Planning your garden to support specialized pollinators

Looking out my window, as the ground is covered with snow and I am getting ready for another snowstorm coming tonight, it seems ironic that I have been spending many hours these days ordering seeds and planning my garden. While I am thankful that the winter brings some rest to the soil in my garden, planning this season brings me happy memories of the scents and buzzes in my yard during the growing days… which reminds me that I should also plan for my little buzzing pollinator friends when I plan what to grow this season. In today’s blog, I want to chat about how we can plan for many types of pollinators, with a special focus on planning for specialists and not just for generalist pollinators.

Specialist pollinators – never heard of them?

As we mentioned in a previous post, pollinators visit plants to feed on nectar and/or to collect pollen to feed themselves or their offspring. However, pollen is not just there for pollinators to feed on; pollen is central to plant reproduction, so plants tend to make it both attractive to pollinators but hard to digest. For this reason, and in order to be able to properly digest the pollen, pollinators are often specialized in their pollen choices. This is because being able to digest the compounds that plants add to their pollen to make them hard to eat requires some level of adaptation, which often involves a trade-off with the ability to eat anything. There are, of course, many levels of specialization, and, while many pollinators feed on many plant families, others are more specialized than that, and feed on only specific plant genera or even species! For us gardeners, this means that if we want to support many different pollinators, we need to make sure that we are also providing for those very specialized pollinators as well!

Luckily for us, the floral choices and pollen specialization is known to some extent for Maryland and Eastern USA bees (see this site to learn more). For this reason, we know that many specialized bees in our region are also rare or uncommon… another reason to try to provide resources for them!

Who are pollen specialists in our region?

Many known pollen specialist bees in our region belong to bee genera Andrena, Colletes, Osmia, and Melissodes, which have many species considered rare or uncommon in Maryland and Mid-Atlantic.

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How do pollinators find plants and flowers?

As we know, pollinators help plants spread their pollen among flowers, and many plants do indeed need them to be able to reproduce and set seeds. We also know that by planting flowers and providing nesting habitats, we can help pollinators’ populations and thus assist with plant pollination. However, how do pollinators find plants? In this post, we will talk about that topic, which can help us become even better at helping pollinators and the plants they pollinate.

The big picture – pollinators need to be in the area

In order for pollinators to find plants, pollinators need to be present in the general region. In fact, although the vast majority of pollinators can move and travel from place to place, all of them have limitations on the distance they are able to travel. For example, hummingbirds can travel for miles (in Maryland, they are migratory), while large bees are able to travel relatively large distances for an insect (~500m-1km), and smaller insects will not be able to travel that far. This means that if, say, we lived in the middle of a very developed area with very few pollinator-friendly resources (few flowers, lots of cement, no green areas, etc.), planting a pollinator garden will attract few pollinators at first. This is due to the fact that it is likely that few pollinators are present in that area, and thus it will take a while for certain groups to arrive and establish in our garden.

It is for this reason that many communities tend to try to establish joint pollinator-friendly actions, and encourage many people in the region to participate (e.g., becoming Bee City USA-certified, creating “pollinator highways or corridors”). By increasing the regional number of pollinator-friendly resources, the whole region becomes more pollinator-diverse, and any supplementary action is more likely to improve pollinator support. As we talked about in a previous post, if you are interested in promoting pollinator-friendly habitat on your property, it may be a great idea to talk to your neighbors or your City, and see if others may also want to participate. In terms of pollinator-friendly activities, the saying “the more, the merrier” is very much true!

pollinator habitat sign in a garden
Pollinator-friendly actions are very effective when they are coordinated across regions. Photo: A. Kokai.

The local picture – different pollinators prefer different plants

As we mentioned in other posts, not all pollinators are made equal, and this is also true in terms of what plants will be found by what pollinators. For example, hummingbirds tend to visit tubular and reddish flowers, while syrphids prefer open flowers, and bees tend to visit flowers that they can access with their mouth parts (see this post to learn more). 

