Creating a Hotel for Native Bees

I enjoy gifts that lead to interesting projects.

For a gardener, there are many possibilities for such gifts, definitely including plants that would complement the landscape, or add something that is both compatible and unusual.

This year, my most intriguing gift is a “hotel” for native bees. It qualifies as a hotel by including nesting facilities for multiple occupants.

This gift leads to an interesting study of the bees themselves, and of ways to entice them into using the hotel.

Some 4,000 species of bees are native to North America. This column is not suitable for an overview of native bees, but it is worth noting at the outset that the familiar honeybee (Apis mellifera) is not native to this part of the world, but emigrated from Europe to the United States.

Native bees are excellent pollinators, more efficient than honeybees in that work. An important difference between these groups: most native bees are solitary, nesting in cavities or the ground, while honeybees are social, nesting in hives.

According to the Xerces Society (which seeks to conserve bees and other invertebrates) about thirty percent of our 4,000 species of native bees nests in cavities that they find or create in nature. The other seventy percent nest in the ground. There are also few other native species, like the Western Bumblebee (Bombus occidentalis) that are social creatures that nest in larger cavities. 

My native bee hotel would be of interest, hopefully, to cavity-nesting species that live in central California. My initial expectation was that this nest would attract mason bees, but the Xerces Society lists only a few species that are found primarily in Washington and Oregon. There are many other species, however, including the Blue Orchard Mason Bee (Osmia lignaria),that might live in California.

Another cavity-nesting native bee species that could appear in my garden is the carpenter bee (Xylocopa californica), which, according to Las Pilitas Nursery, will spend weeks digging a 1/2 by 4–to–6-inch hole into a tree for their nest site. We might expect that carpenter bees would be attracted to a nest that is already for occupancy.

In addition, many other species of native bees might welcome this native bee hotel.

While there will be variations in nesting by different native bee species, the common model has the newly hatched female bee emerging from its nesting place in the early spring, and busily mating and seeking a cavity for its eggs. The bee gathers pollen to stock the nest, then deposits an egg, and seals the nest with a wall of mud. The bee repeats this process so that a single cavity could include five–to–eight chambers, each with its own egg and pollen stash.

During the summer, the eggs develop into larvae, then into pupae, and finally into adults, which remain in the nest until the following spring.

To support this process, the hotel manager, i.e., the garden host, should install the nest complex in a east- or southeast-facing location, where it will enjoy morning sunlight. It could be three–to–six feet above ground for convenient observation, and near a good supply of flowering plants (preferably California natives) and mud for construction of nest chambers.

I will plant a selection of California native annuals near this nesting unit, for the bees’ easy access to pollen. I will also provide a supply of muddy clay soil, for the bees’ use. My garden’s clay occurs in a rather deep layer, so I will need to import some clay soil, and keep it moist in a container, such as a large saucer for a plant container.

A good source of seeds for pollinator flowers is the Early Blooming Beekeeper’s Mix, offered by Renee’s Gardens. This mix includes twenty-two varieties, many of which are California native plants.

Bee experts recommend moving the nesting unit into a dark, unheated garage or shed during the winter months to protect the bees from predators. The gardener should then return the unit to the garden in the early spring, well before flowers bloom. As the days warm, the gardener can watch for the new generation of bees as they emerge from the nests.

Plants Preferred by Other Insects

In a recent column, we described the first report of the Royal Horticultural Society’s “Plants for Bugs” study, which focused on Pollinators. In this column, we’ll look at the second report of this study, “Gardens as Habitats for Plant-Dwelling Invertebrates” (August 2017), which is about insects and invertebrates that live in our gardens, and the plants they love.

Let’s first review the related taxonomic issues.

Gardeners should appreciate the difference between “bugs” and “insects.” The title of the RHS study uses “bugs” to refer informally to a large and diverse group of invertebrates. To be precise, true bugs are members of the order Hemiptera, which is within the class Insecta. There are some 50,000 to 80,000 species of true bugs, all of which have sucking mouthparts. Examples of true bugs include cicadas, aphids, planthoppers, leafhoppers, and shield bugs. By the way, none of the true bugs are pollinators.  

The Insecta is a class within the phylum Arthropoda, which includes 6 to 10 million species, all of which are hexapod (six-legged) invertebrates.

To put the true bugs in perspective, they represent only a tiny fraction of the Insecta.

Returning now to the RHS study, the first report addressed garden insects that are Pollinators, which include bees, butterflies, and a few other insects.

The second report addresses other significant categories of garden insects, which include these groups:

  • Herbivores. Invertebrates that feed on living plants, using chewing mouthparts (e.g., caterpillars) or sucking mouthparts (aphids are a familiar example).  
  • Predators. These are invertebrates that eat other invertebrates. These include lacewings, beetles, e.g., ladybirds, some true bugs, spiders, and parasitoid wasps, which kill their hosts.
  • Omnivores. These invertebrates feed on both plants and other invertebrates. This group includes the harvestman (a spider relative), earwigs, and aphids.
  • Detritivores. Invertebrates that feed on decomposing organic matter. Examples include springtails, woodlice, and some beetles.
Common Lacewing (Chrysopa species) photo by JJ Harrison, shared via Wikimedia Commons

These groups, along with the Pollinators featured in the first report of the RHS study, constitute a vital component of the garden’s balanced ecosystem. In addition to breaking down dead plant material, these plant-dwelling invertebrates provide food for other wildlife, including birds, reptiles, and small mammals.

