Alkali bees, the alfalfa specialists

Honey bees aren’t always the best bug for the job, and that is certainly the case when it comes to pollinating alfalfa. Honey bees do not particularly like alfalfa pollen and colonies don’t thrive when it is the sole source of their protein. But what really bothers the honey bee is the tripping mechanism of the the alfalfa flower. When the bee reaches down into the center of the flower to sip the nectar, the flower releases its pollen with an explosive spring that delivers a good whack to the honey bee. To avoid the direct hit, honey bees learn how to “steal” the nectar from the side of the flower. While this works for the bee, it fails to pollinate the flower.

As it turns out, a solitary ground-nesting bee, Nomia melanderi, is the very best pollinator for this important crop. The tripping flowers bother her not, and she pollinates the fields with gusto. Slightly smaller than a honey bee, with a bluish-, yellowish-, or greenish-and-black stripped abdomen, these bees live in vast communities. Even though each female raises a family by herself in her own home, she is part of an impressive neighborhood.

These bees are known as alkali bees because they prefer to nest in soil with a salty crust which covers a damp, saline soil beneath. The salty soil holds moisture, discourages plants and their annoying roots, and is inhospitable to some ground-dwelling organisms. The perfect home is not easy to find, but once established, the alkali bees will thrive for year after year if the soil remains undamaged and a good nectar source remains nearby.

In the northern states, alkali bees produce only one generation per year and in Washington, the active period coincides perfectly with the alfalfa bloom. Although alkali bees are known to be polylectic—meaning they will forage on a variety of different plants—when a good supply of alfalfa is nearby they can thrive on it alone.

In the late spring, the males emerge first. They spend their days patrolling the nest areas looking for females, and they often spend nights in groups in the nearby foliage. Once a female mates—at two or three days old—she begins the task of preparing her nest.

As shown in the diagram at right, the female digs a tunnel straight down in the moist soil, two to eight inches deep. She prefers a pre-dug hole, so if she finds one, she cleans it up and makes her home in that. She divides the vertical tunnel into seven to twelve branches, each terminating in an oval cell. The female packs the cell walls with fine particles of soil and then coats it with a moisture-resistant material she secretes. Once complete, she provisions the cell with a pollen sphere on which she deposits one egg. She then seals the cell and moves on to the next.

Each female lives four to six weeks and the entire active season lasts about 60 days. To learn about the large threat facing these bees in Washington state, please see my earlier post: “Alkali bees face death by highway.”