Bumble bees are among the most recognizable and hardworking pollinators. Their fuzzy bodies and ability to perform “buzz pollination” make them highly efficient at fertilizing a wide variety of plants, including tomatoes, peppers, and cranberries. These insects are facing a global decline, with several species experiencing population drops. This widespread mortality is not attributable to a single cause but rather a complex web of environmental pressures that threaten the survival of the genus. Understanding these specific threats is the first step toward conservation efforts.
Toxic Exposure from Pesticides
Modern agricultural chemicals represent a potent threat, particularly systemic insecticides that permeate every part of the plant. The neonicotinoid class of insecticides—including imidacloprid, clothianidin, and thiamethoxam—is toxic to bumble bees, even at doses too small to kill them outright. These systemic chemicals are absorbed by the plant from treated seeds or soil and are present in the pollen and nectar that foraging bees collect.
Exposure to these sub-lethal concentrations directly impairs the central nervous system of the bees. This neurological interference disrupts a bee’s ability to navigate, reducing its foraging efficiency and capacity to learn new floral sources. Such impairment means workers struggle to bring back enough food, leading to colony starvation and reduced performance.
The reproductive success of the colony is also compromised by chemical exposure. Studies show that treated colonies experience reduced growth rates and a significant drop in the production of new queens compared to unexposed colonies. Since new queens establish colonies the following spring, this reproductive failure translates directly to long-term population decline. Furthermore, colonies exposed to neonicotinoids deliver a reduced pollination service, evidenced by crops like apples containing fewer seeds.
Pathogens and Parasite Infestations
Biological threats from disease-causing organisms are a major factor in bumble bee mortality, often spreading rapidly through colonies and between species. One common internal threat is the gut parasite Crithidia bombi, a trypanosome that colonizes the bee’s hindgut and interferes with nutrient absorption. Infection with this parasite reduces the number of new queens a colony can produce and lessens the survival success of queens attempting to establish a new nest after overwintering.
Another internal pathogen is the microsporidian Nosema bombi, a fungus-like organism that also infects the digestive system. Infections with Nosema are linked to smaller colony sizes and a reduction in the number of sexual offspring, including male bees and future queens. There is concern that this pathogen can “spill over” from commercially reared bumble bees, used for greenhouse pollination, into wild populations.
External parasites and parasitoids also contribute to mortality and colony failure. For example, the tracheal mite Locustacarus buchneri is an endoparasite that lives within the air sacs of the adult bee, causing lethargy and reducing foraging ability. Additionally, parasitoid insects such as conopid flies lay their eggs directly onto adult workers while they are foraging. The fly larva develops inside the bee’s abdomen, consuming the host and eventually leading to the death of the adult bee.
Loss of Forage and Nesting Sites
The physical destruction and fragmentation of natural landscapes remove the basic resources a bumble bee needs to survive and reproduce. Suburban sprawl and intensive agriculture convert diverse meadows and fields into simplified environments that cannot sustain healthy bee populations. Bumble bees require continuous access to a variety of flowering plants from spring through fall to ensure a steady supply of nectar and pollen.
The practice of monoculture farming, where vast areas are dedicated to a single crop, creates a nutritional desert outside of a short three-week bloom period. This lack of diverse forage leads to malnutrition, which weakens the bees’ immune systems and makes them more susceptible to pathogens. A diet lacking in diverse proteins and micronutrients is a major indirect cause of death.
Bumble bees are also dependent on specific, often undisturbed, structures for shelter. Most species nest underground in abandoned rodent burrows or dense clumps of grass, and new queens require sheltered spots in the soil to overwinter. Land use changes, like tilling or constant mowing, destroy these essential nesting and overwintering sites, preventing the successful establishment of new colonies each spring.
Climate Volatility and Extreme Weather
The fuzzy body of a bumble bee allows it to fly in cooler temperatures, but also makes it vulnerable to extreme heat. Bumble bees are adapted to temperate climates, and prolonged periods of high temperatures can cause them to overheat and die, especially when foraging in direct sunlight. Heat waves also indirectly affect survival by desiccating native floral communities, causing a sudden drop in the availability of nectar and pollen when the colony needs it most.
Changes in seasonal timing, known as phenological mismatch, also disrupt the bumble bee life cycle. Unnaturally warm winter or early spring temperatures can cause overwintering queens to emerge from hibernation too early. If a sudden, late frost occurs, these queens—the founders of the next generation—may be killed before they can establish a nest.
Less predictable rainfall patterns, including prolonged drought, reduce the nectar flow and overall biomass of flowering plants. A lack of moisture prevents plants from producing the sugary nectar bees rely on for energy, directly impacting a worker’s ability to fly and leading to starvation within the colony. These weather-related stressors compound the challenges posed by habitat loss and disease.