The survival of bees, both managed and wild, is a global concern due to their impact on food security and ecosystem health. Approximately one-third of the world’s food crops rely on animal pollination, with bees performing the majority of this essential work. Despite their importance, bee populations, particularly managed honey bee colonies in North America and Europe, have experienced significant losses over the past few decades. Understanding the specific factors that kill these pollinators is the first step toward developing effective conservation and management strategies.
Biological Threats: Mites and Disease
One of the most immediate threats to honey bee colonies is the external parasite Varroa destructor mite. These mites attach themselves to adult bees and developing brood, feeding on the bee’s fat body tissue, which severely depletes the bee’s protein and energy reserves. The mite’s presence is also destructive because it acts as a vector for debilitating viruses, most notably Deformed Wing Virus (DWV). The mite injects the virus directly into the bee, bypassing natural defenses and causing rapid replication.
A heavy Varroa infestation leads to Parasitic Mite Syndrome, causing bees to emerge with crippled wings, shortened abdomens, and reduced lifespans. Without intervention, a colony can collapse within two to three years as the mite population grows exponentially and the viral load becomes overwhelming. Mites also compromise the bee’s immune system, making it less capable of fighting off other common infections.
Another major biological threat is the microsporidian parasite Nosema ceranae, which infects the digestive tract of adult bees. This parasite damages the cells lining the midgut, interfering with the bee’s ability to digest pollen and absorb nutrients. The infection reduces the lifespan of adult workers, forcing younger bees to prematurely take on foraging duties, known as precocious foraging.
This disruption in the normal aging process and division of labor causes a severe imbalance in the colony’s population structure. Infected nurse bees lose the ability to produce royal jelly, which is necessary to feed the larvae and the queen. This slows or halts the production of a new workforce, and the steady decline in the adult bee population ultimately leads to colony failure.
A third serious biological killer is American Foulbrood (AFB), a fatal bacterial disease caused by the spore-forming organism Paenibacillus larvae. This disease targets developing honey bee larvae after they ingest spores present in their food. Once inside, the spores germinate and multiply rapidly, consuming the larval tissue and killing the insect after its cell has been sealed by worker bees.
The dead larva decomposes into a dark, gooey mass before drying into a hard scale that is nearly impossible for the bees to remove. Each scale contains millions of highly resistant bacterial spores that can remain viable for decades on hive equipment. Since AFB kills the new generation of bees, the colony workforce cannot be replenished, causing the hive population to dwindle and die out.
Chemical Exposure: The Impact of Pesticides
Pesticides, particularly systemic insecticides, are recognized as a direct cause of bee mortality and sublethal impairment. The neonicotinoid class of insecticides, or “neonics,” are especially concerning because they are absorbed by plants and distributed throughout the entire structure, including the pollen and nectar that bees consume. Bees are thus exposed to the toxin simply by feeding on a treated crop, even if the pesticide was applied as a seed coating months earlier.
The effects of pesticide exposure can be categorized into acute and chronic responses. Acute exposure, often from direct overspray or contact with high residues, results in immediate death, typically of foraging bees. Chronic exposure involves low, sublethal doses consumed over time, which do not kill the bee outright but degrade its health and behavior.
Neonicotinoids act as neurotoxins that interfere with the bee’s central nervous system, causing behavioral and cognitive impairments. Even at low concentrations, they disrupt the bee’s ability to navigate, making it difficult for foraging bees to find their way back to the hive and resulting in a gradual loss of the workforce. Chronic exposure also suppresses the bee’s immune system, making it more vulnerable to mites and diseases.
Sublethal doses of neonicotinoids can also impair the reproductive success of a colony. Exposure can reduce sperm viability and quantity in drones, and affect the egg-laying capacity of the queen. These long-term effects on individual bees translate into a decline in colony health, productivity, and the ability to survive winter.
Environmental Stress: Nutrition and Habitat Decline
The health of a bee colony is dependent on the availability of diverse, high-quality food sources, which are threatened by changes in land use. Modern agricultural practices, particularly monoculture farming, involve planting vast areas with a single crop species. While these crops may provide an abundance of food for a brief period, they offer only a narrow nutritional profile.
Pollen is the bee’s primary source of protein, lipids, vitamins, and minerals; its content varies drastically between plant species. A diet restricted to a single type of pollen, such as from a large almond or corn field, often lacks the complete spectrum of amino acids required for health. This nutritional deficiency results in a weakened immune system.
Poor nutrition compromises the bee’s ability to resist pathogens and detoxify chemicals, lowering its threshold for surviving parasitic or pesticide challenges. This stress is compounded by the destruction of natural habitats adjacent to agricultural lands. This eliminates the diverse wildflowers that normally provide bees with a balanced diet outside of the cash-crop bloom. Habitat fragmentation also reduces nesting sites for native bee species.
Environmental factors like extreme weather events further exacerbate the vulnerability of bees weakened by poor nutrition. Unpredictable high temperatures or prolonged droughts can reduce the quality and quantity of available nectar and pollen, placing additional stress on the colony during growth. When a colony is already struggling with a limited diet, these external stressors can accelerate its decline toward collapse.
The Synergy of Mortality Factors
Modern scientific consensus indicates that bee mortality is rarely the result of a single factor, but rather the cumulative and compounding effect of multiple interacting stressors. The combined effect of two or more threats is greater than the sum of their individual effects. A bee can withstand a certain level of exposure to a single challenge, but the combination of multiple factors proves fatal.
For instance, a bee whose immune system is already suppressed by a poor, monoculture-based diet becomes more susceptible to infection by Nosema ceranae. If that same bee is also exposed to a sublethal dose of a neonicotinoid, its ability to navigate home is impaired, its lifespan is shortened, and its capacity to fight the parasite is diminished.
The interplay between the Varroa mite and viruses is a prime example of this synergy. The mite not only weakens the bee by feeding but also acts as a biological needle to inoculate the bee with a virus like DWV.
The presence of certain agricultural fungicides, which are not directly toxic to bees, can also synergize with insecticides. This dramatically increases the toxicity of the insecticide and causes high mortality even at low concentrations. This complex web of interactions—where nutrition, parasites, and chemicals reinforce each other’s negative impacts—ultimately drives the decline and death of bee colonies.