The global decline in bee populations—including managed honey bees, native solitary bees, and bumblebees—is a serious concern due to their role as pollinators for a significant portion of the world’s food crops and wild flowering plants. Bees provide a fundamental service that underpins both ecological health and agricultural productivity. The current struggle of these insects is not due to a single problem but results from multiple interacting threats that compromise their ability to survive and reproduce. Understanding the factors that lead to bee mortality is the first step toward mitigating population losses observed worldwide.
Chemical Exposure: The Role of Pesticides
Synthetic chemicals, particularly insecticides used in large-scale agriculture, represent a significant and direct threat to bee health. The most studied group is systemic insecticides, notably the neonicotinoids, which are absorbed by the plant and present in the pollen and nectar consumed by bees. These neurotoxins target the insect central nervous system by overstimulating nicotinic acetylcholine receptors in the brain, leading to nerve damage and functional impairment.
Exposure to these chemicals can be categorized as acute, causing immediate mortality, or chronic, where the effects are sublethal but long-lasting. Chronic exposure, even at low concentrations, impairs a bee’s cognitive functions, such as learning, memory, and navigation. Foraging bees may struggle to find their way back to the hive, reducing the food supply for the colony. This chronic stress can also cause developmental issues in the brood and reduce colony performance.
While herbicides, such as glyphosate, are designed to kill plants, they also contribute to bee decline. Herbicides decimate the diverse wildflowers and weeds that provide bees with essential forage, leading to nutritional deficiencies. Studies show that glyphosate can directly affect a honey bee’s gut microbiome, making them more susceptible to disease. Sublethal exposure also reduces foraging efficiency and alters biogenic amines in the bee brain, which are neurotransmitters involved in learning and behavior.
Biological Invaders: Pests and Pathogens
Beyond chemical exposure, bees face severe threats from living organisms that parasitize them or cause infectious disease. The most devastating biological threat to managed honey bee colonies globally is the external parasite Varroa destructor mite. These mites feed on the fat body tissue of adult and developing bees, weakening their immune systems and causing a loss of body weight.
The primary mechanism of harm is the mite’s ability to act as a vector for various viruses, most notably Deformed Wing Virus (DWV). When a mite feeds, it injects viral particles directly into the bee’s open circulatory system, resulting in a much higher viral load. Colonies with high Varroa infestation frequently experience high DWV prevalence, leading to bees emerging with shriveled wings, reduced lifespan, and impaired cognitive function. This combination of parasitism and viral infection often causes the collapse of entire colonies, particularly during overwintering.
Other pathogens also contribute to colony mortality. American Foulbrood (AFB) is a highly contagious bacterial disease that affects honey bee larvae, causing them to die in their sealed cells. A fungal infection caused by the microsporidian Nosema targets the digestive tract of adult bees, reducing nutrient absorption. While distinct from the Varroa-DWV complex, these diseases prey on colonies weakened by poor nutrition or environmental stress, highlighting the interconnected nature of bee health threats.
Environmental Degradation and Lack of Resources
The physical and structural changes to the environment represent another large-scale factor in bee decline, primarily by restricting access to the necessary resources for survival. Habitat loss and fragmentation, often driven by urbanization and large-scale industrial agriculture, remove the diverse floral resources that bees rely on for food and nesting sites. Bees require a continuous supply of pollen and nectar from various plants to maintain a balanced diet and strong immune system.
The expansion of monoculture farming, where vast areas are dedicated to a single crop, replaces diverse ecosystems with a nutritional desert. While a single crop, such as canola, may provide abundant food for a short period, the lack of other flowering species means that bees face periods of starvation outside of that brief bloom. This lack of diverse, continuous forage leads to malnutrition, which weakens the bees’ immune responses and makes them more vulnerable to parasites and pathogens.
The consequences of climate change, such as extreme heat events and unpredictable weather patterns, also disrupt the delicate timing of bee life cycles. Shifts in temperature can cause flowers to bloom earlier or later than usual, creating a mismatch between when bees emerge and when their food sources are available. A lack of clean water sources, especially during droughts or heatwaves, adds another layer of environmental stress. These resource limitations combine to stress individual bees and entire colonies, ultimately leading to reduced reproduction and increased mortality.