These floral preferences are due to the different pollinators’ abilities to see different colors, the presence of specific attractive floral scents in different plant species, and the ability of different pollinators with different body and mouth part shapes to handle and feed on flowers, and the matching of pollinator presence and flowering time. The practical consequence of this is that if we want to help many different pollinators find their preferred plants, it is necessary to grow different types of plants in our green spaces. By doing this, we would always provide resources that will be preferred to at least one pollinator, and by providing different types of resources, we can make sure that many different types of pollinators are supported by our plants. In order to do this, there are different floral mixes that exist that allow us to plant diverse floral resources appropriate for our region, which lets us build a diverse and welcoming floral bed for many pollinators.

Planting diverse floral resources will attract many different types of pollinators. Photo: C. Celley/USFWS.

The super-local picture – pollinators need to see the plant to access it

This will sound silly, but pollinators need to be able to have access to the plant to find it. For example, if a plant is not clearly displayed or hidden by many other plants or structures, it will be hard for pollinators to find it… even if the pollinator is present in the area and the plant in question is a preferred plant. This means that for us to help pollinators, we need to make sure that our plants are findable by the pollinators. Picking appropriate parts of our green spaces to plant our pollinator-friendly plants is thus key! For example, plants that require full sun to grow should be planted in those conditions and not under the shade of other plants or behind structures.

To know what these specific conditions are, there exist several resources (for example, see this useful and simple resource (PDF) published by the City of College Park, MD). These resources allow us to pick the best growing spot for our plants, making them easily findable by their pollinator friends.

Finally, pollinators are more likely to find plants if there are several of them! This is particularly true for smaller herbs, which may not display many flowers. By increasing the number of plants planted in an area we are also making the plant species more easily findable to the pollinators.

By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park. See more posts by Anahí. Anahí also writes an Extension Blog in Spanish! Check it out here, extensionesp.umd.edu, and please share and spread the word to your Spanish-speaking friends and colleagues in Maryland. ¡Bienvenidos a Extensión en Español!

Spooky Behaviors of Pollinators: The Curious Lives of Parasitic Bees

Fall is here and along with the pumpkins and falling leaves, there is one thing that pops up everywhere: Halloween! And because I can’t add Halloween decorations to a blog post, this blog will have a “conceptual” Halloween twist. Today, I want to talk about something that may seem spooky to many, but that to me showcases the stunning diversity of (pollinators’) life. In today’s post we will talk about bees that are a bit “special”: parasitic bees! Come along and marvel with me about these incredible creatures that coexist with us right here in Maryland!

Parasitic bees? What!?

Yes, you read it right. Although most of the bees we know are solitary and build and provision their nests, there are several groups that have taken an evolutionary path a bit different from their relatives. These bees have evolved parasitic behaviors, exploiting the nests and food from other bee species, and in the process actively killing the host’s brood. Because they display behaviors similar to cuckoos, birds who lay eggs in other birds’ nests and have their chicks reared by the host parents (check out this video), these bees are known as cuckoo bees.

Parasitic (right) and non-parasitic (left) bees look very different. One of the main differences is the fact that parasitic bees do not have structures to collect pollen (like hairy legs with pockets), as we can see in these photos. Photos: J. Gallagher.

Cuckoo bees look different from non-parasitic bees

Because these bees have evolved to not collect nectar or pollen for provisioning (adults do eat nectar and pollen, though), and do not build nests, they have also lost the morphological structures that allow bees to do so. Cuckoo bees thus lack all the structures commonly present in bees that collect pollen (e.g., little pockets on their legs, hairs), and all the structures that allow bees to collect materials and build nests. Unlike non-parasitic bees, who often can lay only one egg per day, cuckoo bees can lay many eggs on the same day. This adaptation allows them to take full advantage of a suddenly-available nest they can parasitize. Finally, as one can imagine, host bees are not super happy about having other bees come and exploit their nests… and they defend them! For this reason, cuckoo bees are strongly “armored”, with thick and bulky structures that can protect the parasitic females against the likely attacks from the host bees. And, last but not least, some cuckoo bees can camouflage using body odors that are similar to the host, which allows them to enter the nests without being “smelled”. Cool, heh?

But how do they do it?

A trait common to all these bees is that they have high levels of specialization on what other species they parasitize, meaning that one parasitic species will often parasitize a relatively small group of closely-related non-parasitic bees. For this reason, there are different methods cuckoo bees use to parasitize their hosts.

