Carpenter bees are native pollinators

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

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,

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,

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

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.

Please Pass the Cranberry Sauce and Give Thanks for Pollination

Thanksgiving is a time to gather with loved ones and usually involves first preparing and then ingesting a lot of delicious goodies. Each family has recipes and traditions related to Thanksgiving, and even foreigners (like myself) may join in and create new traditions. Independently of who we are and our origin, the meals we prepare include a number of common foods: cranberries, pumpkins, sweet potatoes, pecans, and potatoes.

For this year’s Thanksgiving, I want to take us on a trip to recognize and thank nature and some of our little winged friends, without whom we would not be sharing all that deliciousness with our loved ones. Plus, after reading this you will know some cool fun facts you can share with others during your Thanksgiving meal!

cranberry sauce and the bumble bee that pollinates cranberries
Photos: Wikipedia Commons, Lotherington

Cranberry Sauce

Cranberries are the fruits of a plant closely related to blueberries and huckleberries, which all are native to North America. Like blueberries and huckleberries, cranberries need pollinators to produce fruit. The reproductive organs (anthers – where pollen is produced, and carpels – where the ovules are hosted) in a cranberry flower mature at different times, which means that a flower can’t self-pollinate and needs a pollen vector to produce fruit.

As you may know, no pollination means no fruits, and no fruits means no cranberry sauce. Luckily, nature provides and pollinators are around! We know today that several different bees visit cranberry flowers, with bumblebees being some of the best pollinators. Some other wild bees (for example, mining bees, Andrena) also contribute to the pollination of this plant, and honeybees can pollinate as well but not as efficiently as bumblebees and other native bees.

pumpkin pie and a bee that pollinates pumpkins
Photos: Wikipedia Commons

Pumpkin Pie

Pumpkins, squashes, and zucchini are all closely related vegetables that also require pollinators to produce fruit. Unlike cranberries, pumpkin plants produce separate and distinct female and male flowers. Because in these plants the female and male reproductive organs are physically in different parts of the plant, pollination (and thus fruit production) requires a pollen vector. Again, we would have no pumpkin pie if our pollinator friends were not around!

So, who pollinates pumpkins? Pumpkins have very specialized pollinators that do the best job at pollinating. In the US, this specialized pollinator is the squash bee Peponapis, which feeds their larvae a strict diet of squash pollen. Furthermore, unlike many other bees, both the males and females of the squash bee pollinate, since mating happens in the flower. Even though the squash bees are by far the best pollinators of pumpkins, other bees (including honeybees) can occasionally visit and pollinate the flowers… but really it’s these little cuties that we need to thank for all the delicious pumpkin!

pecan pie and pecan tree flowers
Photos: Wikipedia Commons

Pecan Pie

Like pumpkins, pecan plants can’t automatically self-pollinate and need a pollen vector to produce the yummy nuts we eat. This is not only because their female and male flowers are separated spatially on the plant (like they are for pumpkins); they also flower at different times of the year on the same plant.

To be pollinated, the female flowers of a pecan plant need to receive pollen from the male flowers of another plant which is flowering at the same time. For this reason, pecan flowers need a vector of pollination, which here is not an insect but the wind! Pecan flowers are indeed adapted to wind pollination, displaying hanging bunches that shake with the wind, releasing and catching a lot of the pollen in the air.

sweet potatoes and a bee that pollinates the flowers
Photos: Steven Depolo, Wikipedia Commons

Sweet Potatoes

The part of the sweet potato plant that we eat during Thanksgiving is the tubers, which are roots. And since what we eat is not a fruit, pollinators have no role to play for this Thanksgiving ingredient… at least not directly. But the sweet potato plant still needs pollinators to produce seed and breed, because their flowers are unable to be successfully pollinated by the same flower’s pollen.

Pollinators are then really important for the successful maintenance of the genetic diversity of this plant. Sweet potatoes belong to the morning glory family, and as its name suggests, flower in the morning hours. They are pollinated by many different types of bees (from large bumblebees and carpenter bees to smaller bees such as sweat bees), which visit the flowers for nectar and pollen.

mashed potatoes and a bee that pollinates potato flowers
Photos: Ernesto Andrade,

Mashed Potatoes

As for sweet potatoes, the part of the potato plant we eat is not the fruits but the tubers. Pollinators are not needed for obtaining these tubers, but the plant requires pollinators to be able to breed and maintain genetic diversity. Since they are in the same family (the nightshades), potato flowers look similar to those of tomatoes and eggplants.

