Bees pollinate a substantial portion of global crops, including fruits, vegetables, and nuts, playing a profound role in food production and ecosystem function. However, bee populations have experienced concerning declines globally. Understanding the factors contributing to bee mortality is crucial for their preservation.
Pesticides and Chemical Exposure
Pesticides pose a significant threat to bee health, with various types impacting them. Neonicotinoids, a widely used class of insecticides, are systemic chemicals absorbed by plants. They become present in pollen and nectar, making them toxic to bees that consume these resources. Bees are exposed through direct spray, residues on foliage, or contaminated water. Neonicotinoids can persist in soil and be continuously taken up by plants for extended periods, harming pollinators weeks after application.
Sublethal effects from pesticide exposure are a major concern, even at levels not immediately fatal. Neonicotinoids can impair a bee’s navigation, communication, and foraging efficiency, and weaken their immune system. Organophosphates, another class of insecticides, target the insect nervous system, inhibiting a bee’s ability to locate pollen and return to the hive.
Carbamate insecticides shorten the lifespan of honey bees and impact their immune and detoxification systems. Exposure to multiple chemicals, including herbicides and fungicides, can lead to synergistic effects, where the combined toxicity is greater than the sum of their individual effects. This complex interplay of chemicals further compromises bee health, making them more susceptible to diseases and other stressors.
Diseases and Parasites
Biological threats represent a substantial challenge to bee populations, leading to widespread mortality. Varroa mites ( Varroa destructor ) are external parasites that feed on adult and developing bees, weakening them and transmitting viruses. These mites are a major vector for diseases like Deformed Wing Virus, which causes malformed wings and reduced lifespans. High mite populations, especially heading into winter, can severely compromise colony health and increase winter losses.
Bacterial diseases also devastate bee colonies. American Foulbrood (AFB), caused by Paenibacillus larvae, is a highly contagious bacterial disease that affects honey bee larvae, typically killing them after their cells are capped. Spores of AFB are remarkably resilient, remaining viable on beekeeping equipment for up to 70 years, making eradication challenging. Infected larvae decompose into a gooey mass with a distinct foul odor, and the disease can quickly lead to colony death.
European Foulbrood (EFB), caused by Melissococcus plutonius, is another bacterial infection that targets young bee larvae, usually before their cells are capped. Unlike AFB, EFB does not form persistent spores, and colonies can sometimes recover, though it significantly weakens them by reducing the number of emerging adult bees. Weak or stressed colonies are particularly susceptible to EFB, with outbreaks often linked to factors like poor nutrition or unfavorable climatic conditions. Nosema, caused by microsporidian parasites, infects the digestive tract of bees, impacting their ability to absorb nutrients and making them more vulnerable to other illnesses, especially when under nutritional stress.
Habitat Loss and Environmental Stress
Habitat loss and environmental degradation significantly impact bee populations. Habitat loss, driven by urbanization and the expansion of agriculture, reduces the availability of diverse floral resources (nectar and pollen) and nesting sites. Monoculture farming, growing a single crop over large areas, creates “food deserts” for bees outside that crop’s bloom period, leading to nutritional deficiencies and weakened colonies. Insufficient food can lead to starvation, declining health, and increased vulnerability to diseases and other stressors.
Climate change introduces further environmental pressures. Rising global temperatures can disrupt the delicate seasonal timing between bees and the flowering plants they rely on. Flowers may bloom earlier or produce less nutritious nectar and pollen under extreme weather, creating a mismatch for bees. Extreme weather events like prolonged droughts, excessive heat, and unseasonal cold snaps, destroy bee habitats and food sources or disrupt foraging patterns. Droughts, for example, limit water access for bees and can hinder plant growth and nectar production.
High temperatures can also force bees to alter their behavior, such as reducing foraging activity to avoid overheating, which impacts their ability to collect resources. Clean water is important for bees, and droughts can limit this resource. These environmental stressors collectively weaken colonies, making them less resilient and more susceptible to other factors contributing to their decline.
Natural Predators
While other factors contribute more significantly to large-scale bee population declines, natural predators do cause individual bee mortality. Various birds consume bees, including kingbirds, mockingbirds, swifts, thrushes, and woodpeckers. Some birds, like the summer tanager, have even developed techniques to remove bee stingers before consumption. Honey buzzards may attack feral bee colonies, and woodpeckers can feed on bee larvae, especially when other food sources are scarce.
Other insects also prey on bees. Spiders, praying mantises, and robber flies are known to capture and consume bees. Hornets and certain wasps can be significant predators, often targeting foraging bees or even raiding entire hives. While these predatory interactions are a part of natural ecosystems, their impact on the overall health and survival of healthy, established bee colonies is generally less pronounced compared to diseases, pesticides, and habitat loss. Bears can also prey on honey bee colonies, primarily for the honey and brood within the hive.