How to Operate a Successful Bee Hotel

Home , Eastern carpenter bee, Isodontia mexicana, Megachilinae, Monobia quadridens

Making space for wood and stem nesting bees in home landscapes, parks, and gardens

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Elsa Youngsteadt Assistant Professor & Extension Urban Ecology Specialist Department of Applied Ecology North Carolina State University

Meredith Favre Graduate Student—MS Department of Entomology and Plant Pathology North Carolina State University

DRAFT VERSION in progress and not yet peer-reviewed, Feb 27, 2021 TABLE OF CONTENTS

What is a bee hotel? ...... 1 The benefits of bees ...... 1 Who lives in a bee hotel? ...... 2 How bees and wasps use a hotel ...... 3 Meet the residents ...... 5 Family Megachilidae ...... 2 Blue orchard bee ...... 5 Other mason bees ...... 6 Alfalfa leafcutting bee ...... 7 Other leafcutting bees ...... 8 Wool-carder bees ...... 10 Hoplitis and Heriades ...... 12 Chelostoma philadelphi ...... 13 Family Colletidae ...... 14 Yellow-faced bees ...... 14 Family Apidae ...... 15 Small carpenter bees ...... 15 Solitary wasps ...... 15 Potter and mason wasps ...... 16 Thread-waisted wasps ...... 18 Square-headed wasps ...... 19 Bee hotel design and placement ...... 21 Nest tunnel materials ...... 21 Tunnel size ...... 24 Shelter and protection ...... 26 Shade and orientation ...... 27 Navigation aids ...... 29 Other landscape features ...... 32 Overwintering and cleaning ...... Draft 33 What if bee hotels aren’t really good for bees? ...... 35 Appendix 1 Bee hotel checklist ...... 37 Appendix 2 Plants that produce twigs and stems used by nesting bees in eastern North America ...... 38 Appendix 3 Plants whose leaves and fibers are used as construction materials in the nest interior ..... 39 Appendix 4 Plans for a simple bee hotel shelter ...... 40 Appendix 5 Additional resources ...... 47 References ...... 48 WHAT IS A BEE HOTEL?

Bee hotels are the insect equivalent of a birdhouse. Also known as bee condos, bee houses, or nest blocks, these structures provide nesting space for certain solitary bees and wasps. These species would ordinarily nest in hollow plant stems, holes in dead wood, or other natural nooks and crannies. A bee hotel simulates this nesting habitat by providing a bundle of hollow reeds or stems, or holes drilled in a wooden block. These nesting tunnels can be sheltered from weather and predators using a variety of shelters (Fig. 1).

Fig. 1. Bee hotels can take a variety of shapes and sizes. They may include drilled blocks (a); a combination of drilled blocks and hollow (b, d); observation nests with viewing windows (c, g); or bundles of stems and reeds (e, f). Photos: A) Jonathan Giacomini, B) Adam Dale, C & G) George Pilkington/Nurturing Nature, D-E) E. Youngsteadt, F) Jo Zimny

Managing a bee hotel can be simple and rewarding, but it does require some planning, maintenance, and commitment. This document will guide you through the process of choosing a hotel design and location, identifying its occupants, andDraft maintaining a safe environment for them. We will emphasize practices relevant to small-scale bee hotels used to enhance pollination of home gardens, to provide habitat, or for observation. Large-scale production of bees for commercial pollination is not covered.

The benefits of bees While we would likely not starve without the pollination services of bees, we would certainly not enjoy the variety of food and the amount of nutrition that we do today. Staple crops such as wheat, corn, and rice are wind-pollinated and thus do not rely on insect pollinators to reproduce. However, bees and other animal pollinators are essential for many fruits, nuts, and berries, which are packed with nutrition and provide the majority of vitamins A and C in our diet1-2. Approximately 87% of flowering plants and 75% of crops benefit from an animal pollinator for production of seeds or fruit3-4. Bees as a group are estimated to be directly responsible for 3-8% of our total food supply5-6. However, these estimates do not include indirect effects of bee pollination, such as enhancing alfalfa yield which is essential forage

1 for livestock such as cows, which are raised to produce milk or meat. Overall, our food supply would be smaller, less colorful, and less nutritious without bees.

Pollination by honey bees vs native bees While honey bees are the main pollinators in commercial agriculture, native bees and other animal pollinators also provide important pollination services and enhance pollination even while honey bees are present. Depending on the scale of the agriculture, native bees can be sufficient for pollination needs and can be more efficient at pollinating specific crops than honey bees7,8. For example, 250-300 blue orchard bees (Osmia lignaria) can pollinate the same acreage suggested for 1 to 2.5 strong honey bee hives, which on average contain about 30,000 bees per hive9.

Native bees vary in dietary habits, and this variety can make them effective as a group for pollinating a diversity of plants. Some native bees are dietary generalists, like bumble bees, which are active throughout much of the year and utilize various plants throughout the seasons. In contrast, some bees are dietary specialists and collect pollen only from select plants, sometimes only visiting species from a specific plant family. The squash bee, Eucera (Peponapis) pruinosa, is a dietary specialist and solely collects pollen and nectar from plants in the genus Cucurbita, such as squash and pumpkins. The diversity of plants which native bees visit, and their efficient means of pollination, makes them collectively at least as important as any one generalist species, like honey bees.

How bee hotels can support native bees Bee hotels support populations of solitary bees and wasp species where nesting habitat is limited. Native bee populations depend on the availability of appropriate nesting sites, materials for nest construction, and food resources for themselves and their developing brood within their foraging range10. While honey bees and larger bees, such as carpenter bees, can fly several miles from their nesting sites, smaller native bees tend to stay close to home10-11,12. Solitary bees commute between foraging sources and nests multiple times per day11. Urban or agricultural development can create longer distances for bees to travel between nests and foraging resources. The extra travel time then reduces the amount of pollen a mother bee can collect to feed her offspring11,13, and leaves the unguarded nest vulnerable to parasites14. Several studies suggest that bee populations or pollination services can increase after nesting resources are added to a habitat15-17.

Bee hotels, while supporting native bee and wasp populations, are also entertaining and provide opportunities to learn about pollinators close up. By installing these nesting sites in our yards and observing their residents, we can learnDraft about native pollinator diversity and how the variety of food and flora we enjoy in our gardens, markets, parks, and green spaces are linked to pollination services provided by hotel residents. Bee hotels are also incorporated into some citizen science projects and create an opportunity to assist with research on native bee ecology and behavior.

Who lives in a bee hotel? North Carolina is home to about 560 species of bees, each with its own nesting requirements. Bee hotels cater to a small fraction of this overall diversity. Although they’re called bee hotels, many species of solitary wasps also use these nesting spaces. Quite unlike yellowjackets and paper wasps, these wasps are mild-mannered and beneficial neighbors (see page 15).

Bees differ in their nesting habits. Different species may nest in soil, in plant parts (such as hollow stems or wood), or in assorted other voids and cavities (such as rodent burrows or stone walls). They may

actively dig their own nesting tunnels (builders), occupy pre-existing spaces (renters), or lay their eggs in other bees’ nests (parasites).

Figure 2 summarizes the nesting habits of North Carolina bees according to where and how the nests are made. Bee hotels attract species that rent or parasitize nests in stems and wood—at most about 100 species, or 18 percent of the total. The vast majority of species—about 75 percent—nest in the soil and wouldn’t consider occupying a bee hotel! Honey bees and bumble bees use larger cavities and are not attracted to bee hotels.

Figure 2. Of the 558 species of bees recorded in North Carolina, Figure X shows the percentage using each nesting strategy. (Larger bubbles represent more species.) Bees that nest in stems and wood are the species most likely to occupy a bee hotel.

How bees and wasps use a hotel The occupants of bee hotels are entirely solitary—that is, they do not form colonies like those of honey bees, bumble bees, or paper wasps. Instead, each nest is “owned” by a single female, who lays her own eggs and gathers all the food needed for each offspring. There are no workers and no queen. When a bee or wasp occupies a nesting space, even if she’s just a “renter,” she modifies the interior. She may line it with leaf pieces or glandularDraft secretions, and she usually constructs a separate chamber for each egg. The chamber contains one egg, and enough pollen and nectar for that offspring to develop into an adult.

Figure 3. Cross section of a nesting tunnel in a bee hotel. Construction begins at the back of the nest (left) and proceeds toward the entrance. The oldest offspring are toward the back, each in a private chamber with enough nectar and pollen to develop into a mature larva and finally a pupa. (Some bee larvae spin cocoons before they pupate, as shown here.) The mother bee continues to build new chambers and lay new eggs for as long as she can, and often uses more than one nest tunnel during her lifetime. All stages are shown here for illustration, but a real nest would contain offspring that are more similar in age.