Like in those other vegetables, pollinator visits, and more specifically something called ‘buzz pollination,’ needs to occur for successful pollination. In this type of pollination, the insect visits the flower and buzzes loudly, which shakes the flower, releasing the pollen, which they then transfer to a different flower during their next visits. Among these buzzy bees, bumblebees and mining bees (Andrena) are very efficient at pollinating potatoes.

[VIDEO: Buzz pollination of a bumblebee on a potato flower. Note how the pollen is released from the anthers — the four yellow long organs — and sticks to the bee abdomen where the stigma — the female flower organ in the middle of the flower — rubs the bee abdomen where it collects the pollen and gets pollinated. Video by thyreodon.]

Happy Thanksgiving!

By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park

Where are all the pollinators?

Yard in the fall with leaves on the ground
Leaving fallen leaves, plant twigs, and wood in your landscape can help pollinators survive the winter. Photo: Anahí Espíndola

Even though I love the heat of summer, I have to admit that fall makes me happy in a different way. I like the trees that turn into beautiful colors, the crisp air, quiet nights, and the days that slowly become shorter and make me want to drink tea and eat cookies. There is however one thing that makes me a little sad about fall, and it’s that all those beautiful pollinators I love so much are now gone! But are they? Actually, have you ever asked yourself where pollinators go in the fall? Well, thanks for asking — today is your lucky day! In today’s blog post we will talk about what happens to pollinators in the fall, and what we can all do to continue helping them during this quiet time.

When the season reaches an end, pollinators find themselves in a hard spot. They could hopefully collect food (nectar, pollen, etc.) during the spring and summer, but now all the flowers are gone and decisions need to be made if they are to survive until next year. When the season reaches an end, pollinators have basically two options to make sure they or their progeny survive until the next season: migrate for the winter, or stay and protect themselves against the cold.

How to Help Pollinators in the Fall

#1 – Let them go!

Some famous pollinators migrate. You may be familiar with the impressive and beautiful Monarch butterfly migration, which happens every year, and which allows Monarch populations that are far North reach latitudes where the climatic conditions are more benevolent to their survival. Other less famous pollinator migrations are those of hummingbirds, which also migrate to less harsh conditions at the beginning of the fall.

The best thing to do to help migratory pollinators is to help them migrate! This may sound a bit counterintuitive but providing plant resources for migrant pollinators for too long can be a bad idea, because that may make them stay in the region for longer, expose them to parasites for longer, and not reach their final destinations in time.

Let’s take the Monarchs as an example. Monarch caterpillars feed on milkweed, and there are both native and non-native species of milkweed they can develop on (Figure 1).

Tropical Milkweed
Figure 1 – Monarchs can develop on many species of milkweed. Non-native milkweed such as the tropical milkweed (Asclepias curassavica; left) can hinder their migration. Favoring native milkweed species (such as the common milkweed; right) is a way to help Monarchs stay safe in the winter.

The native milkweeds naturally start drying out by the end of summer. This changes the chemistry of the plant and “tells” the caterpillars that the end of the season is coming. This is one of the triggers for the caterpillars to start transforming into adult butterflies. Without that trigger from the host plant, the caterpillars continue feeding. This is what happens when they develop on non-native milkweeds that stay green for longer. In fact, caterpillars that develop on those non-native species become adults later in the season, and when they finally emerge, it is too late in the season and they are unable to migrate and make it to the next spring. One of the best ways to protect these migrants is to let them finish their natural cycle and leave in time (I know it’s hard to see them leave!), which in the case of Monarchs requires favoring planting native milkweeds over their non-native counterparts.

#2 – Let them diapause!

Most of the pollinators in our region, however, are adapted to spend the winter right here. Where are they? As I mentioned in another blog post, the vast majority of pollinators in our area are insects. Insects can’t move, fly, or feed if the temperatures are too low. To deal with very low temperatures, insects in temperate regions like ours enter a physiological stage called diapause.