Larvae in many cuckoo bees are equipped with impressive mandibles that they use to attack and kill other larvae developing in the parasitized brood cell. Look at these weapons! Image: Rozen et al., 2019; American Museum Novitates.

The first main way is parasitizing brood cells that have been already closed. In this group, the females enter a foreign nest where closed cells are present, open the cell(s) where they want to lay the egg, kill the host’s egg with their sting or mandibles, then lay an egg in the (now empty) cell and close it. Other species that also parasitize closed cells are those in which the females open the cells, but instead of killing the host egg, just lay theirs in the cell before sealing it back. In this case, it is not the female but the larvae that will kill the host egg/larva. These parasitic larvae have strong mandibles that allow them to attack the resident larvae and kill them, keeping all the resources for themselves. Finally, other cuckoo species do not wait until the host cells are closed. The females of these species enter nests where cells are still open, and lay their very small and hard-to-see eggs in the open cells. The host female often oversees them and closes the cells with the parasitic egg in it. The parasitic larva develops in the closed cell and also uses its strong mandibles to attack and kill the host larva while in the cell.

Do parasitic bees exist in Maryland?

Yes! Although these life histories may seem like they are coming from another planet, we do not need to travel to exotic places to be able to encounter these species! They also occur right here!

The small Macropis cuckoo bees Epeoloides pilosula are very rare in Maryland and protected in most of their North American range. Photo: M. Veit.

A very cool species that is very likely present here in Maryland is the Macropis cuckoo bee Epeoloides pilosula, which parasitizes nests of the oil-bee of genus Macropis. Because of the level of specialization of both the oil-bee (on their host plant; see here to learn more) and its parasite, E. pilosula is very rarely encountered and is currently protected at different levels in Eastern North-America.

There are several species of Nomada or Nomad cuckoo bees in Maryland. These species often parasitize nests
of ground nesting bees. Photo: M. Lucas.

Another very neat example of local cuckoo bees are the parasites of Andrena and other mining bees: the parasitic bees of genus Nomada. The rule of lack of hairs and structures to collect provisions for the nest is very much true for this species! There are about 30 species of this genus known to be present in our state, and many of them are rare. While the spotted cuckoo bee Nomada maculata is somewhat regularly found in the state, Nomada bethunei is known only from a couple localities. Most of these Nomada species are, however, rare and often under conservation threats.

Will parasitic bees drive other bees to extinction?

Parasitic and non-parasitic bees have been co-evolving for millions of years, and it is very unlikely that this type of interaction would drive species to extinction. Indeed, the parasitized species also have evolved ways to protect their brood (something for another post). Interestingly, however, because parasitic bees are so specialized on their hosts, it is they who may be even more at risk of extinction than their hosts! Indeed, cuckoo bees are rare, hard to find, and are likely to have populations die out as soon as their host species disappears from a locality. From this respect, and if we want to protect the diversity of this super cool group of pollinators, providing resources for them and their hosts (see this and this post to learn some ways to do this) is key to maintaining the populations of these rare and fascinating parasitic bees!

By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park. See more posts by Anahí.

Anahí also writes an Extension Blog in Spanish! Check it out here, extensionesp.umd.edu, and please share and spread the word to your Spanish-speaking friends and colleagues in Maryland. ¡Bienvenidos a Extensión en Español!

Native Solitary Bees: Don’t Make Your Landscape Too Tidy This Fall

ground nesting bee on leaves
Cellophane Bee (Colletes thoracicus) is a solitary bee and valuable pollinator that nests in the ground. Photo by Hadel Go

Cooler evening temperatures might have you switching gears from planting and maintaining your landscape to fall decorating and garden cleanup chores. While cleaning up diseased and pest-infested plants in your yard and garden are important to prevent problems in the future, consider leaving healthy plants that could add visual interest this winter and provide nesting sites for many beneficial critters.

With more than 400 species of native bees in Maryland, these amazing little pollinators are a wonderful addition to your landscape. They are small and not aggressive. Some are specialists, which means they must have certain plants to feed on, while others are generalists and will visit a wide variety of plants. For amazing photos of native bees, check out the USGS Bee Monitoring Lab on Flickr.

