OIL-DIGGER BEES

Genus Centris 

 

Centris bees are difficult to miss.  They are hefty black-and-beige bees that buzz loudly as they zoom through gardens and trees at the National Butterfly Center.   Fast and adept fliers, they are often glimpsed pausing suddenly in mid-air to hover noisily over a blossom. 

 

According to Wilson & Carril, authors of The Bees in Your Backyard, bees of the genus Centris are sometimes referred to as “oil-digger” bees, because they extract flower oils from plants in addition to, or instead of, gathering pollen and nectar.  The bees collect, modify and use the oils to line their nest chambers.

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The hind legs of female Centris bees are covered with bushy pollen-collecting scopal hairs, and bell outward at the bottom like pantaloons.  The front and middle legs of female Centris bees are outfitted with small bristle-like hairs called setae, which form combs (made up of narrow lines of setae) and pads (made of branched and sometimes hooked setae).  These combs and pads are specially adapted to aid in the collection of floral oils. Some of the setae are blade-like and used to cut open flower receptacles that contain oil. Centris bee females transfer oils from their front and middle oil-collecting legs to their hind-leg scopal hairs in mid-flight.

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Male Centris bees tend to hover around nest entrances waiting for females.  In the oil-digger species Centris pallida, males dig up females as they emerge from their underground nests.
 

 

Food plants

Centris bees are associated with plants that produce abundant floral oils, such as manzanita, creosote, and Krameria.  Almost all female Centris bees collect oils, at least at times, from plants in the family Malpighiaceae – which includes such flora as Barbados cherry (Malphigia glabra), a plant native to Hidalgo County.  Both manzanita and Barbados cherry bloom in the spring at the National Butterfly Center -- when they do, they are mobbed by such large numbers of Centris bees that the bushes seem to hum.

 

​Centris bees also can be observed visiting a range of other plants for pollen or nectar.  At the National Butterfly Center, Centris bees feed on  crucity, mallows, palo verde, pigeonberry, sunflowers and snoutbean.

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Identification Information: 

 

Most Texas Centris bees are fairly large (around ½ inch or longer), with black abdomens; shaggy black legs; and thoraxes covered with light-brown hairs.  Both females and males have pale hairs on their heads and often sport pale-yellow facial markings on the clypeus and labrum (the parts between and above the mandibles).  This trait helps in part to distinguish them from Anthophora digger bees (shown in this guide's preceding section):  among Anthophora  digger bees, only males have pale marks on their faces.  

 

At least eight different Centris species inhabit Texas:  Centris aterrima, C. atripes, C. caesalpiniae, C. cockerelli, C. hoffmanseggiae, C. lanosa, C. mexicana and C. rhodopus.  Two of these, Centris atripes and Centris nitida, were documented at the National Butterfly Center in the fall of 2018.

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The black-footed oil digger bee (Centris atripes) is a common visitor to the National Butterfly Center in April and May, the months when manzanita and barbados cherry are flowering.  Males of this species tend to buzz around the trumpet-shaped blossoms of the ornamental shrub known as esperanza; males also can be found sleeping in esperanza blossoms.  Male black-footed oil-digger bees also frequent plants in the Verbenaceae family, such as pigeonberry and Berlandier's Fiddlewood.  Nests of this bee have been sighted in NBC areas bordering a canal that parallels the Rio Grande.

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Until recently, the shining oil-digger bee (Centris nitida) was rarely found north of the Mexican border.  Centris nitida is generally thought of as a neotropical species that typically ranges from southern Texas through Bolivia.  (The species has made a recent appearance in southern Florida, where it is considered invasive but apparently harmless.)  Centris nitida oil-digger bees are encountered occasionally at the National Butterfly Center.  According to Texas bee expert Jack Neff, the bees' appearance signals that Malpighiaceae-family plants are in bloom somewhere in the vicinity.

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The minute traits that distinguish these two species from other Centris bee species are set forth in detail below.