Solitary Bees: Champions of Pollination and Why Bee Hotels Can Be Harmful

Solitary bees are some of the most fascinating and essential pollinators in our ecosystems. Unlike the social honeybees and bumblebees, solitary bees live alone, with each female typically building and provisioning her own nest. There are over 250 species of solitary bees in the UK alone, including mason bees, leafcutter bees, and mining bees. Despite their unassuming nature, solitary bees are vital for pollinating wildflowers, fruits, and vegetables. However, well-meaning efforts to support these bees, such as providing bee hotels, can sometimes do more harm than good.

Understanding Solitary Bees

Solitary bees do not form hives or produce honey, but their pollination efficiency often surpasses that of honeybees. For example, a single red mason bee (Osmia bicornis) can pollinate as many flowers as 120 honeybees in a day.

Solitary bees nest in a variety of places. Some prefer holes in wood or hollow plant stems, while others burrow into sandy soil. They lay their eggs in these nesting sites, providing each one with a pollen-and-nectar “lunchbox” for the developing larvae. After sealing the nest, the adult bee’s work is done, and the next generation emerges the following year.

The Rise of Bee Hotels

In recent years, bee hotels—man-made structures designed to mimic natural nesting sites—have become popular among gardeners and conservationists. These structures are often marketed as a way to help declining bee populations. When well-designed, bee hotels can provide safe nesting sites for solitary bees. However, many commercially available bee hotels are poorly constructed and pose significant risks.

The Problems with Bee Hotels

1. Pests and Diseases
   Overcrowded or poorly maintained bee hotels can become breeding grounds for pests such as mites and parasitic wasps. These pests can infest the nests, killing the developing larvae. Additionally, diseases can spread rapidly in densely packed hotels, further endangering bee populations.
2. Exposure to Predators
   Bee hotels that lack adequate protection may expose bees to predators like birds or wasps. For instance, if the entrances to the nesting tubes are too large or the hotel lacks proper cover, predators can easily access the nests.
3. Unsuitable Materials and Design
   Many bee hotels sold in garden centers are made from inappropriate materials, such as plastic or untreated wood that rots quickly. Some have sharp edges that can injure bees or tubes that are too short, too wide, or poorly ventilated, making them unsuitable for nesting.
4. Poor Placement
   Incorrect placement of bee hotels can deter bees or even harm them. For example, placing a bee hotel in a shady, damp area can lead to mold growth, while excessive exposure to rain or wind can make it uninhabitable.
5. Over-concentration of Bees
   Solitary bees naturally spread out their nests to avoid competition and disease. Large, crowded bee hotels force bees to nest in close proximity, creating unnatural conditions that can stress the population and increase vulnerability to pathogens.

How to Help Solitary Bees Responsibly

1. Choose Quality Bee Hotels
   If you purchase or build a bee hotel, ensure it is made from natural, untreated wood or bamboo, with smooth, replaceable tubes that are at least 15 cm long and 6–10 mm in diameter. Avoid hotels with wide, shallow tubes or plastic parts.
2. Maintain Bee Hotels Regularly
   Clean and replace nesting tubes annually to prevent the buildup of pests and diseases. Close monitoring and care are essential to maintain a healthy environment for bees.
3. Provide Natural Nesting Sites
   Encourage natural nesting by leaving dead wood, hollow plant stems, or bare patches of soil in your garden. Solitary bees are adept at finding and using these resources.
4. Plant for Pollinators
   Provide a variety of nectar- and pollen-rich flowers throughout the year. Native plants are especially valuable for supporting local bee species.
5. Avoid Pesticides
   Even organic pesticides can harm bees. Commit to chemical-free gardening to create a safe haven for pollinators.
6. Monitor and Learn
   Observe the activity in your bee hotel and educate yourself about the types of solitary bees in your area. Citizen science projects often welcome data on solitary bee activity, helping to advance conservation efforts.

How Solitary Bees Nest and Lay Eggs

Solitary bees have a fascinating life cycle, particularly when it comes to laying and provisioning their eggs. The female bee is entirely responsible for building the nest, collecting food, and ensuring the survival of her offspring. Here’s a step-by-step look at how solitary bees use nesting tubes and the critical details of their nesting behavior:

1. Selecting a Nesting Site
   The female bee begins by scouting for a suitable location to build her nest. Depending on the species, this could be a hole in wood, a hollow plant stem, or a pre-drilled tube in a bee hotel. The site must be safe, dry, and well-protected from predators and harsh weather conditions.
2. Laying Eggs and Filling the Tube
   Once the female selects a tube, she works systematically, starting from the back and moving toward the entrance:
   • The First Egg: The female lays her first egg at the very back of the tube.
   • Provisioning the Egg: After laying the egg, she gathers a mixture of pollen and nectar, which she shapes into a compact ball called a “pollen loaf.” This serves as food for the developing larva.
   • Sealing the Chamber: The female uses mud, leaves, or resin to seal off the egg and its food supply into a chamber. This partition keeps each egg and larva separate, reducing the risk of disease spread or competition for resources.
3. Repeat Process
   The process repeats until the entire tube is filled with a series of individual chambers. Each chamber contains a single egg and its accompanying food source. The female strategically lays female eggs at the back of the tube and male eggs toward the front. This arrangement ensures that males, which mature earlier, can emerge first, leaving the females safely behind to complete their development.
4. Finishing the Nest
   After the tube is filled, the female seals the entrance with a final plug made from mud, leaves, or resin, depending on the species. This protects the entire nest from predators and environmental damage.

Is There a Time When the Tubes Are Empty?

