The survival of bees, including managed honeybees and wild native species, is a matter of global ecological and agricultural concern due to their role as pollinators. The decline in bee populations is not attributable to a single cause, but rather a complex entanglement of interacting threats that weaken the insects and their colonies. Understanding the specific factors contributing to bee mortality requires looking beyond immediate deaths to the chronic stressors that compromise the insects’ health. These stressors collectively contribute to annual colony losses experienced by beekeepers and the diminishing numbers of wild pollinators.
Biological Threats: Parasites and Pathogens
The most significant biological threat to managed honeybee colonies is the ectoparasitic mite Varroa destructor, considered the greatest driver of honeybee health decline globally. These mites primarily consume the fat body tissue, which is functionally similar to the mammalian liver. This feeding damages an organ responsible for immune function, detoxification, and nutrient storage, leaving the bee severely compromised.
The mite also acts as a vector for various viruses, most notably the Deformed Wing Virus (DWV), which causes symptoms like shriveled wings in newly emerged bees. The physical injury from the mite’s feeding allows the virus to bypass the bee’s natural defenses, leading to high viral loads that overwhelm the colony. When Varroa infestation levels exceed a threshold, the resulting viral replication can trigger the collapse of the entire colony, particularly during the overwintering period.
Fungal pathogens like Nosema apis and Nosema ceranae also contribute to mortality by infecting the digestive systems of adult bees. When bees ingest spores, the fungus invades the cells of the mid-gut, absorbing nutrients and impairing digestion. This gut damage shortens the lifespan of individual adult bees and can cause nurse bees to lose their ability to produce royal jelly, which is fed to the developing brood.
The cumulative effect of Nosema infection reduces a colony’s strength, leading to fewer foragers and lower honey production. Nosema ceranae is concerning because its infection can lead to a rapid decline, sometimes causing colony death in just a few days as foraging bees are too weak to return. The combination of parasitic feeding damage and internal pathogen invasion creates a severe biological burden that managed colonies struggle to survive.
Chemical Exposure from Pesticides
The widespread use of chemical inputs in agriculture introduces toxins into the bees’ environment, affecting their health even at doses that do not cause immediate death. Insecticides, particularly the systemic class known as neonicotinoids, are neurotoxic compounds that interfere with the insect nervous system. These chemicals are absorbed into the plant and can be present in pollen, nectar, and water droplets, exposing bees during foraging.
Sublethal exposure to neonicotinoids impairs a bee’s cognitive functions, leading to reduced foraging ability, disorientation, and difficulty navigating back to the hive. This neurotoxicity also suppresses the bees’ immune system, making them more vulnerable to parasites and pathogens. For example, neonicotinoid-treated bees show a greater susceptibility to bacterial and fungal infections, including Nosema ceranae.
Complicating the issue is the synergistic effect of combining insecticides with other agrochemicals, such as fungicides and herbicides. Although fungicides are not designed to kill insects, many compromise the bee’s detoxification system by inhibiting crucial enzymes. When a bee ingests both a neonicotinoid and a fungicide like propiconazole, the fungicide effectively blocks the bee’s ability to process the insecticide. This can increase the toxicity of the insecticide hundreds of times. This interaction highlights how the co-occurrence of common agricultural chemicals creates a toxic cocktail that heightens the risk of mortality for both honeybees and wild bee species.
Ecological Pressure: Nutritional Deficiencies and Habitat Loss
The modern agricultural landscape presents challenges to bee nutrition and habitat availability, leading to chronic weakness. Monoculture farming, the practice of planting a single crop over a large area, is a primary contributor to nutritional deficiencies in bees. While a blooming monoculture crop may offer an abundance of pollen and nectar for a short time, it lacks the necessary variety of nutrients that a diverse diet provides.
Bees require a mix of pollens from different plant species to obtain the proteins, lipids, and micronutrients necessary for a strong immune system and healthy development. A limited diet from a single crop, such as corn or soy, results in malnutrition and a weakened ability to fight off disease. This poor nutrition makes bees more susceptible to the effects of pathogens and pesticides, linking ecological pressure to the biological and chemical threats.
Habitat loss and fragmentation further exacerbate the nutritional crisis by destroying the natural forage areas and nesting sites wild bees rely on. The removal of hedgerows, wildflowers, and other non-crop vegetation eliminates the diverse, season-long supply of food that bees need to sustain their colonies. The short bloom times of many monoculture crops mean that once the bloom is over, bees are left with vast areas offering no food, forcing them to starve or travel long distances for resources.
Systemic Stressors and Climate Impacts
Large-scale environmental and human-driven factors place systemic stress on bee populations, lowering their resilience to the more direct threats. Climate change introduces unpredictable weather patterns, such as prolonged droughts and extreme heat, which directly impact bee physiology and food sources. Drought conditions can cause floral nectar and pollen production to dry up, leading to starvation in both managed and wild colonies.
Warmer temperatures also disrupt the seasonal timing between the emergence of bees and the blooming of flowers, a phenomenon known as phenological mismatch. If flowers bloom weeks earlier due to warmer springs, the bees may not be ready to forage when the food is available, resulting in a period of nutritional deficit. Extreme weather events, including heavy rain and intense heat waves, can also limit the hours bees can forage, reducing their ability to collect necessary resources.
Compounding these environmental stressors are the demanding practices of commercial beekeeping, particularly the long-distance transportation of honeybee colonies for crop pollination. Managed colonies are routinely trucked across the country to pollinate crops like almonds, subjecting them to intense physical stress and rapid environmental changes. This constant movement and overwork weaken the bees, reducing their disease resistance and longevity. The concentration of hives in pollination staging areas also facilitates the rapid spread of parasites and pathogens between colonies, increasing the risk of collapse.