes altered behavior when gathering food, glands malfunction and reduce semen production. This leads to the deterioration of the bees’ health that is known as varroasis (Food and Agriculture Organization of the United Nations, 2006).
Varroa destructor infestation is often accompanied by viral infection. The exoskeleton of healthy bees protect from viral infection. However, the penetration of the parasitic mite through the exoskeleton transfers viruses from its salvia and into the bee. Transmission also occurs when the mite feeds on the broods. The resulting viral infection increases the development and severity of varroasis. Viruses that are pathogenic to honeybees include Acute bee paralysis virus (ABPV), Deformed wing virus (DWV), Israel acute paralysis virus (IAPV), Kashmir bee virus (KBV), Sacbrood virus (SBV), Slow bee paralysis virus (SBPV), Chronic bee paralysis virus (CBPV), Lake Sinai virus (LSV), Apis mellifera filamentous virus (AmFV), and Black queen cell virus (BQCV).
The viruses that are often transmitted by V. destructor include DWV, SBPV, ABPV, IAPV and KBV. The threaten A. mellifera population is attributed to the combination of V. destructor and viral pathogens that create double negative impact on the honeybee health that further propels the colony collapse. Reports during the last century claim V. destructor and viral threats on Western honeybee A. mellifera in Europe, USA, New Zealand, Africa, and Asia. The combination of other factors such as unfavorable climatic conditions, toxic level of pesticides and reduced genetic diversity, increase the parasite virulence and susceptibility of host. Failure to treat infestation result in colony collapse and increase transmission to neighboring colonies that creates a chain reaction of widespread infected colonies (Levin et al, 2019).
The second category of potential stressor for the decline of bee population involves environmental stressors. Environmental stressors include climate change, habitat fragmentation, malnutrition and pesticide exposure (Potts et al, 2010).
Climate change is referred to the shift in the weather pattern that influences the overall climatic condition in terms of temperature, humidity and rainfall. The average global temperature is increasing due to the greenhouse effect created by increasing greenhouse gases which known as global warming. High level of greenhouse gases may be originated from natural events (volcanic eruption, melting of permafrost and methane from cattle) or human activities (Sivaramanan, 2015).
Among many direct impacts (change in temporal activities, reduced population genetics, phenology alterations) of climatic shift one obvious is narrow climatic niches that forces local extinction (Potts et al, 2010).An example of local extinction of bees in warmer climate can be seen in a study conducted by Northwestern University and the Chicago Botanic Garden, where the temperature of mason bee nest was increased to predicted high temperature that resulted in 35% deaths in the first year and 70% death in the second year whereas 1% and 2% percent death was found in the controlled group (Morris, 2018).
Habitat availability and bee diversity have a direct correlation where habitat loss has a direct influence on the health of honeybee. Habitat fragmentation occurs as a result of land use for agricultural and urban development. Habitat fragmentation creates a lack of nesting grounds for local and migratory species result in more energy needed for distant foraging. The availability of diverse food that is needed to meet the nutrimental requirement for healthy honeybee is reduced causing the honeybees to become susceptible to other stressors (New York State Pollinator Protection Plan, 2016).
Healthy honeybees require diverse quality pollen and nectar for growth and development, especially during the immature stage. Studies have shown that the severity of parasite and pathogen of bee and the bees’ sensitivity to pesticide is reduced when they consume diverse quality floral nutrient. Habitat loss often results in decreased diversity of flowers and on monoculture farms there are only one type of flowering plant available as food. The deficiency of diverse nutrients is then compensated with artificial substitute feed that lack the same health benefits compare to the food obtained naturally. The bees that are given the synthetic supplements may increase in numbers but they will not be as physiologically robust as those that feed on diverse natural pollen (Gifford, 2011).
Beekeepers uses pesticide (miticides) to control mites infestation in honeybee colonies, however, the bees are now indirectly affected by the exposure of pesticide during the parasite infestation. Pesticides are also use to control insect pest on plants. The Neuro-active insecticide neonicotinoids is one of the most lethal chemical that compromises the bees’ immune system and affect its neurological behavior. The pesticide method of application is usually via aerial spray however, a new technique is implemented where the seeds are treated with the chemical prior planting. This allows for the chemical to be incorporated into the seed and as it grows the chemical is distributed throughout the entire plant. Honeybees feeding on the treated plant ingest the pesticide which was intended for crop insect pest (Gifford, 2011).
The third category of potential stressor for the decline of bee population is the lack of genetic diversity and vitality. The genetic bottleneck effect is observed when queens of commercial honeybees are selected to breed other queens intended to start colonies. This decreases the genetic diversity of honeybees which makes them susceptible to parasite and pathogen invasion. High level of genetic variation is necessary in worker bees to have innate behavior to remove diseased brood from the infected hive. Hence, honeybee colonies resilience depend on healthy queens with high genetic variation to increase brood vitality and worker bees with increased fitness that enables them to effectively carry out their hygienic duties within the colony. This prevents the heavy invasion of parasite and pathogen that is the primary cause of colony collapse disorder which leads to honeybee population decline (Gifford, 2011).
In conclusion, the decline of honeybee population threatens the smooth functioning of the ecosystem that relies on the bees’ pollination services. The potential drivers of this decline are attributed to three groups of stressors: parasite and pathogen, environmental stressors and lack of genetic variation. The identification of is crucial for informed decision making concerning management programs to improve the health of bees and currently low population.
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