As with all members of the animal kingdom, pollinators need food, water, and shelter in order to support life. Successful pollinator habitats include diverse flowering plants, food resources, and safe spaces for creating nests. As you begin cleaning up your yard and garden this fall, remember that these solitary bee species and many other beneficial critters rely on dead plant stems, fallen leaves, and other items that are often traditionally removed from the landscape.

dead plant stalks with new growth emerging at the base
Leave perennial plant stalks standing for the winter. Photo: C. Carignan

According to Colorado State University’s factsheet, Attracting Native Bees to Your Landscape, 90% of native bee species found around the world are solitary. Approximately 70% nest underground in the soil and about 30% nest inside hollow stems of plants and in tunnels left by other insects. Solitary female bees are responsible for collecting food, usually pollen, to include with each egg that she lays throughout the spring and summer. These eggs hatch into larva that spend the winter as pupa, which then turn into adults the following spring. Adult females die with the first fall frost. So in order to continue their life cycle, it is very important that their nesting sites are not destroyed in the fall.

To create friendly bee nesting habitats, provide dead wood like tree stumps or firewood for wood boring bees, plants with hollow stems (brambles and other perennials) for bees that need a tunnel-like structure, and areas of full sun, bare (un-mulched) soil, which ground nesting bees use for their nests.  Landscape fabric prevents ground-nesting bee’s ability to tunnel into the soil.

bee nesting box
Bee nesting box. Photo: Pixabay

Bee houses have gotten some attention in recent years and there are mixed messages about adding bee housing structures. Some evidence suggests that if not properly maintained, these well-intended additions could actually create a negative effect on populations. The houses provide a nice nesting area that results in large numbers of larva/pupa congregated close together, which could be easily targeted by predators, diseases, or parasites. For guidance on bee house maintenance check out this great factsheet from the Xerces Society.

Remember our unseen friends this fall and leave some of your plant materials in place to provide nesting and sheltering sites. Sit back, relax, and delay some of those cleanup chores until spring!

By Ashley Bodkins, Senior Agent Associate and Master Gardener Coordinator, Garrett County, Maryland, edited by Christa Carignan, Coordinator, Home & Garden Information Center, University of Maryland Extension. See more posts by Ashley and Christa.

Swarms, Swarms, and More Honeybee Swarms!

Swarm of bees on a branch
Photo by Anne Arundel County Master Gardener Beekeeping Project

On May 12, Nancy Allred, Interim Master Gardener Coordinator, Anne Arundel Co. called to say that one of the two Master Gardener Beekeeping Project hives at Hancock’s Resolution Park in Pasadena had swarmed. Swarming is the natural process that a honeybee colony uses to reproduce itself, so this is an exciting event. My husband and I packed up our bee jackets and drove to Hancock’s Resolution to check on the swarm.

Hancock’s Resolution is a historic farm park operated by the Friends of Hancock’s Resolution (FOHR) a 501(c) (3) non-profit organization. Under an agreement with Anne Arundel County Recreation and Parks, FOHR operates the farm and offers programming that interprets the site’s historical and agricultural significance. This remnant of a 410-acre middling plantation located in northern Anne Arundel County is on Bodkin Creek, near the mouth of the Patapsco River, and was once a well-situated market farm with close access to the Chesapeake Bay and the port towns of Baltimore and Annapolis.

Master Gardeners tend a demonstration garden and are on hand to discuss 18th-century farming practices, answer questions about plants, and invite visitors to participate in garden activities (however, activities and access are currently on hold due to COVID-19). Since 2009, Master Gardeners have tended the hives and provided pollinator educational programs that currently feature an observation hive that provides a view of the inner workings of a beehive.

Master Gardener beekeeper at Hancock's Resolution Park
Master Gardener beekeeper at Hancock’s Resolution Park – Photo courtesy historichancocksresolution.org

We were pleasantly surprised to see that the swarming bees had settled on the lower limb of a tree right next to the hives. Often swarms land just beyond the reach of your tallest ladder, so we were very lucky.