Some species have just one generation per year. You’ll see the adults of these species actively nesting in the bee hotel for only a few weeks. The rest of the year, those nests will appear inactive. In fact, the offspring are still inside, waiting for the right time to emerge and make their own nests the following year. An intact plug or cap at the nest entrance, often made of leaves or mud, means someone is still inside!

Nesting spaces are not a situation where one size fits all. Each stem- or wood-nesting species requires a tunnel of a specific diameter, so the diameter of the tunnel determines which species can nest there. Moreover, each species uses different materials to remodel the nest interior, so even if the tunnels are right, a bee won’t use a nest if she can’t find the right construction materials nearby. It’s not always practical to add all possible nesting resources to a habitat, but understanding the requirements of different species can help you attract the ones you want, or understand why other species may not show up. Appendix 3 lists plants that provide interior construction materials for bees.

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MEET THE RESIDENTS

The accounts below highlight some of the common occupants of bee hotels in North Carolina and their nesting requirements. Months highlighted in the calendar strip indicate when adults are most often active (foraging and building nests).

Family Megachilidae Most bee species that occupy bee hotels are members of the family Megachilidae, a group that includes mason bees, resin bees, leafcutting bees, and wool carder bees. Members of this family share a noteworthy characteristic: The female bees transport pollen on a brush of hairs (scopa) on their belly (Fig. 4), rather than on their legs, as most other bees do.

Fig. 4. Megachile female with a full load of bright yellow pollen on her scopa. Photo: Debbie Roos.

Blue orchard bee (Osmia lignaria) One of the most recognizable residents of bee hotels is the orchard mason bee, also known as the blue orchard bee (BOB). This native species has been managed commercially in North America since the 1990s18. Although they visit many kinds of flowers, BOBs have a strong preference for flowers of spring- blooming fruit trees and redbuds. BOBs are reared for commercial pollination in large nest blocks and are transported to spring-flowering crops such as almonds, apples, cherries and peaches. Although BOBs are widely available for purchase through garden and bee supply catalogs, home gardeners rarely need to purchase the bees. If conditions are suitable, local wild populations will occupy bee hotels. Draft Jan Feb March April May June July Aug Sep Oct Nov Dec

• Generations per year one • Favorite flowers in NC apple, ash, blackberry, boxelder, oak, redbud, willow19,20 • Natural nest substrate abandoned tunnels drilled by other insects (often beetles) in deadwood or soil21-23 • Construction materials Mud • Tunnel diameter ideal: 19/64” (7.5 mm); range: (1/4” – 1/2” (6 – 13 mm)20,21,24

• Body size

• Recognition Dark metallic blue color, relatively large body size (compared to other mason bees), and use of mud partitions

Fig. 5. Blue orchard bee female foraging on raspberry flowers. Photo: Jim Cane.

Fig. 6. Interior of a BOB nest tunnel, showing mud construction, pollen provisions, and eggs. Photo: Jim Cane

Other mason bees (genus Osmia) In North Carolina, the genus Osmia includes not only the BOB, but 21 other related species, of which 18 are native and 3 are introduced25. Several of these species are common residents in bee hotels. In the eastern United States, populations of native mason bees (including the BOB) have declined over the past two decades, while the introduced Taurus mason bee (Osmia taurus) has become more abundant25,26.

Jan Feb March April May June July Aug Sep Oct Nov Dec Draft • Generations per year one • Favorite flowers in NC varies by species; most have broad diets, several are common on white clover • Natural nest substrate abandoned tunnels drilled by other insects in deadwood or soil; hollow stems27,28 • Construction materials varies by species; mud, leaf pulp, small pebbles, sand grains, and (for one species) wood fibers scraped from the nest interior27,28 • Tunnel diameter 3/16 – 5/16” (3.4 – 8 mm)21,28,29

• Body size

• Recognition Most native Osmia species are metallic dark blue to blue-green; the two most common introduced species (Osmia cornifrons and O. taurus) are golden-brown in color and similar in size to honey bees and BOBs.

Fig. 7. A small native mason bee at her nest entrance in a woodpile.

Fig. 8. Nest interior of Osmia pumila, a small native mason bee who uses chewed leaf pulp to build the nest partitions.

Alfalfa leafcutting bee (Megachile rotundata) The alfalfa leafcutting bee (ALB) is another one of the few solitary bee species managed intensively for commercial agriculture30. Originating from Eurasia31, the ALB was introduced into the US by 1940s32, and significantly augments alfalfa seed production through its pollination services15. Alfalfa is grown globally for hay to feed livestock, especially dairy cows. ALBs are efficient pollinators of alfalfa16 and are also used in western Canada for producing hybrid canola seed. They have been reported to pollinate species 33-34 Draft35 36 of annual clover , lowbush blueberries , and cranberries , and can be used to pollinate crops in confined spaces, such as glasshouses16,37 and cages38-39. They also forage on flowers such as sweet clover (Melilotus officinalis).

ALBs are active during summer months and will nest in hotels of various shapes and sizes. Hotels can be 13 9 40 made of wood, with evenly spaced holes between ⁄64 and ⁄32 inches in diameter .

Figure 9. The alfalfa leafcutting bee Megachile rotundata. Photo: Ilona L, Flickr.

Fig 10. Alfalfa leafcutting bee foraging on alfalfa. Photo: Wikipedia User Pollinator

Other Megachile Draft Megachile species, also known as leafcutting bees, get their name from their use of leaves to outfit their nests. There are 37 known species in NC (a few of which are introduced), with the most famous being the alfalfa leafcutting bee featured above. Leafcutting bees are diverse in nesting habits and foraging material. They commonly cut circles out of leaves from plants such as roses, redbud, and grape to line their nests17, while others use resin or chewed leaf pulp. (See Appendix 3 for a list of plants used.) Many Megachile species have multiple generations per year.

Jan Feb March April May June July Aug Sep Oct Nov Dec ● Favorite flowers in NC asters, legumes, sunflowers, evening-primroses41 ● Natural nest substrate hollow stems, holes in dead wood, cracks and crevices; some nest in the ground

● Construction materials resin, petals, leaves, leaf pieces, or leaf pulp, sometimes mixed with mud or sand. Favorite leaf species include roses, redbud, and grape; see Appendix 3. ● Tunnel diameter 13⁄64 – 5/16” (5 – 8 mm)

● Body size

Fig. 11. Female leafcutting bee foraging at milkweed. Notice the pollen collecting-hairs (scopa) loaded with pollen on the underside of her abdomen. Photo: Peter Pearsall/FOMR

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Fig 12. Female leafcutting bee lining her nest with leaves. Photo: Bernhard Plank

Figure 13. Nests of leafcutting bees, removed from their nesting tunnel. Photo: Subbu Subramanya

Wool carder bees and potter bees (Anthidium) Wool carder bees and potter bees use plant fibers, mud, resin from conifers, and even pebbles to build 42 Draft their nests . Their nesting cavities often have the appearance of being stuffed with wool (actually plant fibers), which makes them easy to identify. There are only 2 species known in NC, one of which is native.

Jan Feb March April May June July Aug Sep Oct Nov Dec

● Favorite flowers in NC deadnettles, legumes, mints, and figworts43 ● Construction materials plant fibers, mud, resin from conifers, pebbles42 ● Tunnel diameter 3/8 – 5/8” (9.5 – 15.6 mm)

● Recognition These bees are yellow and black. Males can be distinguished by their yellow faces. Female Anthidium species, like other members of the family Megachilidae, carry pollen using hairs located on the underside of their abdomens, giving them the appearance of having yellow bellies as they are out foraging.