During this stage, the insect physiological rate is reduced and all development is put in a pause until conditions are more favorable. Even though we usually think about these stages when the season comes to an end for us (the fall), it is interesting to note that many of our pollinators reach this stage at the beginning of the summer and they maintain it until the following spring. The practical consequence of this is that if we want to protect pollinators, we don’t just need to provide food for them; we also need to make sure that wherever they decide to spend their diapause is safe from disturbances.

So, how to do this? First, it’s important to realize that each pollinator species enters diapause at different times and places and at different developmental stages (e.g., larva, pupa, adults). Our native bees diapause in nests (solitary or communal), which can be built in different places, depending on the species. The majority of our native bees are ground-nesting bees and they can enter diapause as early as the beginning of the summer and as late as the fall. For nesting, these bees usually prefer loose soils such as those that are sandy or rocky (Figure 2). Making sure that we are not disturbing the ground in places where we see nests will be key for them to survive until the following year. Practically speaking, this means that if you see bees digging holes in the ground of your garden, you may not want to till that part of it.

Ground nesting bees
Figure 2 – The Rufous-Chested Cellophane Bee (Colletes thoracicus) establishes nests in the ground. They can be identified by the mounds left around their entrances. These bees are solitary and non-aggressive, so if you see them nest, don’t disturb that area and rejoice in the bees’ return next spring! Photo by CsabaVadasz.

The second most common place for bee nesting is in cavities. These cavities can be plant twigs and branches, cracks in rocks or walls, or even, in some regions of the world, empty snail shells! If you would like to help these bees in your garden or yards, just leave the remains of all your dry plants through the winter. Chances are that some bees have chosen your dry plants as a place to set their nest (Figure 3). These bees also are the ones that like nesting in homemade bee hotels, and it is really fun to see them emerge early in the spring from the little tubes.

Bees nesting in twigs
Figure 3 – Some bees nest in twigs and branches, like these nimble Ceratina Bees (Ceratina strenua) which have chosen a raspberry branch as a perfect place to raise their progeny. Waiting until the spring to clean your dry raspberry branches is a good way to keep these bees protected throughout the winter. Photo by Terry Miesle.

Other bees prefer to build their nest completely above ground. You may have seen little mud “amphoras” or other structures made of little rocks that hang from walls. If you see these nests close to your house, try to not disturb them and keep an eye on them next spring!

Other pollinators, like moths and butterflies, diapause in leaf litter, on wood, or in the ground. They usually do so by enveloping themselves in dry leaves, by digging themselves in the ground, or by attaching their chrysalis onto sticks and branches (Figure 4). To protect these pollinators, you can leave parts of your yard or garden soil undisturbed, keeping at least some of your dry leaves on the soil.

Black swallowtail butterfly chrysalis and polyphemus moth cocoon
Figure 4 – Some moths and caterpillars diapause in cocoons, such as this Black Swallowtail (left; photo: Megan McCarty) or this Polyphemus moth (right; photo: Leckie Seabrooke). Note how the moth has enveloped itself in dry leaves.

Finally, some moths and butterflies diapause as adults, hiding in wall cracks or small orifices. In these cases, it can be hard to spot them before it’s too late. To avoid that, you can observe around your garden or yard (and around the house) to try to find them. This way, you will know what places you should not disturb when you are doing yard or garden work.

For more information on this topic, check out Where Do Maryland’s Butterflies Overwinter? and Bewitching Butterflies and Moths with Fall and Winter Habitat.

By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park

Pollinators and Food Gardens

The Food and Agriculture Organization of the U.N. estimates that at least 75% of the world’s food crops depend, at least in part, on pollinators. Honey bees and a host of native bees, beetles, and butterflies are essential for pollinating vegetable and fruit crops on farms and in gardens across Maryland. For example, we would have no cucumbers, squashes, muskmelons, or pumpkins without bees.

Squash bees pollinate all cucumber family crops. Photo credit: Jim Jasinski, Ohio State University Extension;
Squash bees pollinate all cucumber family crops.
Photo credit: Jim Jasinski, Ohio State University Extension;

It takes 8-12 bee visits to fertilize enough ovules (baby seeds) to produce full-size fruits. Gardeners get smaller, fewer, and misshapen fruits when bees are unavailable to pollinate some other crops like eggplant, okra, pepper, strawberry, raspberry, blackberry, and blueberry.

