Plump, fuzzy, and friend of the flower, bumble bees are often found buzzing along while carrying pollen from plant to plant. Bees, as well as bats, birds, and butterflies, belong to a group of animals and insects responsible for pollinating an estimated three-fourths of the Earth’s plants.  

Bees in the United States play a key role in crop and flower pollination. Bee populations have recently experienced staggering losses in number; some estimates state that as many as 1 in 4 bumble bee species in the United States may be at risk of extinction. Six stressors may be causing this population decline:pathogens, habitat loss, agrochemicals, climate change, non-native bee populations, and small populations that limit genetic diversity. 

Understanding how these six risk factors impact each species is a challenge to experts, as the United States is home to an estimated 4,000 native bee species. Within this expansive group of native bees, “bumble bee” is the common name used to describe over 250 species worldwide that are social, fuzzy insects who belong to the genus Bombus. Scientists at the Fort Collins Science Center have recently focused their efforts on better understanding and monitoring bumble bee populations around the country.

Life Cycle of the Bumble Bee 

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Understanding the bumble bee life cycle allows researchers to learn when bumble bees may be most vulnerable. Native bumble bees select a new queen and nest location every year, unlike non-native bees (including honey bees) that may keep the same hive location and queen bee for multiple years. The annual renewal of bumble bee colonies is made up of five distinct stages, which include:  

• Nesting, where the queen bee ventures out alone in the early spring to find a suitable cavity to build a nest, either above or below ground;  
• Colony Growth, when the queen lays successive cohorts of eggs to grow the colony;  
• Reproductive Production, the production of males and future queens, typically in late July and early August;  
• Reproductive Dispersal and Mating, when males and future queens leave their home hive to mate with bees from other colonies; 
• Overwintering, where queen bees find suitable places to hibernate until spring, when flowers are available and the queen can begin nesting. 

Life Cycle + Stressors 

The health of bumble bees directly impacts the future of the colony. If all parts of the life cycle are completed with adequate nutrition, the colony is set up for success as it gathers protein-packed pollen and nectar in preparation for producing offspring.   

Bumble bee populations may be especially vulnerable to risk factors during certain stages of colony development, including nesting and colony growth. Habitat and plant loss at the hands of landscape changes may directly harm colonies, as most bumble bees are generalist pollinators who visit a wide variety of flowers, shrubs, and trees for pollen. Queens also depend on a variety of plants while nesting and beginning the colony; habitat loss may force queens to fly further to find more food or risk a loss of health by not eating enough of pollen and nectar.  

Habitat Change Research 

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USGS scientists recently reviewed plant surveys from 262 sites located in grassland, forest, and wetland habitats across Illinois to learn how plants visited by bees have changed in the last 20 years. Bumble bees depend on a mix of high-quality forests, grasslands, and wetlands; loss of these habitats may mean fewer plants available for pollination by bees. Changes in plant cover (amount of area covered by plants) and plant richness (number of plant species in a defined area) can compound with other stressors to put some species at greater risk of extinction.  

Bumble bees rely on forest plants in the early spring, when queens emerge from overwintering to search for a nest location. Land surveys show a decline in key early season species in forests, including wild geranium and Virginia waterleaf. A continued loss of key plants may worsen other risk factors, as poor nutrition is associated with reduced overall health in colonies.  

While forests provide food for bumble bees in the early spring, grasslands and wetlands provide the bulk of food for colonies in midsummer. Scientists found that plant cover and richness in grasslands remained stable, but total grasslands area declined by ~7.5% over the last 20 years. This is consistent with large-scale losses throughout the Midwest over the last century to make room for increasing agricultural production.  

Scientists, as part of the recent plant survey effort, also focused on plants pollinated by rusty-patched bumble bees. The rusty-patched population has declined by 87% in the last 20 years. This decline, thought to be caused by pathogen exposure, is made worse by poor nutrition caused by plant loss. It’s likely that other pollinators from the Midwest are experiencing a similar phenomenon; while changes in plant availability aren’t the primary cause of population declines, less plants available may worsen the effects of other stressors.  

In the Field 

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Multiple risk factors and the challenge of tracking native bees that move nest locations every year make it difficult to monitor the health of native bee populations. USGS scientists are up to the task – butterfly nets in hand, they can be found in fields around the Midwest, listening for the distinctive buzz of a passing bee. 

USGS scientists, along with partners at the University of Wisconsin-Madison, Minnesota Zoo, U.S. Fish and Wildlife Survey, the USDA-Agricultural Research Service, University of Minnesota, and the Illinois Natural History Survey are spending time in 2021 to learn more about the status of native bee populations across the United States. During the summer months, scientists will be visiting Wisconsin, Iowa, Illinois, Minnesota, and Virginia to visit public forests, parks, and prairies to monitor local bee populations. These researchers will note what bee species are found, as well as where bees forage and nest.  

Information gathered this field season will guide future research at USGS and be shared with other federal agencies and partners invested in the wellbeing of native bees. When asked about present fieldwork, ecologist Ian Pearse shared that, “By the end of the year, we hope to have filled in critical gaps of knowledge about the early season habits of declining bumble bees.”