Fig 14. Anthidium manicatum foraging. Photo: Gail Hampshire (Flickr)

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Fig 15. Anthidium manicatum female foraging plant fibers from lamb’s ear (Stachys) for nesting materials. This species is invasive in the United States. Photo: Jacy Lucier

Hoplitis and Heriades These two genera of resin bees and mason bees are closely related to Osmia mason bees. They are small and relatively understudied as pollinators. Ten species total (7 Hoplitis, 3 Heriades) occur in NC and may occasionally turn up in bee hotels. [All traits from 21,28] Jan Feb March April May June July Aug Sep Oct Nov Dec

Hoplitis Heriades Favorite flowers some species are common on clover; various in NC others are specialists on Nemophila (baby blue eyes), Phacelia, or Amorpha (false indigo bush) Natural nest abandoned tunnels drilled by other pithy stems or twigs substrate insects in deadwood; hollow or pithy stems Construction varies by species; mud, leaf pulp, resin, sand materials pebbles Tunnel diameter 1/8 – 1/4” (2.8 – 6.4 mm) Body size

Draft Fig. 16. Male Hoplitis. Photo: Daniel S. Cooper

Fig. 17. Female Heriades closing her nest entrance with resin. Photo: G. Bohne

Chelostoma philadelphi This small mason bee is the only member of its genus in North Carolina. Mother bees collect pollen from mock orange (Philadelphus), and possibly other plants, to feed their larvae44-46. At least in Raleigh, NC, this species is a fairly common urban resident, but its life history is not well studied.

Jan Feb March April May June July Aug Sep Oct Nov Dec • Generations per year one • Favorite flowers in NC mock orange (Philadelphus), raspberry46,47 • Natural nest substrate abandoned tunnels drilled by beetles in deadwood47 • Construction materials mud, leaf pulp, small pebbles21,47 • Tunnel diameter 1/8” (2 - 3 mm)21,47 • Body size

Family Colletidae

Yellow-faced bees (Hylaeus) Yellow-faced bees get their name from the bright yellow coloring on their faces. These bees are quite small and wasp-like in appearance, lacking pollen-collecting hairs found on other bee species. Instead, they carry pollen and nectar back to nests in their crop (an organ like a stomach). Yellow-faced bees build their nest partitions out of “spit” that hardens into a cellophane-like membrane. Fourteen species of Hylaeus have been found in NC. Draft

Jan Feb March April May June July Aug Sep Oct Nov Dec ● Favorite flowers in NC general; many wildflowers ● Natural nest substrate hollow stems, with and without pith ● Construction materials spit ● Tunnel diameter 1/8 – 5/16” (3 – 8 mm)

Figure 18. Male and female Hylaeus bees. Males can be distinguished from females by the more extensive yellow coloration on their face. Photo: Judy Gallagher

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Fig 19. Hylaeus bees emerging from their nest. Photo: Rob Cruickshank

Family Apidae

This large and varied bee family includes honey bees, bumble bees, and carpenter bees, among others. Most members of this family in NC nest in the soil, and thus do not use bee hotels. Some bumble bees (genus Bombus) do nest in aboveground cavities such as bird houses or wall voids, but they are not attracted to bee hotels. In the eastern US, bumble bees rarely use purpose-made nesting structures, although a couple of species regularly occupy birdhouses that contain old birds’ nests.

Large carpenter bees (genus Xylocopa) are notorious for drilling their own nesting tunnels into wood. They occasionally use bee hotels if these include large-diameter holes (at least 3/8” or 1 cm)48. Even so, a bee hotel is unlikely to lure carpenter bees away from more spacious nesting opportunities in decks, porches, and sheds.

Fig. 20. Large carpenter bee at her nest entrance in a fence post—not a bee hotel.

Small carpenter bees (Ceratina) Small carpenter bees are close cousins of large carpenter bees, but differ in size and habit. Six species occur in NC, and the largest are barely more than 1/4” long. They cannot damage wooden structures. Instead, they nest inside stems with soft, pithy centers; the bees burrow into the pith to make nesting space. They can only use stems that are already damaged to expose the pith.

Because small carpenter bees use pith itself as a construction material inside the nest, the empty blocks and reeds used in most bee hotels are not attractive to them. However, by incorporating pithy stems as nesting materials in the hotel, or by simply managing suitable plants on your property, you may be able to support and observe these species.Draft They may prefer stems that are vertically oriented28.

Jan Feb March April May June July Aug Sep Oct Nov Dec

• Favorite flowers in NC small carpenter bees visit a wide variety of flowers, including cucurbits, blackberries, mustards, and others • Natural nest substrate pithy stems such as elder, sumac, rose, blackberry and raspberry; selected stems are usually vertical • Construction materials pith from stem interior • Tunnel diameter 1/8 – 5/16” (2.2 - 7 mm)28 • Body size

• Recognition Small carpenter bees are metallic blue to blue-green, similar in color to many mason bees. The slender body shape and relative lack of hair sets the small carpenter bees apart. You’ll never see them carrying pollen on their bellies—only on their legs. While foraging away from the nest, they might be confused with metallic sweat bees, but the latter do not nest in bee hotels.

Fig. 21. Small carpenter bee foraging. Photo: Judy Gallagher

Fig. 22. Interior of a Ceratina nest; pollen provisions, eggs, and partitions made of pith scraped from within the stem. Photo: Gideon Pisanty

Solitary wasps You might notice some wasps nesting in your bee hotel. Native solitary wasps are pollinators as well as predators of garden pests such as caterpillarsDraft and crickets, which they paralyze and store in cavities to feed their young42. These solitary wasps are less aggressive than their social counterparts (such as yellowjackets, paper wasps, and hornets) and thus are less likely to sting you than the caterpillars.

Cavity-nesting wasps are diverse and come in a variety of sizes, shapes, and colors. The most common wasps seen in bee hotels in the southeastern US include mason wasps and potter wasps (Eumeninae), thread-waisted wasps (Sphecidae), and square-headed wasps (Crabrionidae)48. Wasps tend to appear later during the year than native bees, predominantly during the summer months (May-August), and typically have multiple generations per year21,48.

Potter and mason wasps Potter and mason wasps get their name from their use of sand and mud to construct their nests. These wasps are in the same family (Vespidae) as the social wasps (hornets, yellowjackets, and paper wasps),

but belong to a distinct subfamily (Eumeninae). Potter and mason wasps typically have two or more generations per year42. Many are predators of leaf-eating caterpillars, such as leaf-rollers and tiers, although some feed on beetle larvae42. They vary widely in body size, color, and hole size preferences for making their nests. For example, species like Pachodynerus erynnis and Ancistrocerus species prefer to nest with hole diameters of 3/16 – 1/4 inches (4.8 - 6.4 mm), while larger wasps like Monobia quadridens, the largest euminine wasp in the eastern US, use 1/2 inch (12.7 mm) diameter holes42,48. In general, potter and mason wasps are active in NC from May through October48.

Listed below are mason and potter wasp species commonly observed in NC.

Ancistrocerus antilope Photo: Judy Gallagher Euodynerus megaera Photo: Judy Gallagher

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Pachodynerus erynnis (Red and Black Mason Wasp) Photo: Judy Gallagher

Monobia quadridens (Four-toothed Mason Wasp) Photo: Robert Webster

Jan Feb March April May June July Aug Sep Oct Nov Dec

● Natural nest substrate old carpenter bee holes, mining bee cavities in clay42 ● Construction materials mud, sand ● Prey caterpillars, beetle larvae ● Tunnel diameter 3/16 – 1/2” (4.8 -12.7 mm)

Thread-waisted wasps Thread-waisted wasps (family Sphecidae) are distinguished by their extremely thin connection between their abdomen and thorax, even by a wasp's standards! One distinctive group of sphecids is the grass- carrying wasps (Isodontia), whose nests are easily identified by tufts of grass protruding from their entrances. Six species are found in North America, three of which have been observed nesting in bee hotels42. Isodontia mexicana is common in NC and can be seen flying while carrying grass blades to their nests. These wasps are a shiny black in appearance, with smoky red-brown wings in addition to their family’s signature thread-like waist. Another small sphecid wasp found in the eastern US is Solierella plenoculoides, which naturally nests in goldenrod galls49 and uses diverse materials to line the nest, such as sand, bark, twigs, pebbles48.

Sphecid wasps prey on nymphs and adults of crickets, katydids, and seed bugs (lygaeid nymphs)42. S. plenoculoides are active slightly earlier in the season, starting in April up until July, while I. mexicana is typically observed May through August48.

Jan Feb March April May June July Aug Sep Oct Nov Dec

● Natural nest substrate hollow stems, abandoned carpenter bee nests for I. mexicana50 ; goldenrod galls and other man-made cavities for S. plenoculoides48 ● Construction materials grass, twigs, sand, bark, small pebbles48 ● Prey crickets, grasshoppers, seed bugs 48 ● Tunnel diameter 1/4 – 1/2” (6.4 - 12.7 mm)

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Fig 24. Grass-carrying wasp Isodontia mexicana. Photo: Dan Mullen

Fig 25. Isodontia mexicana nest, with grass tufts in cavity entrance. Photo: Julien Hoffmann

Square-headed wasps (Trypoxylon) Wasps in the genus Trypoxylon are also known as square-headed wasps or key-hole wasps. Eight Trypoxylon species use cavities in the eastern US. These wasps hunt spiders to provision nests for their developing larvae42. T. politum, the organ pipe mud dauber, is the most well-known wasp belonging to this group, although it is unique in that it constructs nests from mud, rather than seeking pre-existing cavities. Square-headed wasps typically nest in empty nests of mud daubers or paper wasps, capping entries with mud once a cavity is finished48.

