Colony-level pollen collection reflects visitation of managed bumble bees (Bombus impatiens) in strawberry fields and surrounding landscapes without reducing pollen limitation.

Although managed bumble bees (Bombus impatiens) forage on strawberry flowers, their visitation rates are too low to alleviate pollen limitation, and their foraging is heavily influenced by surrounding landscape composition, often leading them to alternative floral resources in diverse agricultural settings where adding colonies may not be economically beneficial.

The Gist

Imagine you are a strawberry farmer. You've heard that bringing in a team of "hired help" (managed bumble bees) will make your berries bigger and sweeter. So, you rent a bunch of bumble bee colonies and place them right in your fields. You expect them to get to work immediately, buzzing from flower to flower, ensuring every strawberry gets perfectly pollinated.

But here's the twist: The bees showed up, but they mostly ignored your strawberries.

This study is like a detective story investigating exactly what these hired bees were doing, where they were going, and whether they actually helped the farmer. Here is the breakdown in plain English:

1. The Setup: The "Hired Help" vs. The Wild Neighbors

The researchers set up 10 strawberry fields in upstate New York. In each field, they placed commercial bumble bee colonies (the "hired help"). They also watched to see what the local wild bees were doing.

The Result: The wild bees were the true stars of the show. They visited the strawberry flowers constantly. The hired bumble bees? They were barely there. Out of every 100 bee visits, only about 7 were from the bumble bees you paid for. The rest were wild bees doing the heavy lifting.

2. The Big Question: Did the Hired Bees Actually Help?

The farmers' main worry is: "If I pay for these bees, will my strawberries grow better?"

To find out, the researchers played a game of "control vs. chaos."

• The Control: They hand-pollinated some flowers (using a tiny brush to move pollen manually, like a surgeon).
• The Chaos: They left other flowers to be pollinated naturally by whatever insects showed up.

The Verdict: The hand-pollinated strawberries were significantly bigger and heavier. This means the fields were pollen-limited—they desperately needed more help. However, adding the bumble bee colonies did not fix the problem. Even in fields where bumble bees visited a little more, the strawberries didn't get bigger. The bees just weren't doing enough work to make a difference.

3. The Secret Spy: The "Wax Detective"

Here is where the study gets really clever. You can't follow every single bee 24/7 to see what they eat. So, the researchers used a "CSI" approach.

They knew that bumble bees build their nests out of wax. As they feed their babies, pollen gets stuck in that wax, like crumbs in a cookie jar. By taking a tiny sample of the wax from the bottom of the hive, they could analyze exactly what the entire colony had eaten over the whole season.

The Findings:

• The Diet: The bees were eating almost everything except strawberries. Only about 4% of the pollen in the wax was from the strawberry family (Rosaceae).
• The Distraction: The bees were mostly eating pollen from pasture grasses and clover (like a giant salad bar next door).
• The Landscape Effect:
  • If the farm was surrounded by pasture, the bees ignored the strawberries even more (they preferred the clover).
  • If the strawberry field was huge, the bees paid a little more attention to the crop (because there was more of it to choose from).
  • But if the field was small and surrounded by pasture, the bees treated the strawberries like a side dish they barely touched.

4. The Analogy: The "All-You-Can-Eat Buffet"

Think of the landscape like a massive all-you-can-eat buffet.

• The Strawberries are the main course the farmer wants the bees to eat.
• The Pasture/Clover is a delicious, free dessert bar right next to it.

The researchers found that the "hired help" (bumble bees) walked past the main course, went straight to the dessert bar, and ate their fill there. Even though the farmer paid them to eat the main course, they were too busy enjoying the free dessert (the pasture) to do the job.

5. The Bottom Line for Farmers

The study concludes that in diverse landscapes (where there are lots of other flowers and pastures nearby), renting bumble bee colonies might not be worth the money.

• Why? Because the bees will naturally drift toward the wilder, more abundant flowers in the surrounding landscape.
• The Risk: Bringing in managed bees can sometimes cause problems, like spreading diseases to wild bees or competing with them for food. If the bees aren't even helping your crop, you are paying for a service you aren't getting, while potentially hurting the local ecosystem.

In short: Nature often provides its own pollinators (the wild bees). If you add a "hired help" team in a landscape full of other flowers, they might just get distracted by the scenery and forget to do their job. The best strategy might be to protect the wild bees and the natural habitat around the farm, rather than trying to force the bees to work where they don't want to.

Technical Summary

1. Problem Statement

Managed pollinators are frequently introduced to agricultural systems to supplement wild pollination services. However, their efficacy is often inconsistent. A critical gap in understanding is how landscape composition influences the foraging behavior of managed bees and whether their presence actually translates to improved crop pollination.

• Specific Context: The study focuses on the common eastern bumble bee (Bombus impatiens), a commercially available managed species, in open-field strawberry (Fragaria × ananassa) production in upstate New York.
• The Conflict: While B. impatiens is an effective pollinator in greenhouses, its behavior in open fields is less understood. There is a concern that managed bees may forage on alternative floral resources in the surrounding landscape (e.g., pastures, forests) rather than the focal crop, rendering supplemental colonies economically ineffective or ecologically redundant.
• Knowledge Gap: Traditional methods (visual transects) only capture short-term, individual-level foraging. There is a lack of data on whole-colony foraging patterns over an entire blooming season and how these patterns correlate with actual crop pollination outcomes (pollen limitation).