When swarming, the colony splits into two distinct colonies. The queen and about half of the workers (females) leave the hive in search of an appropriate site for their new home, often a large cavity in a tree. Typically the swarm temporarily settles on a tree or structure near the hive with the workers surrounding the queen in a cohesive “teardrop” of bees. Scouts fly off to find a suitable site for the colony, which might take a day or two or more. Once they identify a suitable site, the swarm takes to the air and follows the scouts to the new site. Meanwhile back in the original colony, the workers are busy raising several queens, one of which will become the new queen of the hive.

Tending the swarm
Photo by Anne Arundel County Master Gardener Beekeeping Project

In many cases, beekeepers are thrilled to see a swarm because if they can catch the swarm, they can add another colony to their apiary or replace a colony that died during the winter (in the last few years in Maryland, beekeepers have reported losing approximately half of all colonies each year — but that’s a topic for another article).

As Brian and I looked at the swarm that had settled next to the beehives at Hancock’s Resolution, we considered catching the swarm and setting up a new colony.  However, all of the Master Gardener hives at Hancock’s Resolution and Quiet Waters Park made it through the winter just fine, and our own colonies had also survived, so we had no place to install the new swarm. Unfortunately, you can’t just put the bees back into the hive that they came from since that hive is already down the path of raising a new queen.

Since we had nowhere to put the swarm, I called a fellow beekeeper, Charles DeBarber, to come collect the swarm. Charles has worked with John Conners and myself with the beekeepers at the Correctional Institution for Women in Jessup, including to help remove a swarm that had moved into an abandoned building there. Charles wears many hats, including maintaining about 20 hives at Filbert Street Community Garden in nearby Curtis Bay, and he sets up hives for other community gardens in the area. He was thrilled to have another colony to donate to a community garden.

Charles arrived at Hancock’s Resolution 20 minutes after I called him and a few minutes later had the bees in a small box called a nuc complete with frames of honey and honeycomb to make them feel at home. The swarm was very gentle and only focused on protecting the queen who was somewhere in the middle of the ball of bees. Charles was able to collect the swarm without protective gear (don’t try this at home!) and no one got stung during this process. When Charles left, he was headed to a community garden to move the “ladies” into their new home.

Tending the swarm
Photo by Anne Arundel County Master Gardener Beekeeping Project

But that wasn’t the end of this story. The next day Nancy called again to say that ANOTHER swarm had appeared in the same location. Was it from the other hive, the same hive, or another hive in the neighborhood? There’s no way to know, but we suspect that it was from the other hive. Once again, we made the trip out to Hancock’s Resolution, called Charles, and got the second swarm (which was even larger than the first) dropped into another box to be delivered to its new home.

But, still, there’s more to the story.On May 22, ten days after Nancy called about the first swarm, she called again reporting a THIRD swarm. Once again, Charles showed up and retrieved the swarm and took it to a local community garden that was grateful to have such hardworking pollinators near their garden plots.

In the meantime, the Master Gardener Beekeeping Project honeybees at Hancock’s Resolution and Quiet Waters Park are thriving, collecting lots of nectar and pollen, raising young, and making sure that the hives have enough honey and pollen to survive the next Maryland winter.

It is a wonderful thing to support and help pollinators thrive. The transfer of pollen from the male organ to the female organ of a flowering plant – is essential to life on earth, for without pollination most people and non-human animals would not have enough food. Over 90% of all known flowering plants, and almost all fruits, vegetables and grains, require pollination to produce crops. Happily, there is a pollinator volunteer workforce that includes bees that does this job for us.

If you find a swarm on your property, you can find a beekeeper via Maryland Beekeepers Association to come and save it.

Pam McFarland, Anne Arundel County Master Gardener, University of Maryland Extension. Edited by Dan Adler.

Why Do Pollinators Visit Flowers? Hint: It’s Not Just for Nectar and Pollen

Other than because I think they are pretty, I love looking at plants and their flowers. In fact, one of my pastimes has become figuring where and what is the reward that pollinators get out of their visits to their favorite flowers. You may be now thinking that my pastime is a bit nonsensical, since it is pretty clear that pollinators get pollen and nectar from flowers, so why bother checking? Well, actually, that is only partially true; did you know there’s a myriad of rewards that pollinators can get from their flower visits?