Jan Feb March April May June July Aug Sep Oct Nov Dec

• Natural nest substrate abandoned beetle borings, old mud dauber and paper wasp nests51 ● Construction materials mud ● Prey spiders ● Tunnel diameter 1/8 – 1/2” (3.2 - 12.7 mm)48

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Fig 26. Trypoxylon wasp. Photo: Hectonichus (Wikimedia)

Fig 27. Trypoxylon collinum wasp. Photo: M Whalen

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BEE HOTEL DESIGN AND PLACEMENT

There is a tremendous variety of bee hotels (and bee hotel plans) available online, and it can be hard to choose a good one. Much of what we know about artificial nest design comes from research in just a few commercially managed species, or short-term studies of other species. Some common recommendations are based on expert opinion, which may be sound, but lacks a documented evidence base. This patchy knowledge can be frustrating, but it also means there’s room to innovate and tinker with your own bee hotel. To help you make informed and logical decisions, this section explains key considerations for bee hotel design and placement based on research and our experience in Raleigh, NC.

Nest tunnel materials The functional core of a bee hotel is the nesting material itself. This can take four basic forms—drilled blocks, grooved boards, observation nests, and bundled tubes. Each has advantages and disadvantages, described below. Some bee hotel shelters include multiple cubbies that hold different nesting materials, so you don’t have to make an either-or decision among the four types. In fact, providing a variety of nesting materials may help attract a greater diversity of bees52,53. Pinecones, loose bark, and moss, however, are not (or very rarely54) used by nesting bees and wasps. Even though you’ll see these materials in some commercial insect houses, we don’t recommend them in a bee hotel55.

Regardless of the type of nesting material you choose, keep your bee hotel structure modest in size, fewer than about 100 nesting tunnels. Research in blue orchard bees shows that nest boxes with more than 100 holes are less attractive56,57. In contrast, introduced alfalfa leafcutting bees in commercial fields prefer very large, conspicuous nesting structures with tens of thousands of tunnels58. Most native species, however, are not so gregarious59. Although an optimal bee hotel size has yet to be determined, logic and ease of maintenance suggest that smaller structures will appeal to more species and be more successful.

Drilled blocks are made by drilling holes into chunks of logs, branches, or untreated wood. These are often quite attractive to nesting bees and wasps, but are difficult to clean or inspect unless each hole is lined with a paper or cardboard insert. The nest tunnels don’t go all the way through the block; each tunnel has an open end and a solid, closed end. Nest entrances should be smooth and free of splinters that could deter bees or damage their wings55. Draft

Fig. 28. Drilled “blocks.” Photo: Manfred Richter, Pixabay

Grooved boards, when stacked, simulate drilled blocks, but facilitate seasonal cleaning and inspection because each nest tunnel can be exposed by unstacking the boards. (Opening the grooves damages active nests, and should only be done in the winter when nests are complete and bees are dormant.)

Grooves usually go the entire length of the grooved board, and the stack of boards needs to be placed with one end snug against a vertical surface to effectively close one end of the tunnels. In our experience, grooved boards are less attractive than drilled blocks or bundled tubes, but other users have reported success55,60,61.

Fig. 29. Grooved boards stack together to form round nesting tunnels, here lined with paper tubes. Photo: Scott Famous

In both drilled blocks and grooved boards, the nest tunnels should be at least 3/4” from the edge of the block or stack. Following this guideline reduces parasitism by the wasp Leucospis affinis, a common parasite of mason bees, leafcutting bees, resin bees and, occasionally, solitary wasps62,63. A Leucospis female approaches the nest of her host from the outside, and can use her ovipositor (egg-laying appendage) to penetrate up to 3/4” through the nest wall to lay an egg inside a bee’s nest chamber24. The wasp herself is only about 0.4” long at most, but she can extend her ovipositor about twice her own body length into solid wood, a process that requires about 10 minutes to lay a single egg24,64. Once the Leucospis larva hatches, it feeds gradually on the living bee larva for a few days until only an empty husk remains64. Draft

Fig. 30. Female Leucospis affinis wasp. You can see the sheath of the ovipositor lying lengthwise across the top of her abdomen like a sword or needle. The wasp herself is just over 1/4” long, but she can extend her ovipositor up to 3/4” through solid wood to lay an egg inside a bee nest. Photo: Matt Bertone

Numerous other species of flies, wasps, and mites can also parasitize bee nests; in many cases, parasite prevention is an issue of maintenance, rather than design (see page 33).

Observation nests consist of a single, deeply grooved board covered by a plexiglass viewing panel. The plexiglass is concealed behind a door to keep the nest interior dark except when someone wants to look inside. Observation nests can be a great opportunity to peek at nesting activity in real time. Of course, they’re more expensive to purchase (or more complicated to construct), and generally offer only one or two rows of nests.

Fig. 31. Observation nest exterior (left) and view behind the door (right). Photos: George Pilkington.

Bundled tubes can include a variety of purchased or found plant stems. Nesting tubes are closed at one end. Reeds or bamboo, cut at a node, are popular because they naturally have one open end and one closed end. Pre-cut reeds, sorted by diameter, are commercially available for use in bee hotels. Hollow and pithy stems and twigs can also be harvested from plants in a home landscape. Appendix 2 lists plants whose stems are used by nesting bees in eastern North America.

DraftFig. 32. Bundled tubes can include harvested and purchased stems (left) and twigs (right). Photos: E. Youngsteadt and Pixabay

If you collect pithy stems for a bee hotel, you can leave the pith intact or you can ream it out, leaving a short plug of pith (maybe 1/2”) to seal the back of the tube. Expect different species to use reamed and un-reamed tunnels55. Bees and wasps use dried stems, not fresh green ones, and cut ends should be free of splinters or ragged edges65.

Do not use cardboard, paper, glass, or plastic tubes in nesting bundles. Drinking straws can be tempting because they are about the right size, but nests in plastic straws are often ruined by mold, while those in paper straws are often parasitized60,66,67. Glass tubes also accumulate moisture and mold21. Paper and cardboard tubes are commercially available for use in bee hotels, but, because they are so susceptible to parasitism, we do not recommend using them in bundles60,67. Paper and cardboard tubes can, however, be used to line drilled blocks or grooved boards. In this setting, the tubes are well protected from parasites, and they facilitate seasonal cleaning of the drilled blocks.

Fig. 33. Sumac, sunflower, and raspberry are among the plants that produce pithy stems used by nesting bees. These can be used in the landscape to promote natural nesting (bees use damaged twigs or the previous year’s dead perennial stalks), or twigs and stems can be harvested and bundled for use in bee hotels. Photos: Anne McCormack, J. Biochemist, Lucy Bradley.

Best practices for nest tunnel materials • Use only natural, breathable materials such as wood, twigs, and plant stems, not plastic or glass. (A plastic panel in an observation nest is OK when the rest of the nest is wooden.) • All tunnels should have smooth entrances, free of splinters or cracks. • Paper and cardboard tubes are OKDraft to line drilled wooden tunnels, but not as stand-alone materials. • Avoid pinecones, moss, and bark, which are not generally used by nesting bees and wasps. • Keep the total number of tunnels in a bee hotel under about 100 • In drilled blocks and grooved boards, nests should start at least 3/4” from the edge of the block or stack

Tunnel size

Diameter. Whether you use drilled blocks, grooved boards, observation nests, or bundled tubes, the diameter of the nesting tunnel determines which species can use it. Bees use nests whose diameter approximately match their own body size. Tunnel size also limits the size of the offspring that develop inside68. Bees who can’t find an optimal tunnel will use other sizes, but these nests are generally less successful69.

Nest diameter affects offspring size and sex in a few ways. First, bees who develop in nest chambers stocked with more food grow up to be bigger adults21,70. Smaller tunnels hold less food, so they produce smaller bees. Second, even in the best of circumstances, male bees are usually smaller than females. The mother bee controls when she’s going to lay a female egg (which is fertilized) versus a male egg (which is not). When faced with a smaller nest, she fills it with the better-fitting offspring: males. Similar patterns are documented in several species of mason bees, leafcutting bees, and wasps24,68,71-74. If only too-small tests are available for a given species, the small, male-skewed brood can be problematic for the overall population. Smaller individual bees are less likely to survive and reproduce74, males are usually less effective pollinators68, and a male-skewed population may not have enough nest-making moms to maximize the next generation.

