Carpenter bees
Click for larger image Close-up of the exposed tunnels made by carpenter bees (Hymenoptera) for their eggs. The grooves in the wood were made by woodpeckers drilling for the larvae.

Carpenter bees, Xylocarpa sp., are large bees that resemble bumble bees. They can be distinguished from bumble bees by their shiny black abdomens with yellow hairs only on the first abdominal segment. Bumble bees have yellow hairs on other abdominal segments as well. They are solitary and do not form colonies. Males have a white- or cream-colored spot on their head between the mandibles that looks like a “nose”. Carpenter bees are pollinators and therefore are considered beneficial. They can, however, damage untreated wood, especially cedar, around the home and garden where they excavate tunnels in which to rear their young. If they are not in a place where they are bothering anyone they can be left alone.

Symptoms and Diagnosis

Carpenter bees excavate tunnels, 4-6 inches long and about ½ inch in diameter in rotten or soft, untreated wood where females lay their eggs and the young develop. Additional damage can occur when woodpeckers peck holes in the wood in search of the developing larvae. The first sign that you may have a problem with carpenter bees is when they fly close to your face when you are close to their nest. Males may appear aggressive but they can not sting. Females can sting if handled.

Life Cycle

Carpenter bees go through complete metamorphosis. Adults emerge from the pupal stage in August or September, feed, then return to their old nests where they overwinter. They emerge in April and mate. Females lay eggs in existing tunnels or seek out new locations. Females have strong mandibles which they use to excavate tunnels in soft woods, such as cedar. They can advance a tunnel by about 1 inch in six days. They lay an egg and pack in pollen and nectar to feed the larvae, seal the cell, and then start another cell. A tunnel will contain several cells. After the female completes egg laying she dies. There is only one generation a year. Adults live only one year.

Integrated Pest Management Strategies

1. Prevention. Keep all exposed wood surfaces well painted with a polyurethane or oil-base paint to deter attack. Wood stains will not prevent damage. Consider using aluminum, asphalt, vinyl, or similar non-wood materials that are not damaged by carpenter bees. Seal existing gallery entrance holes to discourage carpenter bees that are looking for potential nesting sites.

2. Mechanical measures. When dealing with carpenter bees, it is preferable to locate tunnel entrances during the daytime, but treat after dark on a cool evening when carpenter bees are less active. Males lack stingers but female bees can inflict a painful sting if disturbed when provisioning their nest. Wear protective clothing to avoid any stings during treatment.

Deny carpenter bees access to their galleries by sealing each entrance hole. Thoroughly plug the hole with caulking compound, wood putty, or a wooden dowel affixed with wood glue. If possible, also fill the entire gallery system with a sealant. In new nests, the single female often can be swatted and killed, or she can be captured and crushed or otherwise destroyed. Larvae and pupae can be killed by inserting a sturdy wire into the entrance hole and probing into the gallery as deeply as possible.

3. Chemical controls. A chemical treatment using an appropriately labeled insecticide can protect wood for short periods, especially in the spring and summer when carpenter bee nesting activity is apparent. Dust formulations (e.g., 5 percent cabaryl) typically provide residual effects and are effective due to the nature of carpenter bee gallery construction. Precisely inject the dust directly into each nest entrance hole and as deep into the tunnel as possible. Also apply to adjacent wood surfaces. Wait for a few days before plugging entrance holes since adult bees should be allowed to pass freely to distribute the insecticide within the galleries. Newly emerged bees also will contact the dust when attempting to leave their gallery.

For use as a preventive, an insecticide should be applied to wood in early spring before carpenter bees begin excavating nests. The insecticide kills the bees that contact it on the wood’s surface. However, a preventive approach has limitations because of the difficulty in applying a chemical to all exposed wood on the house where bees could nest. Furthermore, such insecticides usually degrade in a matter of weeks or months so repeated applications are needed to maintain a lethal dose of the insecticide. Some pest management companies report good results against carpenter bees by spraying wood with a microencapsulated pyrethroid, Demand® CS insecticide (registered for use only by licensed professional applicators), which contains the active ingredient lambda-cyhalothrin. A number of other pyrethroids (bifenthrin, cyfluthrin, deltamethrin, permethrin, etc.) also are labeled for use against carpenter bees.

Insecticides that act as stomach poisons, such as borates, typically are ineffective against carpenter bees, which do not ingest the wood that they excavate.

Organic Strategies

Strategies 1 and 2 are strictly organic approaches.

More images:

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These are tunnels made by carpenter bees (Hymenoptera) in a trellis. The tunnels were opened by woodpeckers looking for larvae to feast on.
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Typical circular holes made by carpenter bees (Hymenoptera) in a pergola
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Male carpenter bees (Hymenoptera), identifiable by the white spot between their mandibles, look and act aggressive but actually cannot sting. As with all bees only females have stingers.
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Carpenter bee (Hymenoptera) on crepe myrtle (Lagerostroemia); note, the shiny black abdomen characteristic of carpenter bees
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Carpenter bee (Hymenoptera) pollinating a daphne (Daphne).
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Carpenter bese (Hymenoptera) on wild passion flower Passiflora incarnata demonstrating their importance as pollinators.
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Holes in a split rail fence caused by carpenter bees (Hymenoptera)