In today’s post I want to tell you a bit about some of those other rewards; the ones that fascinate me so much. Let’s talk about special floral pollination rewards and where you can see them in real life!

We like essential oils, some pollinators like floral oils!

The first time I heard about floral oils my mind was blown in such a way that I became obsessed with them, to the point that now a large part of my research program focuses on them. Floral oils are a reward that many types of plants offer to their favorite pollinators: oil-bees.

But don’t let me get ahead of myself! Floral oils are a special type of oil – different from essential oils – that are produced and presented to pollinators on different parts of the flowers of some plants. Independently of what exactly they look like, all these plants are visited and pollinated in a very specialized way by oil-bees. Unlike honeybees, these oil-bees are solitary and make their nests in the ground. These oils help these bees line their nests to waterproof (!!) and strengthen them. Along with that, they also mix the oils with pollen and feed that ‘pollen ball’ to their larvae.

Macropis oil bee
The whorled yellow loosestrife (left; photo: Eli Sagor) is one of Maryland’s native plants that offers floral oils to their Macropis oil-bees (right; photo: Don Harvey). Note the shiny load of oils and pollen on the hind legs of this Macropis!

Oil flowers are present all around the globe. In our region, they are represented by several species of the yellow loosetrife plant genus Lysimachia. With their floral oil rewards, these loosestrifes sustain the rare oil-bees of the genus Macropis. At the level of the country, most oil-flowers (and their specialized pollinators) are restricted to the Southern USA, where they are visited by the large bee genus Centris. Some of these plants are the wild crapemyrtle, the prairie bur, and the purple pleatleaf.

Hungry? Please, help yourself!

Along with nectar, pollen, and floral oils, food for pollinators can come in many different shapes and forms. In fact, some flowers even offer parts of their flowers to their pollinators. In cases like this, flowers develop special structures – usually around their petals – with the only function of becoming food for pollinators. Flowers providing this type of reward are usually pollinated by beetles, who can use their strong mandibles to chew on and eat the special structures.

calycanthus
Sweet shrubs display nutritious structures to their pollinators, small sap-feeding beetles of the family Nitidulidae. Photo: Wikipedia commons.

One of the coolest examples of the use of this type of reward is our very own sweet shrub, Calycanthus floridus. This spring flowering plant (flowering right now in Maryland!) attracts small beetles that enter the flower and stay there for quite some time. To maintain and support them while they are helping the plant reproduce, the sweet shrub flowers englobes them during parts of their flowering (this is why sometimes these flowers seem to be opening and closing throughout the day) and present small extremely nutritious structures at the base of their petals. It is on these structures that the beetles can feed on to stay strong and healthy while they are on the flowers. If you have one of these flowers in your yard, or happen to see them in one of your walks, take a second to stop and check them; you may get to meet their little beetle friends! 

Need a hand taking care of the kids? Here I am!

Some other flowers have established even more intricate relationships with their pollinators, and what they provide is not just food, but also a house! Because in these plants the offered reward is a place for the larvae of these pollinators, these interactions are called ‘nursery pollination’. Here, the pollinator visits the plants, collects pollen, and sometimes even actively places pollen on the flower tip. By doing so, the pollinator makes sure that the plant seeds develop. This is important, because their larvae will need some of them to feed on throughout their development.

yucca moth
Joshua trees (left; photo: Shawn Kinkade) are some of the most iconic plants of the US Southwest. These plants offer a brood site to their super-specialized small moth pollinators (right; photo: Judy Gallagher).

Along with this being the reward we see in a plant we love to eat (figs!), one of the most spectacular examples of the use of this reward is found in an iconic plant of the deserts of the US Southwest, the Joshua tree. Indeed, Joshua trees produce flowers that are visited by a group of moths, the Yucca moths. These moths visit the flowers, collect their pollen, and then literally push it into the flower tip to actively pollinate it. Because the moths lay eggs on the flowers, this assures that the flower develops seeds so the larvae have something to feed on. What is fascinating, though, is that these larvae never eat all the seeds, so this really is a win-win relationship between the plant and the moth.