In their four-year Plants for Bugs study, the RHS scientists used suction samplers to collect about 18,000 of plant-dwelling invertebrates. Their samples included 18% Herbivores, 3% Omnivores, 18% Predators, and 61% Detritivores. They also collected about 4,700 uncategorized invertebrates.

They recorded these collections of invertebrates separately from plants that were native to three different areas: the United Kingdom (called ”natives”), other areas in the Northern Hemisphere (“near natives”, and Southern Hemisphere (“exotics”).

As they found with Pollinators, the scientists found that the native plants supported the largest numbers of the plant-dwelling invertebrates. By comparison with the native plants, the near-native plants supported about 10% fewer invertebrates, and the exotic plants supported 20% fewer.

Overall, these observations indicated that native plants are most important in supporting these groups of invertebrates, and the near-native and exotic plants also provide effective support at somewhat reduced levels.

The study concluded that gardens should emphasize native plants, but could include near-native and exotic plants as well. The most important consideration was to develop a dense planting scheme so that the garden could support all kinds of plant-dwelling invertebrates as part of a balanced ecosystem.

Exotic plants are important to include in the garden for Pollinators because Southern Hemisphere plants often bloom during months when Northern Hemisphere plants are dormant, and thus provide Pollinators with food sources for a longer period of the year.

Although this study was conducted in England, its findings could apply reasonably also to gardens of the Monterey Bay area. With that interpretation, we would treat California native plants as the “natives,” plants from the Mediterranean climate areas as “near natives,” and any other plants as “exotics.”

These two studies support the usual assessment that native plants are most supportive of local plant-dwelling invertebrates, while showing that near-native and exotic plants also provide effective supports for the garden’s ecosystem.

These findings might apply as well to local birds, reptiles and mammals, but demonstrating those relationships would require another study. For example, while berry-producing shrubs provide natural food for birds, separate counts of bird visits to native, near-native, and exotic berry-producing shrubs might yield interesting results.

That would be a very challenging assignment!

Plants Preferred by Pollinators

When planning their gardens, many gardeners recognize the relationships between their plants and local wildlife. This ecological perspective has gardening implications for each of several categories of wildlife: birds, mammals, reptiles, and various invertebrates.

An overarching concept for this ecological perspective is the food chain. Insects are low on the food chain, so selecting plants according to the needs and preferences of insects has impacts on the higher levels of the chain. As a basic example, plants that attract insects to the garden increase the food source for birds and reptiles, and an increase in the numbers of birds and reptiles provides food for carnivores.

Gardeners often want to grow flowering plants that attract bees, butterflies, and other pollinators. Those gardeners might intend primarily to support plant pollination and garden aesthetics (butterflies always delight the eye), but their plant selection also supports the food chain. 

As we develop our garden and select plants from this ecological perspective, we might favor plants that are native to our local environment. The rationale for preferring native plants reflects an assumption that the insects and other wildlife in our gardens know and prefer plants that they have encountered throughout their lives, and the lives of their preceding generations. Surely, wild creatures communicate survival knowledge to their offspring at least by demonstration.

For such reasons, I have favored native plants because of their presumed appeal to local wildlife, in addition to the compatibility of native plants with native soil, climate, and plant communities. The logical application to this view is for the gardener to fill the garden with native plants.

This approach to landscaping succeeds. We are not here to negate gardening in the Monterey Bay area with California native plants.

I have become aware, however, of another layer of thought to consider.

In a recent issue of Horticulture magazine, entomologist Eric Grissell described a wildlife gardening study, “Plants for Bugs,” conducted by England’s Royal Horticultural Society.

I visited the project’s website to learn about this project. Its first bulletin for gardeners is titled “Gardens as Habitats for Pollinators” (August 2015). This column addresses the study’s methods and findings related to pollinators.

Honey bee (Apis mellifera) pollinates creeping thistle  photo by Ivan Leidus,
shared via Wikimedia Common

The study’s approach was to observe the numbers of times pollinating insects land on flowers of plants with each of three different origins: United Kingdom natives; other Northern hemisphere natives, i.e., plants from regions similar to the UK; and Southern hemisphere natives, i.e., plants from climates and habitats that are different from the UK.

The study’s original recommendations are oriented to the United Kingdom, but they could be applied reasonably to other gardening environs. Accordingly, I have modified the wording of these statements to relate them to gardening in the Monterey Bay area.

  1. Gardeners should plant a mix of flowering plants from different countries and regions;
  2. Plant selection should emphasize plants that are native to California’s central coast, or to the Mediterranean climate areas;
  3. Regardless of plant origins, the more flowers the garden can offer throughout the year, the greater number of pollinators it will attract and support.

Plants that are native to the garden area are still important, but gardens serve pollinators best when they have large numbers of flowers and a long flowering season, regardless of the origins of the plants.

In a future column, we’ll review this project’s second bulletin, “Gardens as Habitats for Plant-Dwelling Invertebrates” (August 2017).