Cover Story

By Dr. Jennifer Tsuruda, Assistant Professor, and Dr. Karen Vail, Professor, UT Entomology and Plant Pathology Department

Spring is here, and as floral resources become abundant, bees begin buzzing and developing their nests. Most people who spend time outside are familiar with large carpenter bees (Xylocopa species) – their size, loud buzzing, and hovering make them hard to miss. While the behavior and activity of these native bees can be problematic for homeowners, these bee species also provide beneficial pollinator services.

• Large bees, 0.75" (20 mm) or more in length

• Dark/black abdomen [Photo 1]

• Carpenter bees are similar in appearance to bumble bees [Photo 2]

• The adult abdomen is less hairy than bumble bees

• Carpenter bees have a “shiny hiney”, with little to no hair [Photo 1]

Light patch on the “face” of males [Photo 3]

• Males cannot sting but can appear intimidating when hovering around people near nesting sites or females

• Xylocopa virginica and Xylocopa micans both occur in Tennessee. Other species may be present in other regions

REPRODUCTIVE HABITS AND RESOURCE USE

Rather than living in colonies, like honey bees and bumble bees, female carpenter bees create nesting galleries in wood. Carpenter bee entrance holes excavated by females are very clean and circular, around ½" (12 mm) in diameter [Photo 4]. From the entrance hole, the colony-founding female will turn and chew one or more tunnels that are often 4 – 7" long, but can extend several feet after reuse and expansion over multiple years.

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Although the giant resin bee (Megachile sculpturalis) cannot drill through wood like carpenter bees, this non-native, introduced bee species may occupy abandoned carpenter bee galleries or displace carpenter bees and capitalize on these pre-existing nesting sites. Female giant resin bees are similar in appearance to female and male carpenter bees – at least 0.75" (at least 20 mm) in length and dark in color, but are narrower in width [Photo 5]. Native to Asia, giant resin bees pollinate a federally threatened viny plant (Apios priceana, known as Price’s Potato-Bean) that is native to the southeastern region of the United States, as well as other flowering plants.

The tiger bee fly, Xenox tigrinus, is a parasitoid of the carpenter bee and derives its name from the dark wing stripes resembling that of a tiger [Photo 6]. Tiger bee fly larvae hatch from eggs laid in the carpenter bee nest and feed on the carpenter bee larva or pupa in its cell.

These complex interactions within our landscape ecosystems highlight the need for thoughtful and careful management plans for pests and beneficial organisms. We continue to search for simple and effective solutions to pest activity, but carpenter bee nesting and interactions of these species with humans and other pollinators exemplify how different circumstances may warrant different strategies. In spring, when nesting female carpenter bees are chewing into our homes, it is easy to simply classify them as pests, rather than beneficial pollinators. In reality and like many organisms, carpenter bees have both harmful and helpful behaviors. With a bit of effort, we can employ practices to limit their structural harm in ways that allow their ecosystem benefits to be appreciated.

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A tunnel consists of a series of cells, each provisioned with a ball of pollen mixed with nectar, and a single egg [Photo 7]. Each cell is sectioned off with a division of chewed up wood pulp. After hatching, the larva consumes the provisions and progresses through multiple larval instars before pupating into an adult. Adults emerge in the summer and forage on floral resources before overwintering in galleries. In the spring, males typically emerge earlier than females and establish and guard territories and females.

Given the overlapping housing issues for humans and carpenter bees, many homeowners are surprised that carpenter bees can still be considered beneficial insects for their ecosystem services [Photo 8]. Native bees, including carpenter bees, pollinate approximately 15% of agricultural crops in the US. Carpenter bees are also frequent pollinators of home gardens that include tomatoes, eggplants, and other plants needing sonication for the release of pollen (buzz pollination).

General preferences for carpenter bees include unpainted, untreated, weathered wood from varieties of softwood trees, like cedar, spruce, and pine, that are easier to chew. However, lumber from less preferred hardwood species may be utilized if these are the only available resources. Carpenter bee galleries in natural wood can be found in stumps, logs, and dead branches. Structural lumber and fence posts are timber products that are frequently compromised.