2. Methodology

The study employed a multi-faceted approach combining landscape analysis, field observations, sentinel plant experiments, and a novel pollen extraction technique.

• Study Sites & Design:

  • Location: 10 strawberry fields in upstate New York (May–June 2018).
  • Intervention: Commercial B. impatiens colonies were placed in each field at a density of ~2 colonies/acre.
  • Landscape Analysis: Land cover composition (Agriculture, Semi-natural, Pasture/Grass/Hay) was quantified at 500m, 750m, and 1000m radii using the 2018 Cropland Data Layer.

• Visitation Surveys:

  • Visual transects (10 minutes) were conducted four times during the bloom period.
  • Researchers counted visits to strawberry flowers, distinguishing between managed bumble bees and "other bees" (wild bees and honey bees).

• Pollination Assessment (Sentinel Plants):

  • Method: Potted 'Seascape' strawberry plants were used as sentinels.
  • Treatment: Primary flowers were removed. Paired secondary flowers were used for a paired comparison: one was hand-pollinated (control for maximum potential), and the other was open-pollinated (exposed to field visitors).
  • Metric: Pollen limitation was calculated as: $(Weight_{hand} - Weight_{open}) / Weight_{hand}$. A value > 0 indicates pollen limitation.

• Novel Colony-Level Foraging Analysis (Wax Extraction):

  • Innovation: Instead of analyzing individual pollen loads, the researchers extracted pollen from colony wax.
  • Procedure: ~10g of peripheral wax was harvested from a representative colony at each site. The wax was pulverized, and pollen was extracted via an acetolysis protocol (using acetic acid and sulfuric acid).
  • Identification: Pollen grains were counted (targeting 300 grains) and categorized as Rosaceae (the family containing strawberries) or non-Rosaceae. This provided an integrated measure of the colony's foraging effort over the entire season.

• Statistical Analysis:

  • Linear mixed-effects models (LMM) tested the relationship between landscape variables and visitation rates.
  • Beta regression models tested relationships between landscape/visitation and the proportion of Rosaceae pollen.
  • Models controlled for field size and random effects of site.

3. Key Contributions

1. Methodological Innovation: The study successfully adapted a technique for extracting pollen from museum wax specimens (previously used for historical honey bee studies) to quantify whole-colony foraging dynamics in contemporary managed bumble bees. This overcomes the limitations of snapshot visual surveys.
2. Decoupling Visitation from Pollination: The research demonstrates that high visitation rates by managed bees do not necessarily correlate with reduced pollen limitation in the focal crop.
3. Landscape-Dependent Foraging: It provides evidence that managed bumble bees are highly sensitive to landscape context, specifically shifting foraging effort away from crops when alternative resources (like pasture) are abundant.

4. Key Results

• Low Visitation Rates: Managed bumble bees provided significantly fewer visits to strawberry flowers compared to wild bees (mean 1.78 vs. 11.08 visits per 10-min transect).
• No Reduction in Pollen Limitation: Despite the presence of managed colonies, increased bumble bee visitation did not reduce pollen limitation. Open-pollinated fruits were consistently smaller than hand-pollinated fruits across all sites, indicating that the crop was pollen-limited and managed bees were not providing sufficient service.
• Colony-Level Foraging Patterns:
  • Correlation: Fields with higher observed bumble bee visitation had a higher proportion of Rosaceae pollen in the colony wax, confirming that visitation data reflects colony-level foraging.
  • Landscape Effect: The proportion of Rosaceae pollen collected was negatively correlated with surrounding pasture (at 1000m). Colonies in pasture-rich landscapes collected significantly less strawberry pollen.
  • Field Size Effect: Colonies in larger strawberry fields collected a higher proportion of Rosaceae pollen, suggesting that larger crop areas capture a greater share of the colony's foraging radius.
• Landscape vs. Visitation: Interestingly, while landscape composition (pasture) influenced what the colony collected (pollen composition), it did not significantly alter the number of visits observed during the short transects. This suggests that while bees visit the crop, the intensity of foraging on the crop relative to the colony's total effort is diluted by alternative resources.

5. Significance and Implications

• Economic Implications: In diverse agricultural landscapes (specifically those with significant pasture cover), adding managed B. impatiens colonies to strawberry fields may not be economically beneficial. The bees largely forage elsewhere, failing to alleviate pollen limitation.
• Ecological Risks: Introducing managed bees into landscapes where they do not effectively pollinate the target crop poses risks without benefits. These risks include competition with wild pollinators, disease transmission, and the potential for managed bees to act as "ecological traps" for wild queens.
• Management Recommendations:
  • Growers in landscapes with high pasture cover should be cautious about investing in supplemental bumble bee colonies for strawberries.
  • Future strategies should focus on enhancing wild pollinator habitats or increasing managed bee density significantly (beyond standard recommendations) to overcome competition from alternative floral resources.
• Broader Scientific Impact: The study validates the use of wax-based pollen extraction as a robust tool for understanding season-long foraging ecology, offering a new standard for assessing the efficacy of managed pollinators in complex agroecosystems.