Did You Know?

Where do pollinators nest?

While we may think of pollinators mostly when they are visiting flowers in our gardens, they do have a life of their own. When they’re not feeding and pollinating plants, where do they spend their leisure time? There turns out to be a great variety of nesting spots for our pollinators. Most use ground burrows or wood cavities. Some have more specialised nesting strategies, such as snail shells, cliff condos or self-made aerial eyries. To live a life in the air, it seems that you still have to be well grounded.

Go to ground

Around 70% of bees nest in the ground. They lay eggs in burrows right under the surface. As a result, they need easy access to the earth for their nests. Therefore, lawns, paved surfaces and thick mulch have a negative effect on their ability to nest. Using compost instead of wood bark as mulch allows ground nesting without reducing soil quality. You might also disturb ground-nesting bees by tilling the earth, so make sure you check for their presence before you do. Look for tiny holes in the ground and if you’re patient, you may see them flying in and out. 

Some species of ground-nesting bees emerge in early spring. Fruit trees such as plum, cherry, pear and apple profit greatly from these vital pollinators. Did you know that vegetables also benefit from bee activity? Squash bees (Peponapis and Xenoglossa spp.) are American ground-nesting bees that specialise in pollinating cucurbits such as pumpkins. They nest underneath the crops they pollinate. In this behaviour, they resemble mining bees (Andrena spp.), which also burrow in the ground.

One cluster of solitary grey-backed mining bees (Andrena vaga) lived alongside each other for 60 years. These so-called nest aggregations may have as many as 10,000 nests and can reach 10 feet (3 meters) deep underground. Many other ground-nesting bees are social bees with quite extensive colonies. This includes most bumble bees that nest under the ground, preferably in abandoned rodent holes. However, some bumble bee species prefer aboveground cavities for nests. Their ideal locations include hollow trees, rock walls, bird nests and even bird houses.

Caveat emptor

Cavity-nesting bees use pre-existing holes in wood, hollow stems or stone walls. They may recycle holes made by beetles or find bamboo pieces or nesting blocks created by humans. The bees chew plants to create walls that divide the nesting materials into chambers for their brood. They then lay an egg in the chamber, leaving pollen and nectar (which are called ‘bee bread’) as food. They make up around 30% of bees, including carpenter bees.

For cavity-nesting bees to flourish, it helps to leave dead wood in the area. You can also use these materials to create artificial habitats such as nesting blocks and insect hotels. Examples of suitable plants include raspberry and other pithy stems. A bee hotel does need some timely maintenance, lest mites and disease spread in the premises. After a couple of years, it is best to dispose of such material and replace it.

When bees take over an existing location, they are called renter bees. This group includes mason bees, leaf-cutter bees and wool-carder bees. By contrast, some above-ground nesting bees create their own cavities with their mandibles. They are referred to as wood and pith nesters. Examples are large carpenter bees (Xylocopa), small carpenter bees (Ceratina) and wood-borer bees (Lithurgus spp.). One species, Trichothurgus bolithophilus, even nests in dried horse manure.

Build their own

A few bees steer clear of both methods and build their own free-standing nests. They are often called architects. The nests can comprise of resin, mud, stone, cellulose and fur. Some resin bees (Megachile spp.) create a water-resistant nest from plant sap. Serapista species glue plant fibre, animal fur and bird feathers to stems so they can nest. Many bees use a combination of such materials.

Not only bees can build their own homes. Butterflies, moths, and hoverflies, fold their brood into host plants, shrubs and trees. Therefore, it is important to time your garden clean-up accordingly. Wait until late spring (when temperatures are above 10° C) before cleaning up your garden to allow larvae that overwinter to emerge as the weather warms. Additionally, in the fall, consider minimal clean-up by leaving some plant debris, seed heads, and fallen leaves which provides essential habitat and protection for overwintering insects. As soon as the eggs hatch into larvae, the plants provide food and shelter for the young. Some Lepidoptera species are flexible in their requirement for host plant. For example, Papilio polyxenes, the black swallowtail, can pick any plant from the carrot family (Apiaceae) such as dill, fennel, parsley or wild carrot. Other species seek out just one plant. A famous example is the monarch butterfly, which will only brood on milkweed (Asclepias spp.).

As it turns out, butterflies, moths, and hoverflies can be quite particular in their preferences. It is therefore good to know which local species of plant and animal are present. If you pick host plants that are regionally native, they will attract the insects adapted to the taste. A range of plants or trees is also a good way to bring in a variety of species. You can even consult local authorities for the best plants to have for your area.

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