While carpenter bees contribute valuable ecosystem services as pollinators, they also are often considered pests. Our annoyance also stems from sustained infestations, repeated fly-by’s and buzzing, and defecation streaking below nest entry holes [Photo 9]. A single year’s nest building may not result in extensive damage, but carpenter bees continue to expand upon old nests, adding additional galleries in subsequent years. In time, their activity can weaken structural timbers if not curtailed.

Successful management often requires multiple actions, repeated treatments, and persistent efforts.

Commercial traps are available that provide easy entry holes and less obvious exits. Simple designs utilize a wooden block with an appropriatelysized entrance hole and tunnel to attract and lead the bees to a clear compartment – a destination with no exit. Bees struggle to find a way out and eventually exhaust themselves to death. Additional work is needed to determine how effective these are at eliminating female carpenter bees, which are the individuals that build nests, and whether traps measurably decrease nesting in structural wood.

Non-chemical management also can include inserting a strong but flexible wire into galleries to destroy developing bees. Once cells are damaged, the entrance hole should be plugged and sealed with plastic wood, putty, or a similar substance. If possible, paint the patched surface to further discourage re-entry. This method is more likely to be successful for nests with few and short galleries, and seasonal timing should be considered for effective implementation.

Reducing or replacing exposed wood can help prevent initial probing and excavation. This pro-active approach is most practical when building new structures and refurbishing severe damage. Non-wooden materials like vinyl are unlikely to encourage nesting. Lumber from hardwood species is generally less preferred than softwoods, but nesting in hardwood can still occur if preferred wood sources or sites are unavailable.

For wooden eaves, siding, or decks with nesting issues, application of registered pesticides that are labeled for carpenter bee control is an option. Dust, spray, and foam formulations are available but insecticidal dusts can be easily blown into an entrance and further spread throughout the gallery by the bees. In addition, dusts will not be absorbed into the wood like the other formulations, and will be available to the newly emerged adult bees. Consult Extension factsheets for pesticide suggestions1 that can be used by pest management professionals

(https://tiny.utk.edu/W658) and by homeowners (https://tiny.utk.edu/ PB1303). As with the non-chemical wire method, the entrance holes should be sealed after treatment with plastic wood, putty, or a similar substance to prevent re-colonization. However, when applying dusts, wait at least a day before plugging to allow time for the female colony-founding female to contact and distribute the dust within the tunnels.

There are no simple, one-time treatments that are likely to provide effective management of carpenter bees nesting in structures. Simply sealing nesting holes without treating or eliminating developing bees can result in those bees exiting the nest by chewing new holes, thereby creating new openings (and opportunities) in the wood. Pesticide reapplication is usually needed for long-term control. Successful carpenter bee management may take several years because their reproductive life cycle spans the calendar year.

As a caution to applicators, the insecticides that may be used for carpenter bees are not specific to just this type of bee so it is highly important to follow the label and adopt practices that prevent and reduce the likelihood of accidental pesticide exposure to non-target insects (e.g., other pollinators). A second reason for plugging and sealing nest entrance holes is to prevent entry and infestation by other pests, and also helps keep insecticides within the gallery. Without sealing, insecticides can leak into the surrounding environment, where other beneficial insects and birds may be foraging, nesting, or visiting.

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Unlike large carpenter bees that nest in household structures, soffits, and moldings, small carpenter bee species in the genus Ceratina nest in pithy plant stems rather than household structures. Due to their smaller size (less than 8 mm long) and plantnesting behaviors, small carpenter bees are not considered economic pests.

More information on carpenter bee biology and management can be found in the UT Extension publication “W 876 Nature’s Right-Angle Drill: Carpenter Bees” — please scan the QR code or visit: https://tiny.utk.edu/W876

1 CAUTION: Some pesticides mentioned in this publication may not be legal outside of TN. If in doubt, please consult your county Extension office or regulatory agency. Furthermore, ALWAYS READ AND FOLLOW LABEL DIRECTIONS FOR THE PRODUCT YOU ARE USING.

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This article was originally published in Tennessee GreenTimes, Spring 2022 and is reprinted with permission.