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Mason Bees and Their Nests

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In honour of World Bee Day, the High Park Nature Centre is kicking off a new blog series, Big City Bees, for the summer chronicling the bees of High Park and Toronto, including the stem-nesting bees and other invertebrates living in the cabinet in OURSpace behind the Nature Centre.

While honeybees are the focus of World Bee Day, they are not native to North America and were imported as agricultural animals when European colonists settled here.  For many of our food crops, honeybees are key pollinators; however, they could have some negative impacts on native bee and plant populations including spreading disease and increased competition for resources.

Redirecting the spotlight away from honeybees, bees are an incredibly diverse group with over 20,000 species worldwide! They range in size from less than 2mm (Perdita minima) to more than 40mm (Megachile pluto).  Beyond the classic black and yellow, there’s a full spectrum of bees: blue bees, green bees, metallic bees, and even purple bees! 

A bicolour agapostemon sitting on some Blue Vervain. Credit: High Park Nature Centre

Native Bees of Toronto

Declared as the first Canadian Bee City in 2016, Toronto is a bee biodiversity hotspot with over 360 species of native bees!  Toronto even has its own official bee: the bicolour agapostemon (the star of an ethereal mural at Bloor St West and Howland Avenue in the Annex)! 

Native bees evolved alongside the native plant communities. While most bees are generalists, pollinating a wide variety of plants, some have specialized symbiotic relationships. As an example, squash bees are early risers, pollinate squash blossoms when they open at dawn then spend the afternoon dozing in the closed flowers.   Since their innate daily schedules align with the opening and closing of squash blossoms, they finish their pollination work before honeybees get to the fields. 

Many food crops indigenous to North America, like tomotoes, potatoes, and blueberries, require buzz-pollination where the bee’s buzzing releases pollen from the anther of a flower. Honeybees are incapable of buzz pollination so native bees (or commercial bumblebees) are essential in producing some of our most popular dishes and a side of fries and ketchup!

Unlike honeybees that live in hives with more than 10,000 individuals, most native bees are solitary.  Roughly 70%, are ground nesting, meaning they lay their eggs and overwinter in nests they excavate undergroundThe other 30% are stem nesting, building their homes in the hollow or pithy centres of flower and shrub stems. 

In OURSpace, the outdoors savannah learning garden and restoration space behind the Nature Centre, there is a bee cabinet.

Installed in 2016, Sonic Solitaries Audio Bee Cabinet is not only an interactive, educational art exhibit, but also a functional nesting space for a variety of bees and wasps. It was created by Sarah Peebles with support from Mary-Ann Alberga (wood burning), Stephen Humphreys (poetry), John Kuisma (work working), and Rob Cruickshank (electronics).  

On either side of the cabinet, there are two latched doors that can be opened to view the nests. The nests are protected by plexiglass. The grooved planks in which the insects build their nest are removable and are frequently cleaned by High Park Nature Centre staff.

A vertical bee nesting cabinet in a garden. The cabinet is constructed of wood with a painted woodburned design of a raspberry flower and a bee polinating it
Bee cabinet in OURSpace. Credit: High Park Nature Centre

Notable previous residents of the Audio Bee Cabinet include mud dauber wasps, mason bees, leafcutter bees and cellophane bees.  

When visiting the bee cabinet, please limit the sun exposure of the nests by briefly opening the panels and securely latching them when finished viewing the bees.  

First Residents of Summer 2023

Showing a cross section of a stem nesting bee box, there are 2 complete nest and the beginnings of a third. There is a mason bee in one of the grooves.
Mason Bee Nests in the OURSpace bee cabinet. Credit: High Park Nature Centre

First spotted on May 16th, mason bees are building their nests in the bee cabinet!

Mason bees overwinter as adults and are one of the first bees to emerge in the spring, pollinating spring blossoms and ephemerals.

After they mate, the female bees start their nests by building cells with mud. Each cell is provisioned with a pollen ball that is a mixture of pollen and small amounts of nectar. The pollen ball will provide food for the developing larva once the egg hatches. The pollen is a protein source and the nectar is a carbohydrate.

Once the female lays all her eggs, she will cap off the stem with a wall of mud to protect the nest from predators.

Often microscopic differences distinguish different native bee species from each other so it can be to identify the exact species that’s currently nesting in the cabinet. We think our first resident is of the genus Osmia.

Creating Habitat for Stem Nesting Bees

Like all pollinators, planting native plants is the most important tool in creating habitat for bees.  Whether you have access to a yard or a container garden on a balcony, every native plant helps provide food or shelter resources for bees. Most native bees have a 500m foraging range so even small pollinator gardens can be incredibly impactful and help bridge gaps between larger spaces.  

Bees need safe overwintering habitat to survive through to the spring. For stem-nesting bees, hollow or pithy stems of branches are essential. In the fall, leave the stems of native flowers intact and keep pruned shrubs branches in the garden over winter. Only remove them once the spring temperature is consistently warm (after the Victoria Day weekend is a good rule of thumb). 

Bee Hotel Beware

Over the last decade, bee hotels have become popular additions to gardens touting that they add shelter space for bees; however, many are not appropriately designed for stem-nesting bees. Below is a quick list of some features to look for if you are planning on adding a bee hotel to a garden.

Bee Hotel Checklist

  • Holes or tubes minimum 8” or 20 cm deep 
  • Opening size no more than 3/8” or 1 cm wide 
  • Made of natural materials, no chemical treatments 
  • Can be cleaned or next spaces replaced annually 
  • A sunny location 
  • Time to commit to cleaning and monitoring

The bee cabinet in OURSpace was developed with input from entomologists and biologists to ensure its stem design was suitable for a variety of insects. The High Park Nature Centre staff frequently clean, rotate and replace the stem planks to ensure they are clean and free of mold to promote healthy bees.  

Well-designed and maintained bee hotels can be a great addition to a garden, though without a nearby food source, they’ll often go unused. Planting native plants, whether a small container garden on a balcony or a sprawling meadow, is key to supporting the diverse native bees in our neighbourhoods.  

In most cases, investing in native plants and keeping the stems intact throughout the winter has more bang for the buck for native bees than installing a bee hotel. Please consider adding native plants to a garden space before installing a bee hotel.  

Get Involved in Bee Conservation

Participating in community science projects is a great way to get involved in conservation effortsKeep an eye out for native bees in your neighbourhood and share your observations on iNaturalist. If you spot bumble bees,  submit your sightings on Bumble Bee Watch.  

Learn More About Bees and Other Pollinators

Interested in getting to know the bees of Toronto and beyond better? Sign up for Pollinators: Nature’s Tiny Superheroes, one our many online naturalist courses!