Large Carpenter Bees
Genus Xylocopa

 

Large carpenter bees are essential pollinators of tomatoes, eggplants and peppers – crops that cannot be pollinated by honey bees.  Plants in the tomato family require vibratile or “buzz "pollination, a skill that large carpenter bees possess -- that is, they vibrate their flight muscles while grabbing hold of flowers, in order to shake pollen from their anthers.  Large carpenter bees' size and attendant ability to carry prodigious pollen loads also make them highly effective crop pollinators. 

 

The National Butterfly Center has an unusually diverse array of large carpenter bee species.  While many northeastern states, for example, harbor only one species of large carpenter bee (the eastern carpenter), the NBC has several.  Some of these are featured below. Two of them (Strand's carpenter bee and Griswold's carpenter bee) are notable for marked sexual dimorphism within a single species. Females tend to be entirely black, while males are covered with golden-brown hairs that make them look a little like flying teddy bears.  In many other large carpenter bee species, both males and females are predominantly black.
 

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Nesting behavior and "nectar robbing" 

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Female large carpenter bees build tubular nests within plant materials or in rotted wood. The bees scrape fibers from the walls of their tunnels to form a kind of particle board used to divide their nests into separate egg chambers. The bees nonetheless do not eat wood; they feed exclusively on pollen and nectar.

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​One interesting aspect of large carpenter bee behavior is “nectar robbing”.  Within most bee species, females are the sole pollen-gatherers, but both male and female bees survive by drinking nectar from blossoms.  Because large carpenter bees are so hefty, they have difficulty entering a variety of flowers to obtain nectar.  Instead of crawling into a flower with a narrow, tubular neck, a large carpenter bee may use its sharp mouth-parts to slice holes in the base of a blossom – the bee then "steals" nectar by lapping it from the flower's anthers through this hole.

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At the National Butterfly Center, large carpenter bees are frequent visitors of plants such as esperanza, scarletbush and Turk's cap, all of which have deep-throated blossoms.  On any given day, if you lingered by any of these plants, you might well see large black bees sidling up to the blossoms to engage in nectar robbing.

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Some carpenter-bee critics complain that nectar robbing allows carpenter bees to take nectar from flowers without coming into contact with the anthers in a way that leads to pollination.  In addition, after a carpenter bee robs nectar from a flower, other pollinators may shun the depleted blossoms. Thus, the "robbed" flower never gets pollinated.

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Nonetheless, there is some evidence that nectar robbing actually may boost pollination and aid other pollinator species. Smaller bees, including honey bees, often take advantage of the holes slit into the sides of blossoms by carpenter bees.  The holes’ existence encourages multiple trips by such smaller bees to the flowers, leading to increased pollen-gathering.

 

As shown in the accompanying photo strip, at the National Butterfly Center, green metallic sweat bees often take advantage of holes slit in esperanza blossoms by carpenter bees.

 

Other studies have shown that this benefit is not reserved for smaller bees alone – butterflies also take advantage of nectar-robbing entrances to gain easier access to flowers.