Meanwhile, too-wide nests have their own problems. Mother bees have to collect more construction materials and modify the interior design to make the nest fit better (thereby wasting time that they could be using to collect pollen and lay eggs)68,75. In some cases, the modified interiors of wide nests may be linked to lower survival of the developing larvae71.

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Fig. 34. Bees will occupy nest tunnels that range in diameter from just over 1/16 inch to about 1/2". Ranges represent the sizes known to be used by species native to NC. Species-specific ideal diameters for the blue orchard bee (Osmia lignaria, 7.5 mm, 19/64”) and alfalfa leafcutting bee (Megachile rotundata, 6 mm, 15/64) are also shown. *Ceratina is shown here for comparison, but requires pithy stems and is not found in wooden blocks.

Length. As with diameter, ideal nest length varies among species. For example, research with alfalfa leafcutting bees find nests of 2-3” long to be most efficient68, while blue orchard bees (and their similarly sized relatives) do well with 6” nests24,75,76. A bee forced to use the “wrong” nest length may produce fewer offspring, or fewer female offspring.

Too-short tunnels have two main disadvantages. First, the mother bee may construct only one or two nest chambers in the tunnel before she has to waste time and energy to seal it up and search for a new tunnel68. Second, mother bees fill their nests from back to front, with female offspring in the back and males near the entrance. When a nest-building mom senses that she is nearing the entrance, she switches to laying male eggs73. If a nest is too short, she lays fewer female eggs before she reaches the male zone near the entrance68,73,75. The downsides of a too-short nest are similar to those of a too- narrow nest.

Longer isn’t always better, either. For example, alfalfa leafcutting bees waste space when they nest in tunnels longer than 2-3”, because they only use short sections of each tunnel68. Moreover, when these bees build long nests with many chambers, the offspring that emerge in the very back can have trouble getting to the entrance through all the other nest parts, debris, or dead siblings that block their way68.

Best practice for tunnel size • To support a diversity of species, and balanced sex ratios within species, provide a variety of tunnel sizes from about 5/64” (2 mm) to 1/2” (12.7 mm) in diameter. For some bees, increments as small as 1/64” can make the difference between a great nest and a mediocre one. • To target specific species (such as the blue orchard bee), use the specific hole diameter they prefer. • A good tunnel length is about 6”, or a range of lengths from about 3” to 8”.

Shelter and protection Nesting tunnels need to be protected from rain and, often, from predators. Shelter designs range from utilitarian to artistic, and there is plenty of room for creativity (Fig. 1). A few basic elements will maximize the safety of your nests in any design.

Excess moisture can lead to mold problems or deterioration of nest materials. To avoid this concern, shelters should overhang nest entrances by at least a couple of inches to protect from rain, and you can put drain holes in the bottom of any shelter (not the nests themselves) to prevent water accumulation65. The shelter and its overhanging roof also provide some shade to help regulate nest temperatures.

Birds can destroy a bee hotel by pulling tubes out of it or breaking nests open65,77. In the Piedmont, we have seen unprotected reed nests suffer near 100% mortality from woodpeckers. Simply placing nesting materials inside a shelter helps, and may be sufficient in some habitats. If you still find nest bundles torn apart, or paper liners pulled out of wooden nests, you can attach a piece of chicken wire over the front of the shelter for protection. We areDraft not aware of an ideal mesh size that excludes predators but allows bees to pass freely without damaging their wings. Experienced bee ranchers simply recommend using the largest chicken wire you can find, and waiting until you know you have a bird problem before you actually use it18. There need to be a couple of inches of space between the chicken wire and the nest entrance to make sure birds can’t reach through the wire.

Fig. 35. This bee hotel illustrates several desirable design features. It has a generous roof overhang to protect nests from rain, and a hardware cloth front to exclude birds. (We do worry that this mesh is a little small for some bees to fly through, though.) It also incorporates two navigation aids (detailed below). It is attached to the side of a larger building, which helps it stand out to bees finding their way home from foraging trips, and it includes nest blocks of different shapes and depths, which helps each bee recognize her own specific address. Photo: Peter, Pixabay.

Best practices for shelter and protection • Always place nesting materials within some kind of shelter. • To protect from rain and moisture, the shelter should overhang the nest entrances, and have a couple of drain holes in the bottom. • If (but only if) birds, rodents, or other predators become a problem, attach a sheet of chicken wire over the nest entrance.

Shade and orientation The climate inside a bee hotel depends partly on the type of shelter and nesting materials, but also on the amount and timing of sunshine it receives. On a sunny afternoon, the temperature inside a nest can easily reach 18 °F (10 °C) hotter than air temperature78, sometimes up to 40 °F (25 °C) hotter than air temperature79. Nest orientation makes a big difference to nest temperature; for example, nest tunnels facing southwest have reached temperatures up to 8 °F (4.5 °C) hotter than identical, adjacent tunnels facing northeast78. Draft

A warm nest can be a good thing, up to a point: For insects, warmer temperatures usually speed development and increase activity (which is why, for example, you can tell the temperature by counting cricket chirps). However, at some point, too-hot temperatures become lethal. Adult bees may be able to avoid dangerous temperatures by moving away from them, but brood inside a nest cannot escape. To ensure the safety of her young, it’s important that a mother bee choose a nest site that gets the right amount of sun at the right time of day.

Fig. 36. The mason bees (Osmia pumila) developing in this nest died as larvae; note the dead, dried-up larva visible in the middle cell. This outcome is common when nests overheat in the afternoon sun.

In North America and Europe, standard practice is to place nest materials inside shelters in sunny locations, with the nest entrances facing south or southeast (toward sunrise)24,29,30,55,80. The rationale is that south- or southeast-facing nests are warmed earlier by the morning sun, but spared the brunt of the afternoon sun18,24,59. This arrangement should provide overall more hours of foraging activity (for adults) while avoiding dangerous heat (for brood)24,59.

However, ideal nest placement may differ regionally, or for different species of bees and wasps. Studies in English and German cities found that bee hotels in sunny locations were more than twice as likely to be occupied than those in shady locations80,81. In contrast, shaded nests were better occupied than sunny ones in Brazil and Costa Rica82,83, and alfalfa leafcutting bees preferred nests in the shadiest parts of shelters in North Dakota78. In Canada, some species of beneficial wasps preferred shade, while others were indifferent84. These studies all used different kinds of nest materials and shelters, however, making comparison difficult. Research on sun versus shade has not been done in North Carolina or other areas with similar climates, and our own observations suggest that at least some species in the piedmont do like a partly shaded nest site. Shelters in very deep shade in wooded areas are unlikely to be occupied, but afternoon shade is probably a good idea in our region21.

Evidence for the ideal compass orientation of a bee hotel is also mixed. Although blue orchard bees seem to agree with the standard southeast-facing orientation61, a recent study of alfalfa leafcutting bees found that the mother bees made more nests and laid more eggs in tunnels that faced northeast78. However, this study didn’t look at the survival of the offspring, so we can’t be sure the mother bees really made the best choices. Different species within the same habitat may choose different nest orientations. One study in Brazil noted that, although different species chose different orientations, none of them preferred nest entrances facing into the wind82. Both sunlight and wind may therefore influence the ideal nest orientation. In NC, prevailing winds are generally from the southwest, and blow least often from the southeast and northwest85. In this case, the conventional southeast orientation also avoids the prevailing wind. Draft

Fig. 37. Wind rose for the weather station at the Raleigh-Durham airport from 1948 to 2013, illustrating that prevailing winds in this region blow from the southwest; nests facing northwest or southeast would be most sheltered. You can check prevailing winds at a weather station near you using the website of the NC Climate Office. [Image: NC Climate Office]

Given patchy and inconsistent evidence, shade and nest orientation are areas where the bee hotelier can reasonably tinker. If you are inclined to make multiple bee hotels, you could try shaded and sunny locations, or different compass orientations, to see which are preferred in your own landscape.

Best practices for shade and orientation • It is customary to place bee hotels in a sunny location with nest entrances facing southeast. This has worked fine for decades and remains the default placement. • Nest entrances facing away from prevailing winds may be preferred. • Ideal nest placement and orientation are probably different across species and regions; there is plenty of room to try different combinations and see what works best in your own landscape.

