Successful pollinator habitat provides resources for the entire life-cycle. While pollen and nectar sources support adult bees and butterflies, you need to also provide adequate nesting habitat if you want pollinators to live in your landscape rather than just pass through. There are many ways to provide nesting resources through natural and man-made features or simply by changing land management practices. Below is an overview of the nesting needs of bees and butterflies.
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70% of native bees are ground nesting, creating burrows in soil. They need access to bare soil and may be impacted by tilling.
30% of native bees are cavity nesting such as leafcutter and mason bees. These species need cavities in dead wood, hollow stems, or brush piles.
Bumble bees create nests in cavities underground or in trees. They prefer abandoned rodent burrows or sheltered areas such as those beneath brush piles.
Butterflies lay their eggs on specific plants or within plant families. These are known as host plants, that provide food and shelter for caterpillars when they emerge.
Ground Nesting Bees
70% of native bees are ground nesting, laying their eggs in burrows just beneath the surface. Some ground-nesting bees are amongst the earliest to emerge in the spring, making them vital pollinators of cherries, plums, and other tree fruits. Another example are the squash bees (Peponapis and Xenoglossa) which nest in the soil where pumpkins and other cucurbit crops are grown and are specialist pollinators of those vegetables.
Some ground nesting bees are amongst the earliest to emerge in spring, making them important pollinators of early fruit trees and early flowering plants (Photo: Justin Wheeler). Squash bees nest in the ground beneath the crops they pollinate. Making sure they are protected from tilling is important.
Nesting Notes: Ground nesting bees need access to bare earth. Thick layers of mulch, expanses of lawn, and paved surfaces common to many suburban landscapes reduce the availability of nesting habitat for these bees. Consider mulching with compost instead of wood bark mulch - it has many of the same aesthetic, weed suppression, and water retention properties as mulch, yet allows for nesting and improves your soil! Tilling can kill ground nesting bees. Reducing, eliminating, or changing the timing and equipment used for tilling can all have a beneficial impact on ground nesting pollinators. Non-flowering buffers may be useful in preventing pesticides from leaching into areas where bees may be nesting.
Cavity Nesting Bees
30% of native bees are cavity nesting. These bees lay their eggs in beetle holes in dead wood, or create nesting chambers in hollow or pithy stems. Some species, such as leafcutter and mason bees, will also use man-made nesting blocks or cut pieces of bamboo for nesting.
The photo at left shows a cross-section of a hollow reed used by cavity nesting carpenter bees. The hollow stem has been divided into brood chambers by walls of chewed up plant material. A “bee bread” of pollen and nectar is deposited into each chamber and an egg laid upon it. In this photo you can see a developing pupa at the back of the chamber, Bee bread in a chamber where the egg did not hatch, and developing larvae (Photo: Katharina Ullman). Right - a mason bee seals a piece of bamboo with mud in a “bee hotel”. This type of material should be used with caution as it may lead to a build-up of diseases or mites if not disposed of and replaced after a reason or two (Photo: Justin Wheeler)
Nesting Notes: Where possible, allow dead trees or snags to remain in the landscape or incorporate this material into habitat areas. Commercial and backyard nesting blocks and “insect hotels” are available or can be easily made, but careful maintenance is required to limit the spread of disease and harmful mites in these materials. Plants with pithy stems, such as raspberries, provide excellent habitat for small carpenter and other cavity nesting bees.
Bumble bees need a cavity in which to build their nest. The queens are opportunists, looking for any suitably sized cavity. Sometimes this is above ground, such as in hollow trees, abandoned bird nests, rock walls, or under a tussock of grass, but they mostly nest underground. An abandoned rodent hole is a favorite, as this space is warm and already lined with fur. Bumble bees generally do not nest in the same location repeatedly unless by chance. After queens have reared their young throughout the spring and summer, they die and newly mated queens abandon the nest and hibernate over the winter beneath leaf litter or at the base of bunch grasses or in other sheltered locations.
Bumble bees use sheltered cavities to build their nests. Left, a bumble bee has found a welcoming hole in the ground, likely an old rodent burrow. At right, this colony has taken up residence in a disused bird house (Photo: Matthew Shepherd)
Nesting Notes: To benefit bumble bees, consider maintaining a small pile of brush such as hedge clippings and woody plant material. This will provide cover for wildlife that will in turn create nesting habitat for bumble bees. Where possible, leave leaf litter in gardens and allow it to build up over time. Doing so not only improves the soil, but provides cover for overwintering queens. Incorporate bunch-forming ornamental grasses where possible to provide additional cover. As with other ground nesting bees, altering tilling practices will limit the potential of harming bumble bees.
Butterflies and Moths
Butterflies and moths lay their eggs on plants, trees, and shrubs known as host plants. These plants provide food and shelter once the caterpillars emerge. Some butterflies and moths are specialists - laying their eggs on only one species of plant. The most widely known example of this type of host plant is milkweed (Asclepias spp.) which is the only plant a monarch butterfly will lay its eggs upon. Other butterflies and moths are less specific - the eastern black swallowtail, for example, will lay its eggs on any plant in the carrot family (Apiaceae), including dill, parsley, fennel, or native wildflowers of the carrot family such as golden alexander (Zizia spp.) It’s important to note that two-thirds of host plants are trees and shrubs. One oak tree for example can support more than 500 species of butterflies and moths!
Milkweed is the only plant a monarch butterfly will lay its eggs upon - no milkweed, no monarchs! (Photo: The Xerces Society / Stephanie McKnight). A well-chosen tree can support hundreds of species of butterflies and moths and also provide shelter and benefits to other wildlife (Photo: Pixabay.com).
Nesting Notes: Understanding what butterflies are present in your area is a good place to start. Planting lupines to support the endangered Karner blue butterfly may be admirable - but isn’t likely to do much good if the butterfly isn’t present in your area - other selections may be more appropriate. Select butterfly host plants that are regionally native. Providing a diversity of plants of different families is also a good strategy for supporting a diversity of butterflies and moths, and don’t forget native trees and shrubs where space permits. Encourage your local tree commission or planting authority to select native trees when installing sidewalk trees and other municipal plantings.