Yes, there is a window of time when nesting tubes may be empty:

• Spring (before nesting begins): Most solitary bee species emerge from their overwintering cocoons in early spring. The nesting tubes from the previous year will either be empty after the bees emerge or still contain unhatched bees. Once the bees emerge, the tubes are vacant until females begin nesting again.
• Late Summer/Autumn (after nesting): By late summer, all eggs have been laid, and the larvae inside the tubes are fully provisioned. At this stage, the female bees die, leaving the sealed tubes as the only sign of their activity. The tubes will remain occupied until the next spring when the new generation of bees emerges.

Seasonal Activity and Maintenance Opportunities

Understanding this time window is crucial for maintaining bee hotels:

• Late Autumn to Early Spring: This is the best time to clean or replace nesting tubes to remove pests and pathogens. Empty tubes indicate the bees have already emerged, and you can prepare the hotel for the next nesting season.
• Spring and Summer: Avoid disturbing the bee hotel during this time, as it’s likely active with nesting females or developing larvae.

By respecting these natural cycles, you can ensure your efforts to support solitary bees are effective and beneficial.

Why Some People Harvest Solitary Bee Eggs for Separate Release

Harvesting solitary bee eggs, also called cocoons, has gained popularity among gardeners and conservationists who aim to improve the survival rates of these important pollinators. The process involves removing the cocoons from nesting tubes, cleaning them, and storing them until the following spring for a controlled release. When done carefully, harvesting eggs has several benefits, though it also requires attention to detail and a commitment to proper management.

Benefits of Harvesting and Separately Releasing Solitary Bee Eggs

1. Reducing Pests and Parasites
   Solitary bee nesting tubes often attract pests like pollen mites, parasitic wasps, and other harmful insects. By harvesting the cocoons, you can inspect each one, removing pests and damaged cocoons to protect the healthy ones.
2. Preventing Disease Spread
   Disease can easily spread in poorly maintained bee hotels, especially if nesting tubes are reused year after year without cleaning. Harvesting eggs allows you to clean the cocoons and nesting materials, reducing the risk of fungal infections or bacterial pathogens that could harm developing bees.
3. Paper Inserts: A Helpful Tool
   Bee hotels with removable paper or cardboard tube inserts can make harvesting easier and safer. These inserts allow you to access the cocoons without damaging them or the nesting structure. After harvesting, the paper inserts can be replaced, ensuring a clean and pest-free environment for the next nesting season. While not strictly necessary, using inserts can significantly improve the hygiene and usability of your bee hotel.
4. Better Nesting Tube Maintenance
   Over time, wooden or bamboo nesting tubes can degrade, accumulate debris, or become infested. Harvesting eggs allows you to clean or replace these tubes, maintaining a healthy and welcoming nesting site for future generations of bees.
5. Synchronizing Bee Emergence
   By storing cocoons in controlled conditions, you can ensure their emergence aligns with the availability of flowering plants. This can be especially useful in areas where early or late frosts might disrupt natural timings, ensuring the bees have adequate food when they emerge.
6. Learning and Monitoring
   Harvesting cocoons provides a unique opportunity to observe solitary bees up close, monitor their health, and contribute to citizen science or conservation projects. It’s an excellent way to learn more about these fascinating insects and ensure their well-being.

The Process of Harvesting and Releasing Eggs

1. Timing the Harvest
   Wait until late autumn or early winter, when the bees have completed their life cycle, and the larvae have developed into cocoons. Ensure the tubes are sealed before beginning the process.
2. Removing the Cocoons
   For bee hotels with paper inserts, gently slide out the paper tubes and carefully unroll them to access the cocoons. For wooden or bamboo tubes, use a thin tool to extract the cocoons without damaging them.
3. Cleaning the Cocoons
   Inspect each cocoon for signs of pests or mold. Wash them in a light solution of water and bleach (1%) to kill any pathogens, then rinse and allow them to dry in a well-ventilated area.
4. Storing the Cocoons
   Place the cleaned cocoons in a breathable container, like a mesh bag or ventilated box, and store them in a cool, dark location. Mimic natural overwintering conditions by keeping them at temperatures between 1–5°C.
5. Releasing the Bees
   In early spring, as temperatures consistently reach 10°C or higher and flowers begin blooming, place the cocoons back into the bee hotel or a release container near a suitable nesting site. The bees will emerge naturally when they are ready.

Drawbacks of Harvesting Eggs

1. Time and Effort
   Harvesting and cleaning cocoons is a meticulous process that requires patience and care. It may not be practical for those who prefer low-maintenance solutions.
2. Risk of Mishandling
   Improper handling or cleaning can damage the cocoons or stress the developing bees, potentially causing more harm than good.
3. Not Always Necessary
   In many cases, providing a well-designed bee hotel in a safe location and allowing natural processes to take their course is sufficient to support solitary bee populations.

Should You Harvest Solitary Bee Eggs?

For most gardeners, harvesting eggs is optional. High-quality bee hotels with paper inserts, regular cleaning, and good placement can provide a safe environment for solitary bees without the need for harvesting. However, in areas where pests, diseases, or environmental challenges threaten bee survival, harvesting cocoons can be a valuable tool for ensuring the health and success of these essential pollinators.

Conclusion

Bee hotels can be a valuable tool for supporting solitary bees, but they must be approached with care and responsibility. Poorly designed or maintained hotels can do more harm than good, potentially contributing to the very problems they aim to solve. By focusing on creating diverse, bee-friendly habitats and practicing thoughtful management, gardeners can genuinely help solitary bees thrive.

Solitary bees may be small and inconspicuous, but their role in pollination is monumental. With informed actions, we can ensure these remarkable insects continue to flourish in our gardens and beyond.