Navigation aids A mother bee regularly flies to foraging locations hundreds of yards (or farther) from her nest, then returns to the exact 1/4” of real estateDraft that is her nest entrance. Impressive feats of navigation are common in insects, and have been best studied in honey bees, ants, and wasps. From these species, we know that bees use landmarks to recognize their nest, familiar routes, and foraging locations86. Bees see in color and use color in landmark recognition87. (Bees cannot, however, distinguish red from black.)

While in transit, bees sense direction using the sun and polarized light, and distance using visual information—the amount of scenery that flashes by during flight88-90. With a combination of landmarks, direction, and distance, a bee can find the way home to her nest from miles away. Some stem- and wood-nesting species may also use individual scent marks to help them recognize the nest from close range30,91-93.

To memorize landmarks30,94, a bee makes a learning flight when she leaves the nest for the first time88,95. A learning flight is easy to spot in action: a bee flying out of her nest turns to face it, hovers for a

moment, and then backs away in widening horizontal(ish) zigzags before leaving the area. Some bees may repeat the learning flight daily, or after they have trouble finding the nest88,95.

On a large and highly uniform nesting block or bundle, landmarks for individual nests are scarce, and bees can have trouble finding the right nest96,97. This waste of time and energy can be avoided by adding patterns of color, contrast, or three-dimensional structure96,97. For example, alfalfa leafcutting bees find their nest faster when drilled blocks are painted different colors and stacked so some are recessed and some protrude, placing nest entrances in different planes97.

Fig. 38. Large-scale nesting operation for alfalfa leafcutting bees. The nest blocks have been marked with colored shapes as landmarks to help bees find their individual nests. Landmarks may or may not be needed in a small-scale bee hotel, but they do no harm. Photo: Peggy Greb, courtesy of USDA ARS.

Similarly, blue orchard bees use patterns of shape, color, and structure to recognize their nests96, and do best if nest entrances are at least 3/4” apart24. This is easy to accomplish in drilled blocks and grooved boards by spacing the holes or grooves. With bundles of stems or reeds, space between entrances can be created by using pieces of varied length, or by pulling some reeds out a little ways and pushing others in, so the entrances are not in the same plane. These aids are probably most important in large, commercial bee shelters, but they certaiDraftnly won’t hurt in a small-scale hotel.

Fig. 39. In this bee hotel, some nesting tubes are pulled forward and others pushed in, creating a 3- dimensional pattern that bees use to identify their nests. Photo: Udo Voigt, Pixabay

In addition to helping bees find their nests within a hotel, color and landmarks can help them find the hotel itself. For example, blue orchard bees prefer to nest in blue shelters over yellow or orange ones56. Alfalfa leafcutting bees are more likely to use nests within large, boldly colored shelters that stand out from the landscape79; a pattern of broad, black-and-white or black-and-yellow stripes is often recommended 18,58,59,79,97. Perhaps for similar reasons, red mason bees (Osmia bicornis, cousins of blue orchard bees) in Leipzig, Germany, were more likely to nest in bundles of bamboo when these were attached to a shed, carport, or balcony than if they were mounted in a tree or shrub. In a small bee hotel, color is probably not essential; after all, bees searching for natural nests are looking for twigs and logs, which have no flashy color at all. Still, the benefits of a bold landmark for some species suggest that painting the hotel yellow, blue, or white, or mounting it on a larger structure such as a shed, might make it easier for bees to spot. Whether mounted to a building or a fencepost, bee hotels are usually installed about 5’ above the ground—high enough that the entrance is not blocked by vegetation, and low enough for convenient observation and maintenance65,69.

Disturbing a bee’s landmarks, or moving her nest, while she is away causes confusion. Upon her return, the bee will hover in front of the nest block, retreat and approach again, or enter the wrong nest91,94. For this reason, nest materials should be left undisturbed throughout spring, summer, and fall while bees are active. If something really must Draftbe rearranged, making the change at night can help ensure that all the residents are in, and will notice the change next time they leave the nest.

Best practices for navigation and nest recognition • Stagger nesting blocks or reeds by about 3/4” so not all nest entrances are in the same plane. • Optionally, consider incorporating yellow, blue, or white paint on the hotel itself or on the faces of nesting blocks. • Mounting the hotel on a larger structure such as a shed may make it easier to find. • A mounting height of about 5’ is often recommended. • Do not move the hotel or nesting materials during spring, summer, and fall when bees are active.

Other landscape features

A nest is just one part of a whole-habitat package that bees need to thrive. To ensure that even the smallest species can reach the resources they need, there should be good sources of pollen, nectar, and nesting materials within about a city block of the nest itself.

A female solitary bee will make hundreds of trips back and forth between her nest and other resources. Depending on the bee species, it can take up to fifty trips to collect enough pollen and nectar to feed a single offspring8,11,98, and another ten or twenty trips to collect enough construction materials (leaves, mud, etc.) to complete a single brood cell24,47. Multiply that effort by an average of two to fifteen offspring24,28, and it’s clear that commuting between the nest and other resources is a big part of a bee’s life.

What counts as a reasonable commute depends on the size and species of bee. Although a few large- bodied bees, such as carpenter bees, can fly for miles99,100, the smaller the bee, the smaller the foraging range101. Most small to medium species may forage no more than 1/10 to 1/4 mile from their nest10,11,101,102. A study of a small, solitary bee species in Germany found that an extra 1/10 mile between nest and flowers could reduce a bee’s reproductive rate by nearly 25%11. Alfalfa leafcutting bees with longer commutes between nest and flowers produced fewer offspring and relatively more males offspring103. And nests left unattended during a mother’s long foraging searches are more susceptible to parasitism104-106. To support all sizes of bees, then, nesting resources and flowers need to be close together, ideally within the same city block or even the same property.

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Fig. 40. Bees eat pollen and nectar, so nests need to be located near flowers that provide these food sources. Many resources are available to help you incorporate pollen and nectar into your landscape; see Appendix 5. Photo: Debbie Roos.

All bees in the family Megachilidae (leafcutting bees, mason bees, resin bees, and carder bees) collect construction materials to complete their nests. Different species use different materials, including leaf pieces, leaf pulp, flower petals, plant resin, plant fibers, mud, sand, and pebbles.

Although no bee species seems to require one specific plant species as a nest lining, they do have preferences. One study used DNA barcoding to identify the leaves used by three species of leafcutting bees in Toronto, Ontario, and found that each bee species used leaves from dozens of plant species. Members of the plant rose family (family Rosaceae) and bean family (Fabaceae) were used most often17. Some leafcutting bees seem to be pickier than others, and may show strong preferences for certain leaf sources107,108. They may select leaves based partly on size108; the chemistry of these construction materials may also protect the nest from microbes or parasites17,109. Although most plants used by bees remain undocumented, Appendix 3 lists known sources of construction materials that could be included in a landscape to enhance nesting opportunities.

To satisfy bees who use mud as a construction material, such as blue orchard bees, some bee hoteliers also provide an intentional source of mud. An incident in Serbia and Montenegro illustrates how mud availability can limit mason bees. After a water main broke, female bees flocked to the new mud puddles, and their nesting rate skyrocketed over what they had accomplished in the weeks before the pipe broke110. Different species prefer different mud textures27,111, and commercial suppliers even sell mud mixes of the ideal consistency for blue orchard bees. In some cases, bees are better than we are at finding mud, and may ignore provided mud in favor of unknown sources23. Given that relatively few bee species require mud27, and that many find it on their own, mud patches are not an essential landscape enhancement for every bee hotel. If, however, you are specifically interested in blue orchard bees, and find that they are not using your hotel or that their numbers are declining, you might consider this approach.

Finally, bees need food! The topic of pollinator gardening to provide nectar and pollen for bees is too big to take on in this nesting guide. Fortunately, it has been covered quite thoroughly elsewhere. Appendix 5 lists some additional resources to help you choose flowering plants that support bees year round.

Best practices for landscape features • Flowers and nest construction materials are available within about 1/10 to 1/4 mile from the nests • Consider including some known favorite leaves in the landscape for leafcutting bees (Appendix 3) • If species that construct nest walls from mud (such as blue orchard bees) are a priority for you, consider providing an intentional source of mud

Overwintering and cleaning your bee hotel To help your hotel residents survive through the winter months, keep the hotel in a cold but protected location, such as an unheated shed.Draft (A winter cold period is important for bee and wasp development.) It’s also OK to leave the hotel in place with chicken wire or hardware cloth for protection.

