Robbing Nectar

A female California carpenter bee, Xylocopa californica, distinguished by the bluish metallic reelections on her abdomen and smoky brown wings, drinks nectar from a well drilled at the base of a sage flower. Note an additional well in the base of the second flower. Photograph copyright Anne M. Rosenthal.

Flowers produce nectar to lure pollinators inside. When entering to partake, an insect brushes past the flower's anthers, which bear the pollen. Some pollen sticks to the insect and is transported on to the next flower. To put it succinctly, nectar is payment for a job, and that job is pollination.

But what if you're an insect too big to crawl inside a nectar-bearing flower – perhaps many times too big? Carpenter bees solve this problem by drilling a well through the base of the flower. In other words, they steal the nectar instead of working for it.

A male mountain carpenter bee, Xylocaopa tabaniformis, drinks from a flower well. Note the bald spot on the bee's middle section (the thorax), likely caused by a mite – possibly the minuscule red flat patch near the line of remaining hairs. Worn wings indicate this bee is a senior citizen. Photograph copyright Anne M. Rosenthal.

The chewing apparatus of the female California carpenter bee in silhouette. Photograph copyright Anne M. Rosenthal.

California carpenter bees come equipped with formidable mouthparts, used by females to excavate nesting tunnels. Since carpenter bees often excavate in fences or other structures, they bear a mixed reputation of pollinator and pest.

Opened by a woodpecker searching for larvae, this California carpenter bee nesting tunnel in a redwood fence contains individual nest cells separated by disks of cemented wood bits. Photograph copyright Anne M. Rosenthal; taken on the property of Irene Brown, PhD., with permission.


Chewing through a flower is child's play by comparison. However, it's still a certain amount of work, and carpenter bees often scout for predrilled nectar wells rather than making new ones. Other insects, such as honey bees, may sip from the wells when the carpenters are not around.

Despite cheating this sage flower by robbing nectar, a female California carpenter bee has collected pollen from – and in so doing pollinated – other flowers. She carries the pollen in short stiff hairs, known collectively as a pollen brush, on her hind legs. Photograph copyright Anne M. Rosenthal.

References:

Milne, L. and M. 1980. National Audubon Society field guide to North American insects and spiders. Alfred A. Knopf, New York.

Powell, J. and C. Hogue. 1979. California insects. University of California Press, Berkeley.