Bumblebee drinking nectar from a basil flower. Photo credit: Jon Traunfeld
Bumblebee drinking nectar from a basil flower.
Photo credit: Jon Traunfeld

What can I do?

  • Plant lots of flowering plants (annuals and perennials, especially native species) in borders around, or in, your food garden. Choose plants with a variety of flower color, shape, and size with different flowering times, to provide nectar and pollen through the growing season. Include herbs in your vegetable garden (earlier blog post on anise hyssop and tulsi basil)
  • There are many vegetable, fruit, and herb plants that don’t require bee pollination to produce the edible part(s) that humans are most interested in. But the nectar and pollen in the flowers of these plants (e.g., arugula, broccoli, radish, cilantro, and basil) may be valuable for pollinators.
  • Place a shallow saucer or container of water in your garden with gravel or pebbles for insects to rest on while drinking (change the water twice per week).
  • Avoid using pesticides (including organic pesticides, like pyrethrins, known to be toxic to pollinators). If spraying is necessary, make the application when pollinators are least active (early morning or early evening), don’t spray when flowers are open, use short-residual insecticides that are least harmful to bees


Honey bee getting water from a blueberry flower.  A variety of native bees assist also assist in pollination. Photo credit: Jason Gibbs, Michigan State University
Honey bee getting water from a blueberry flower.
A variety of native bees assist also assist in pollination.
Photo credit: Jason Gibbs, Michigan State University



Pollinators vital to our food supply under threat- FAO news release:

Attractiveness of Agricultural Crops to pollinating Bees for the Collection of Nectar and/or Pollen- USDA:

How to Attract and Conserve Pollinators and Natural Enemies in Your Garden- HGIC:


By Jon Traunfeld, Extension Specialist


Bees making holes in your yard? They could be gentle pollinators: Meet the cellophane bee

Colletes thoracicus
A solitary ground-nesting bee, Colletes thoracicus. Photo by Hadel Go

I am a horticulture consultant at the University of Maryland Extension’s Home & Garden Information Center (HGIC). Every spring we receive many questions about ground bees that make burrows and holes in lawns and ornamental garden beds. Homeowners are alarmed when they notice many bees flying over an area for several weeks in the spring. They ask questions like: What type of bee is this? Do they sting? Can I mow in the area? When will they go away? How do I get rid of them for good? The bees look intimidating because they tend to aggregate together. It may look like they are tending a busy underground hive, but each bee is typically solitary and digging individual holes. Since a hive is not being defended, the bees are not aggressive.

If you do not know the type of bee you are dealing with, it is concerning. I have a landscaped hill in my backyard that is prime habitat (loamy/sandy soil and good drainage) for ground-nesting bees. For the past several spring seasons, hundreds of bees have flown over an area on my hill. Our entomologist, Mary Kay Malinoski, wanted to identify these bees because many types of bees are important pollinators. Our staff collected samples. The bees were picked up by Lindsay Barranco, a graduate student studying ground bees at the University of Maryland Bee Lab. The bee samples were sent to the USDA Bee Lab for identification.

Home & Garden Information Center (HGIC) staff collecting samples and sitting among the gentle ground-nesting bees. Photo by Marian Hengemihle

The ground bees were identified as gentle ground nesters, Cellophane Bees (Colletes thoracicus), also called Plasterer Bees. They are important pollinators of plants and are not aggressive or defensive. They are solitary bees and are not prone to sting humans. They have short lives. After the females lay the next generation and provide food, they will die off. Encourage everyone to tolerate these gentle ground nesters and important pollinators.

Additional Resources

By Marian Hengemihle, Horticulture Consultant, Certified Professional Horticulturist, University of Maryland Extension Home & Garden Information Center

Do you need help to identify bees or other insects found around your yard or home? Send clear photos of your insects to HGIC’s Ask Extension.

Plant for pollinators: 9 ways to attract and help pollinators in your garden and yard

monarch butterfly

Pollinators of all types – insects, birds, and bats – are in decline due to habitat loss, pesticide use, and diseases. Insects – including Maryland’s 400 species of native bees – provide valuable pollination and a food source for wildlife. Insect pollination is essential for the production of about one-third of our food crops. And some pollinators, such as butterflies and hummingbirds, are simply a delight to see!

You can make a difference for pollinators by incorporating these practices in your garden or yard.

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