If your hotel has been in use 2-3 years, winter is also a good time to prepare your nest materials for cleaning to help reduce parasite and disease levels. In general, expect 10-30% losses of bees and wasps to parasites34-36. Cleaning is particularly important to reduce disease.

Because nest materials cannot be cleaned while residents are inside, the first step toward cleaning is to place the nests in an emergence box, which can be fashioned from various materials. An emergence box has two important features: 1) the box is opaque, and 2) there is one bee-sized hole in the container. Place the nests in the emergence box in fall or winter, and bring it outside after the last frost. During the spring and summer, hotel residents will emerge from their nests and exit via the single hole. Placing your nests in the emergence box prevents bees and wasps from recolonizing them, and gives you the

opportunity to clean or retire the nest materials once every resident has vacated. It can take an entire growing season—from spring to fall—for all residents to vacate.

Bee hotel cleaning procedure The best time to clean or retire nest materials is in the fall, after all the bees and wasps using the hotel have exited the nests and left the emergence box. It is good practice to clean or retire your nest materials if they have been in use for 2-3 years, or are showing signs of mold. To retire natural reeds and stems, simply compost them after they have spent a full growing season in the emergence box. To clean drilled blocks or grooved boards, use a bleach solution to soak the bee hotel38.

1. Separate stacks of grooved boards or other components to be cleaned. 2. Make a solution of household bleach (6%) and water, 1 part bleach: 3 parts water by volume. If mold is present, reverse the ratio (3 parts bleach: 1 part water) to make a more concentrated solution. 3. Submerge boards in bleach solution. Soak for 5 minutes, agitating the boards to remove debris sticking to the sides. If using a concentrated bleach solution, reduce soaking time to 1 minute. 4. Shake boards to remove excess moisture and allow to dry thoroughly. 5. Reassemble hotel and store until late winter or early spring. 6. Around the time of last frost, restock the hotel with fresh reeds and stems (if using). Set it outside for native pollinators to use again!

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What if bee hotels aren’t actually good for bees? You may have come across headlines like “Your Cheap-Ass Bee House Is Probably Killing the Bees” (Gizmodo.com, June 9, 2019), or research showing that bee hotels can be heavily occupied by non- native species29. These critiques have raised valid concerns about possible risks of poorly designed or poorly maintained bee hotels. In the end, though, we really don’t have strong evidence for or against bee hotels as a tool for insect conservation.

To reach a conclusion, we would need to know whether bee hotels consistently increase or decrease populations of wild, native bees, compared to a situation where no artificial nesting materials are provided. And we would need to know whether the apparent pitfalls of bee hotels—such as production of parasites or non-native bees—are actually worse in bee hotels than in natural nests.

Fig. 41. The giant resin bee (Megachile sculpturalis) is native to eastern Asia, but was accidentally introduced to the US and Europe. This species was first noticed in the US in 1994, on the NC State University campus, and has spread throughout the eastern states112,113. The giant resin bee can be an aggressive occupant of bee hotels (and carpenter bee tunnels), sometimes immobilizing its victims with sticky resin114,115. Photo: Christa Rohrbach

These are difficult things to measure, and few studies have attempted to do so. One reason is that bee populations vary wildly from year to year anyway. A well-controlled, long-term study would be needed to detect effects of bee hotels on beeDraft population growth. A few studies in agricultural areas have suggested, but not consistently proved, that wild bee populations do increase when artificial nests are provided over multiple years60,116,117. Stronger evidence is still needed, especially in urban, suburban, and semi-natural habitats.

In addition, nesting success in bee hotels is difficult to compare to nesting success in natural nests, because natural nests of most species are hard to find. They are often discovered opportunistically, one at a time. And they can be buried in logs and other large objects that make them hard to study in detail. Therefore, we don’t really know whether bees nesting in bee hotels are more heavily parasitized, or more likely to catch diseases, than bees living on their own.

Fig. 42. This small mason bee (Osmia) is nesting in a natural beetle burrow in a piece of firewood, but it’s hard to tell how her family is doing. How many offspring are hidden in there? How many will survive? How many will be parasitized? The inaccessibility of natural nests can make it difficult to benchmark the quality of artificial nesting habitat in a bee hotel.

Given the lack of evidence comparing natural and artificial nests, some cautionary tales come from the commercial management of blue orchard bees and alfalfa leafcutting bees. The development of leafcutting bees for commercial pollination began in the late 1950s and early 1960s, and populations of these (non-native) bees initially exploded in the western US under artificial nest management58. Within just a few years, however, pests and parasites began to accumulate, and the fungal disease called chalkbrood surged from one generation to the next in unwashed nests18,58. By the 1980s, the alfalfa leafcutting bee industry in the US had collapsed18. Its downfall emphasizes the need to control parasites and diseases through nest design and maintenance. But rearing tens of thousands of bees of a single species in a single nesting structure in an agricultural field is very different from hosting a variety of species in a small bee hotel in a yard, garden, or park. It’s hard to know exactly how the risks translate.

Overall, we think that current information still supports—or at least does not discourage—the use of thoughtfully designed and well-maintained bee hotels as habitat enhancements and as a means to appreciate your insect neighbors. Draft

Appendix 1. Bee hotel checklist

This quick-reference list summarizes the best practices suggested throughout the document

o Nesting tunnels of breathable materials, not plastic or glass o All tunnels have smooth entrances, free of splinters o Paper and cardboard tubes used only as liners in sturdier tunnels o Total number of tunnels in a bee hotel under about 100 o Nesting tunnel diameters 5/64 – 1/2” o Nesting tunnel lengths around 6” (3-8” is reasonable and other lengths could be good for some species) o Tunnel entrances spaced about 3/4” apart, perhaps with contrasting color patterns or 3D configuration o Tunnels are in a shelter that overhangs the nest entrances by a couple of inches o Shelter floor has drain holes o If (but only if) birds, rodents, or other predators become a problem, a sheet of chicken wire goes over the nest entrances o Shelter is about 5 feet off the ground o Tunnel entrances are oriented intentionally relative to sun and wind; facing southeast is often recommended o Nesting blocks or reeds are staggered by about 3/4” so not all nest entrances are in the same plane o The shelter or faces of nesting blocks may incorporate yellow, blue, or white paint o Hotel may be mounted on a larger structure such as a shed o Nest materials are not moved or rearranged while in use (only in winter or at night if you must) o Landscape includes flowers for nectar and pollen o Landscape includes nesting resources (mud, sand, resin, leaves) o Flowers and nest construction materials are available within about 1/10 to 1/4 mile from the nests o Landscape may include known preferred leaves for leafcutting bees (Appendix 3) o Landscape may include a source of mud, especially of blue orchard bees are desired o Nest materials retired or disinfected every 2-3 years

Threat DraftCountermeasure Woodpeckers or other birds • Protective housing (chicken wire covering on shelter) Pathogens (e.g. mold or fungal • Select breathable nest materials (no glass or infections) plastic) • Retire or disinfect nest materials regularly (every 2- 3 years) Parasites • Select natural materials for hotel cavities (shown to ● Mites be safer37 than paper) ● Parasitic wasps • Provide cavities with thick walls (some parasitic ● Cuckoo bees wasps can penetrate through ¾ inches of wood) • Clean regularly

Appendix 2. Plants that produce hollow or pithy twigs and stems used by nesting bees in eastern North America

Plant species that are native to NC can be used in landscaping to encourage natural nesting, or as a source of nest materials for bee hotels. Some of these native species are rather weedy, and some of the non-native species are invasive and should not be planted. Even these, when present, can be harvested for nesting materials to include in bee hotels. This list is not exhaustive, and you could experiment with any number of dried perennial stems as nesting material. Bees rarely nest in live plant material, but will use dead twigs and dried stems.

Latin name Common name Native to NC* Bees recorded Ambrosia ragweed yes Megachile

Arundinaria† cane yes Brassica mustard no Ceratina, Hoplitis Cirsium thistle mixed Megachile Dipsacus teasel no Ceratina Erianthus alopecuroides silver plume grass yes Ceratina Erigeron canadensis horseweed yes Ceratina Helianthus sunflower mixed Hoplitis, Megachile Lactuca biennis wild lettuce yes (M, P) Ceratina

Phragmites australis† common reed mixed Phytolacca pokeweed yes Megachile Rhus sumac yes Ceratina, Heriades, Hoplitis, Hylaeus, Megachile Rhus glabra smooth sumac yes Heriades Rhus typhina staghorn sumac yes (M, P) Ceratina Rosa rose, wild rose mixed Ceratina, Megachile, Osmia Rubus blackberry, dewberry mixed Ceratina, Hoplitis, Hylaeus, Osmia Rubus idaeus blackberry no Ceratina Rubus strigosus raspberry yes (M) Ceratina Sambucus canadensis elder, elderberry yes Ceratina, Hoplitis, Hylaeus, Osmia Sassafras albidum sassafras yes Ceratina Syringa lilac Draftno Ceratina Verbascum thapsus mullein no Megachile Vernonia ironweed yes Megachile * Plants that are only reported to genus often include a mix of native and non-native species. Where species are native to only part of the state, they are indicated with M (mountains), P (piedmont), or C (coastal plain). † We are not aware of bees using these materials spontaneously, but they have been occupied when they are cut and included in bee hotels.

