Higher Temperatures Benefit Certain Bumble Bee Species, NC State Research Reveals

In a surprising twist amid ongoing concerns about pollinator declines, recent research from North Carolina State University suggests that higher temperatures could actually provide benefits to certain bumble bee species, particularly those that nest underground. This nuanced finding challenges simplistic narratives about climate change's impact on these vital pollinators and highlights the importance of considering colony-level dynamics in social insects like bumble bees.

The study, published in the Journal of Animal Ecology, focuses on the common eastern bumble bee, known scientifically as Bombus impatiens. This species is one of the most commercially important bumble bees in North America, used extensively in greenhouse pollination for crops like tomatoes and cucumbers. Researchers examined how rising ambient temperatures affect nest thermoregulation, foraging behavior, and overall colony performance, integrating field data, lab experiments, and climate modeling.

🐝 The Science Behind Bumble Bee Colonies and Temperature Regulation

Bumble bees, belonging to the genus Bombus, are large, fuzzy social insects renowned for their role in pollinating wildflowers and agricultural crops. Unlike solitary bees, bumble bee colonies consist of a queen, workers, and drones, with the queen initiating the nest in spring and workers handling foraging, brood care, and nest maintenance throughout summer. Colonies can house 50 to 500 individuals, depending on species and conditions.

A critical aspect of bumble bee biology is their ability to actively regulate nest temperature. When the nest is too cool, workers vibrate their flight muscles to generate heat and incubate the brood—immature larvae and pupae. Conversely, in hot conditions, they fan their wings to circulate air and evaporate water for cooling. This thermoregulation is energy-intensive, diverting workers from foraging, which brings in nectar and pollen essential for colony growth.

The NC State research reveals that moderate warming reduces the need for incubation, freeing up workers for more foraging trips. This shift can enhance colony productivity, as foraging peaks during warmer daylight hours when flowers are most active.

Spotlight on the Researchers: NC State's Contribution to Pollinator Science

Leading the collaboration is Elsa Youngsteadt, an associate professor in NC State's Department of Applied Ecology. Her work explores how urbanization, climate change, and landscape alterations influence insect populations and ecosystem services like pollination. Youngsteadt's lab integrates ecological fieldwork with geospatial modeling to predict urban heat effects on pollinators, building on prior studies showing urban yards in Raleigh experience bee diversity drops with rising temperatures.

Co-author Clint Penick, now an assistant professor of insect ecology at Auburn University, emphasizes colony-level perspectives: 'A lot of research on how higher temperatures affect living things has been done on individual animals. But when it comes to social animals, such as ants and bumble bees, you have to look at the entire society.' Other contributors include Francis Mullan and Nicholas Green from Kennesaw State University, and Kevin McCluney from Bowling Green State University.

NC State's Applied Ecology department, home to cutting-edge pollinator research, continues to lead in understanding climate-pollinator interactions, from pollen load heating effects to flower phenology shifts.

Unpacking the Methodology: A Multi-Faceted Approach

The study's rigor stems from four integrated components:

• Baseline temperature monitoring in simulated subterranean and aboveground nests across sites from Georgia to Michigan, revealing underground nests' superior insulation.
• Lab microcolonies of B. impatiens under controlled warmer and cooler conditions to quantify behavioral shifts, like reduced incubation and increased fanning.
• Field observations of insect visits to cucumber flowers (Cucumis sativus), a key crop pollinated by bumble bees, linking ambient heat to foraging rates.
• Climate modeling projecting mid- and late-century scenarios, estimating optimal foraging windows and extreme heat exposure.

This comprehensive design provides robust predictions, accounting for both benefits and risks.

Key Benefits of Moderate Temperature Increases

Under moderate warming—aligned with current trends—colonies gain several advantages. Brood incubation time drops as ambient warmth suffices, allowing workers to prioritize foraging. Models predict expanded daily foraging windows, especially in northern ranges previously limited by cold snaps.

For B. impatiens, which naturally prefers underground nests, soil insulation buffers against daytime highs, maintaining stable internal temperatures. This stability supports faster larval development and larger worker forces, boosting pollen collection efficiency.

Youngsteadt notes, 'We were surprised to see that—in many ways—bumble bee colonies should be doing better under increased temperatures.'

Nest Types: Underground vs. Aboveground Vulnerabilities

Bombus impatiens queens typically excavate nests 1-3 feet underground in abandoned rodent burrows or similar cavities, entering via tunnels up to 9 feet long. These sites offer natural insulation, with daily temperature swings far less extreme than surface conditions.

Commercially, however, bees are reared in aboveground wooden boxes for crop pollination. These experience rapid heating, demanding constant fanning. The study found aboveground nests require 2-3 times more cooling effort during peaks, potentially halving foraging output.

For details on the full study, see the original paper.

The Dark Side: Extreme Heat and Colony Failure Risks

While moderate changes favor bees, extremes tell a different story. In Georgia, aboveground nests endure about 9 hours annually above fanning thresholds (around 36°C nest temperature), where larvae risk death or malformation. Late-century projections escalate this to 200 hours, overwhelming colonies.

Foraging bees also face stress: prior NC State work showed pollen loads raise body temperature by 0.07°C per milligram, pushing foragers toward critical limits (42-44°C) on hot days. Combined, these stressors explain southern range contractions observed in many Bombus species.

Broader Context: Pollinator Declines and Economic Stakes

Bumble bee populations have plummeted—some North American species down 96% since 2000, per surveys. Climate extremes, alongside pesticides and habitat loss, drive this, with temperature anomalies explaining much of the variance.

Economically, pollinators underpin $20-29 billion in U.S. agriculture annually, including $15 billion from honey bees but significant native contributions to berries, squash, and greenhouse crops. B. impatiens alone supports cucumber yields, vital for global food security.

More on NC State's press release here.

Conservation Strategies for a Warming World

To bolster resilience:

• Design cooler commercial nest boxes mimicking underground stability.
• Preserve forest edges for shaded nesting sites.
• Plant native perennials like asters and goldenrods to shorten foraging distances.
• Reduce urban heat islands through green infrastructure.
• Support research via university programs.

Individuals can contribute with window-box natives; farmers with hedgerows. Organizations like Xerces Society advocate habitat corridors.

Future Directions and University-Led Innovations

Outstanding questions include pollen quality under heat, pathogen interactions, and multi-species dynamics. NC State plans expansions, leveraging geospatial tools for landscape-scale predictions.

Higher education institutions like NC State drive solutions, training entomologists and informing policy. Their interdisciplinary approach—from labs to fields—positions academia as key to pollinator survival.

This research underscores hope amid crisis: targeted actions can harness warming's upsides while mitigating downsides.

Photo by Rebekah Vos on Unsplash