To see how this is done, take a look at this video!

yucca moth video
Larvae of the Yucca moths feed on a Joshua tree’s seeds. To make sure that there is something for their larvae to eat, these moths actively pollinate the plants, exchanging a brood site for pollination, and in the process display some of the most fascinating behaviors one can see in pollinators. Check out the video to see it for yourself! Video: University of Nebraska-Lincoln. 

By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park. See more posts by Anahí.

Carpenter bees are native pollinators

carpenter bee on redbud flowers
Carpenter bee (Xylocopa virginica) on an Eastern redbud tree. Photo: Allen Szalanski, Bugwood.org

A sure sign of spring is the emergence of carpenter bees. Have you seen the perfectly circular small holes that these bees chew into wood? I mean honestly, I have trouble drawing a perfect circle freehand, much less chewing one!  

Adult carpenter bees spend the winter hanging out in their nesting site, just waiting for the first signs of spring when they will emerge, find pollen for food, and mate. Nesting sites/holes are most often seen under the eaves of buildings, particularly on unpainted wood. Every spring, we have carpenter bees infesting the eaves of our barn.    

carpenter bee holes in wood
Holes made by carpenter bees. Photo: Ashley Bodkins

People often confuse carpenter bees for bumble bees, but there are some distinct differences between these two types of bees, including nesting locations. Bumble bees live in a social hive whereas carpenter bees are solitary, and a carpenter bee has an abdomen that is shiny black, not hairy.

Carpenter bees are native pollinators, are not aggressive, and are only noticeable in late spring-early summer. They are mostly found around structures made of wood, or around facia trim, beams, etc. 

Male carpenter bees often cause alarm when they dive-bomb and fly erratically around humans that approach nesting sites, but in actuality, these bees are bluffing as they lack a stinger and are harmless. They can be identified easily by the white spot on the front of their heads.

male carpenter bee
The male carpenter bee has a distinct whitish pattern between the eyes. Photo: David Stephens, Bugwood.org

Only female bees have a stinger, which is a modified egg-laying device (ovipositor). Female carpenter bees are docile and are reported to sting only if handled, but a female carpenter bee can sting more than once.

Carpenter bees do not consume wood, but they do create their nesting sites in wood. They feed on pollen and nectar and are important plant pollinators. Their large size allows them to pollinate some flowers that are unsuitable to smaller bees.

carpenter bee collecting pollen
Carpenter bees are important native pollinators. Note the shiny black abdomen, a characteristic that differentiates them from bumble bees. Photo: Ansel Oommen, Bugwood.org

Female carpenter bees use their strong jaws (mandibles) to chew a perfect circle entrance hole and make a small chamber that will become her home. The entrance is a little less than a half-inch wide, close to the same diameter as her body. These channels that she chews are perpendicular to the grain of the wood. Then they will turn about 90 degrees and excavate along the wood grain for 4 to 6 inches to create a gallery (tunnel). It takes approximately 6 days to bore one inch into the wood.  

Female carpenter bees build several cells within each tunnel and each cell contains one egg and enough food for the larva that will hatch. The food, called bee bread, is a mixture of pollen and plant nectar. She places the food into the tunnel, lays a single egg on it, and builds a partition in the tunnel with cemented wood particles.   Each tunnel contains 6 to 10 cells. 

Control of carpenter bees may be desired if there is significant damage to wooden structures. But be aware, carpenter bees do not cause major damage like termites, and again, they are important native pollinators.

If control is desired, you can apply a registered insecticide into the entrance holes. Then the holes should be sealed thoroughly with wood putty or caulking compound. If possible, filling the entire tunnel system with a sealant can also be effective. All exposed wood surfaces should be painted or varnished and sealed. When managing carpenter bees, it may be helpful to treat and seal entrance holes during the early morning or late evening when bees are less active. Additional management options are offered by West Virginia University Extension.

References and Resources

https://extension.wvu.edu/lawn-gardening-pests/pests/carpenter-bee

https://extension2.missouri.edu/g7424

https://ohioline.osu.edu/factsheet/hyg-2074

By Ashley Bodkins, Senior Agent Associate and Master Gardener Coordinator, Garrett County, Maryland, and Christa K. Carignan, Coordinator, Home and Garden Information Center, University of Maryland Extension. See more posts by Ashley and Christa.