Appendix 3. Plants whose leaves and fibers are used as construction materials in the nest interior

Plants that are native to NC can be used in landscaping. Some of the non-native species, such as white clover, are widely established in lawns and gardens where they do little harm, and may provide useful resources.

Latin name Common name Native to NC* Used by Parts used Acer maple mixed Megachile leaf pieces Alcea rosea hollyhock no Megachile petal pieces Alnus alder mixed Megachile leaf pieces Amelanchier serviceberry yes Megachile leaf pieces Betula birch mixed Megachile leaf pieces Betula lenta sweet birch, cherry yes (M, P) Megachile leaf pieces birch, black birch Cassia cassia no Megachile leaf & petal pieces Cercis canadensis redbud yes Megachile leaf pieces Cirsium thistle mixed Anthidium fibers Clematis clematis, curlyheads mixed Megachile leaf pieces Cornus dogwood mixed Megachile leaf pieces & pulp Corylus hazel, hazelnut yes Megachile leaf pieces Crocanthemum georgianum Georgia sunrose yes (C) Osmia leaf pulp Epilobium willow-herb yes Megachile leaf pieces Fagopyrum esculentum buckwheat no Megachile leaf pieces Fragaria strawberry mixed Hoplitis, leaf pieces & pulp Megachile Humulus hops mixed Megachile leaf pieces Medicago sativa alfalfa no Megachile leaf pieces Oenothera primrose yes Hoplitis, leaf pulp Megachile Polygonum knotweed mostly no Megachile leaf pieces Populus poplar mixed Megachile leaf pieces Ribes gooseberry, currant many species (M) Megachile leaf pieces Rosa rose, wildDraft rose mixed Megachile leaf & petal pieces Salix willow mixed Megachile leaf pieces Smilax greenbriar, yes Megachile leaf pieces carrionflower Spiraea spiraea mixed Megachile leaf pieces Symphoricarpos snowberry yes Megachile leaf pieces Trifolium repens white clover no Megachile leaf pieces Vitis grape mixed Megachile leaf pieces * Plants that are only reported to genus often include a mix of native and non-native species. Before planting, confirm that the species you have in mind is not invasive. Where species are native to only part of the state, they are indicated with M (mountains), P (piedmont), or C (coastal plain).

Appendix 4. Plans for a simple bee hotel shelter

Materials • lumber—see specifications below • chop saw or other saw capable of making mitered and beveled cross-cuts • screws—23 total, GRK trim head screws, size #8 x 1-1/4" • T-10 star bit, to drive trim screws (usually included when you purchase a box of screws) • wood glue • power drill • ruler or tape measure • bar clamps or masking tape, to hold pieces in place while fastening • wood filler, to hide screw holes • sandpaper, 120 grit, for smoothing screw holes, board ends, and wood filler • exterior finish of your choice (marine varnish, spar urethane, or primer and paint) • paintbrush, to apply finish • nesting materials in approximately 6" lengths • attachment hardware of choice, such as o two #9 deck screws with corresponding drill bit and manual screwdriver o birdhouse mounting bracket with keyhole openings

Lumber specifications • Clear yellow pine is a mid-priced option that has minimal knots and blemishes, and is unlikely to split while fastening, but does require exterior finish for durability. • Cedar is more durable without finish, but more likely to split during fastening. If you use cedar, you may wish to pre-drill before fastening with screws. Stainless steel screws are recommended, since cedar can corrode other fasteners. • Quantities nominal actual length dimensions dimensions needed 1" x 10" 3/4" x 9-1/4" 14" 1" x 8" 3/4" x 7-1/4" 14" 1" x 6" 3/4" x 5-1/2" 24"

Draft

Cuts

From the 1” x 10” board, cut two pieces • One piece 6-1/4” long • One piece 7” long

From the 1” x 8” board, cut two pieces, both: • 6-3/4” long, beveled to 45° at one end

From the 1” x 6” board, cut three pieces: • Two pieces 6-1/2” long • One piece 9-1/2” long, mitered to create the 45° roof pitch Draft

Assembly overview

Front

Draft

Right and rear, showing screw placements

Left and rear, showing screw placements

Draft

Assembly steps

1. Attach left side (part C) and back (part E) as shown • Line the vertical edge of part E with wood glue where it will touch part C • Bring into contact with part C and use a piece of masking tape or clamp to temporarily stabilize the joint while handling • Attach with screws

2. Attach bottom and shelf (parts D) to left side (part C) and back (part E) as shown • Use a ruler or tape measure to establish the 3-3/4” spacing between bottom and shelf • Line each joint with glue, stabilize with tape or clamp if needed, and fasten with screws

Draft

3. Repeat steps 1 and 2 to attach the right side (part C) to the back (part E), bottom (D), and shelf (D) • Line each joint with glue, stabilize with tape or clamp as needed, and fasten with screws

4. Attach the short side of the roof (part A) to the back (part E) and side (part C) • Line each joint with glue and fasten, as above • Note that screws entering part E are driven perpendicular to the roof, while those entering part C are driven parallel to part C

5. Attach the long side of the roof (part B) to parts A, C, and E • Line each joint with glue andDraft fasten, as above

6. Fill screw holes with wood filler

7. Let the wood filler cure, then sand wood filler and any other rough areas with sandpaper

8. Use a paintbrush to coat all surfaces with exterior finish of choice

9. Mount to side of 4” x 4” wooden post, outbuilding, or other vertical surface using hardware of choice a. If using deck screws o Pre-drill two holes centered on part E, one above the shelf and one below the shelf o Lay a strip of masking tape flat along the back of part E over the two holes, and mark their location on the tape with pencil or by poking holes in the tape o Transfer the masking tape to the mounting surface and pre-drill holes at the marked locations o Attach the hotel to the mounting surface with a deck screw in each hole; these will be driven from the front by reaching into the bee hotel, and will likely need to be driven by hand with a screwdriver, or using a long extension on a power drill/driver. b. If using birdhouse mounting brackets, follow the mounting instructions included with the brackets

10. Fill with nesting materials

Draft

Appendix 5. Additional resources

Books about bees and pollinator gardening

Bosch, J., and W. Kemp. 2001. How to Manage the Blue Orchard Bee as an Orchard Pollinator. Sustainable Agriculture Network, Beltsville, MD.

Carley, D. S. and A. M. Spafford. 2021. Pollinator Gardening for the South: Creating Sustainable Habitats. University of North Carolina Press, Chapel Hill, NC.

Danforth, B. N., R. L. Minckley, J. L. Neff, and F. Fawcett. 2019. The Solitary Bees: Biology, Evolution, Conservation. Princeton University Press, Princeton, NJ.

Levenson, H., and E. Youngsteadt. The Bees of North Carolina: An Identification Guide. 2020. NC State Extension, Raleigh, NC.

Mader, E., M. Shepherd, M. Vaughan, S. H. Black, and G. LeBuhn. 2011. Attracting Native Pollinators. Storey Publishing, North Adams, MA.

Mader, E., M. Spivak, and E. Evans. 2010. Managing Alternative Pollinators. Natural Resource, Agriculture, and Engineering Service, Ithaca, NY.

Wilson, J. S. and O. Messinger Carril. 2016. The Bees in Your Backyard. Princeton University Press, Princeton, NJ.

Lists of pollen and nectar plants

Chatham Mills Pollinator Paradise Garden https://growingsmallfarms.ces.ncsu.edu/growingsmallfarms-pollinatorgarden/

The Xerces Society https://xerces.org/pollinator-conservation/pollinator-friendly-plant-lists

Video footage of nest interior

The Solitary Bees https://vimeo.com/129712987Draft

Commercial suppliers of bee hotel materials

Crown Bees https://crownbees.com/

Citizen science with bee hotels

http://entnemdept.ufl.edu/ellis/nativebuzz/Join.aspx

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