The Lower Colorado River subsection of the Sonoran Desert and the adjacent Mojave Desert are home to two types of native wildflowers, both often called ghost flowers for their pale translucent white coloration. The one most featured in this post is Mohavea confertiflora, the Mojave ghost flower, which has a widespread range across several US and Mexican states, but seldom occurs in large numbers anywhere. Most plants are sparsely scattered here and there across rocky slopes, gravelly plains, and sandy wash channel banks. They aren’t really rare, but they certainly aren’t the most commonly seen desert springtime wildflower either.

Mojave ghost flower (Mohavea confertiflora) grows with masses of Arizona lupines (Lupinus arizonicus) along the Lower Colorado River in the wet winter and spring of 2018-2019.

Mohavea confertiflora is a member of the plantain family (Plantaginaceae). Up until about maybe 10 years ago, they were included in the figwort family (Scrophulariaceae) but new DNA and phylogenetic studies forced a revision of numerous plant families and genera, with many shifts and reclassifications that were confusing even to many botanists at first.

Mojave ghost flowers grow best on sandy gravel substrates in both the Mojave Desert and the drier reaches of the northwestern Sonoran Desert. This plant is about 18 inches across and nearly a foot tall, with about 30 flowers open at once.

The figwort family was arguably too cumbersome and diverse anyway, and had unofficially been used as a taxonomic dumping ground for plants that didn’t really seem to fit anywhere else based upon visible morphological characteristics. Once DNA analysis became much easier and more affordable to do, however, studies suggested that genetic and biochemical relationships between various plant families had to be rethought and that plants that were once seemingly unrelated needed to be placed closer together into different families. Some plant families were drastically reduced in size (like the aforementioned Scrophulariaceae) and a few new families were created. In certain cases entire genera and sometimes even small plant families were eliminated, subsumed into larger ones, with numerous shifts that were challenging for the average naturalist or horticulturist to understand. Even experts were sometimes confounded. But this is all part of the scientific method, to rethink old assumptions and change theories based upon the best new evidence coming in….

The “other ghost flower” – Mentzelia involucrata, better known as the sand blazing star. Photo credited to Wikipedia.

At the outset of this article, however, I mentioned two so-called ghost flowers, so allow me to now introduce the other one, Mentzelia involucrata, heretofore better known as the sand blazing star. Unfortunately, I didn’t catch any photos of this species in my recent flower-seeking travels, so I will rely upon the one Wikipedia photo above to make the next set of points.

Detail of the Mojave ghost flower’s petals. The similarity between the two species is obvious, although there are clear differences as well.

Note how while the sand blazing star is a single pallid color with a few marginally darker veins, the ghost flower has a series of dark reddish-maroon speckles, as well as a larger dark red patch that lies directly underneath an arching, dark yellow central structure that bears both pollen and receptive female parts. This will be important in a bit. I’ll take a moment to point out that while these two flowers look related, that they are in fact from two entirely different plant families. As already discussed the ghost flower is in the plantain family (Plantaginaceae), while the sand blazing star is in the loasa family (Loasaceae). Both plants have been modified in leaf structure, lifestyle, and overall stature and appearance by their desert environment into looking similar, but they are indeed not all that closely related otherwise.

My friends Ken and JoAnn photograph a Mojave ghost flower along the Lower Colorado River Valley just north of Parker, Arizona on Wednesday, March 6, 2019.

But the evolutionary histories of these two separate plants from different families and lineages have become entwined in a rather unique way: The Mohavea is thought to be a flower mimic of the Mentzelia. The sand blazing star is pollinated primarily by small sweat bees of the genus Xeralictus, for which it produces a specific type of nectar that is extremely attractive to the female bees, who use it to raise their brood in their small nests. Other insects also visit the flowers and probably do some degree of pollination for the blazing stars, but the Xeralictus bees are likely the most motivated and effective pollinators for the plants.

Mass opening of numerous blossoms all at once is a common strategy amongst desert wildflowers. It concentrates both plant and insect activity into a short but intense season before normally prevailing drought and poverty conditions recur, which often happens quickly in arid landscapes. They don’t delay with growth and reproduction because they can’t afford to waste time. Harshness is always just around the corner in a desert environment!

Meanwhile, the Mohavea has “learned” to capitalize upon the dedication shown by Xeralictus bees in visiting the Mentzelia flowers, by creating those brick red spots that I mentioned earlier. Observations and studies have shown that the main pollinators of ghost flowers are actually male Xeralictus bees, not females.

How many bees do you see in this photo? If you’re a male Xeralictus bee, this appears to be a “meet market”. (Or “meat market” – use your homonym of choice.)

Why does the ghost flower target male bees? By using the large dark red spot in the center to resemble the abdomen of a feeding female bee. The males often copulate with feeding female bees who are preoccupied with nectar and pollen gathering, and obviously this typically occurs in a blazing star flower where the females hang out. The male bees are uninterested in pale, spotless blazing star flowers that have no females, since their primary job is to mate. But they are very interested in spotted ghost flowers that appear to have females in them!

Another ghost flower grows with expanses of Arizona lupines and a few yellow suncup primroses (Camissonia brevipes). Interestingly, the suncups were also reclassified into a different genus, Chylismia, using the same revisionist DNA and other studies that reclassed the Mohavea into the plantain family. Although to be clear, Chylismia brevipes has remained in its original evening primrose family of Onagraceae, and only changed genera.

Part two of this deception is that the floral structures mentioned earlier in the ghost flower, the yellow arching reproductive parts, are positioned in such as way that the male bees crawling around on the red spot beneath attempting to find the female get pollen dabbed onto their backs. When the male bees repeat their mistakes and visit another flower on another plant, the pollen is transferred, fertilization occurs, and the cycle completes with new seed pods being set. But they didn’t get to mate, sadly.

What else is deceptive about this? The Mojave ghost flower does not produce any nectar or pollen rewards for the male bees. The pollen they do produce is located on the back of the males’ thoraxes, out of reach, and therefore safe from consumption. They don’t need to reward their hapless male visitors, since hormone-addled male sweat bees aren’t really looking to feed anyway, just to hook up. The ghost flowers don’t even bother with trying to attract female Xeralictus bees since their trickery works so well with the males. Through false advertising, the plants can instead invest more energy in seeds, rather than in nectar or feeding extra pollen to females, the way the blazing stars do. Basically, the ghost flowers are free riders on the main relationship between the female bees and the blazing stars. They do this by manipulating the males on false pretenses.

Since the two species of flowers inhabit similar habitats, and their natural ranges overlap fairly closely, this particular association works in an ecological sense. If for any reason the sand blazing stars or the Xeralictus bees were to become rarer or locally extinct, it is probable that the ghost flowers would also follow. However the inverse is not true: If the ghost flowers were to become rare or extinct, it would not likely impact the population dynamics of blazing stars or bees very much. It’s sort of a one-way street.

This is but one example of the many fascinating hidden relationships to be witnessed in the ecological realm, so often unseen by humans unless we observe closely. Let’s just hope that no one disappears without a trace, effectively “ghosting” anyone involved. 😉

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Mark A. Chappell’s gorgeous images of sand blazing star.

I saved this screenshot of a beautiful website featuring the images of photographer Mark A Chappell, a faculty member at the University of California Riverside, because it shows a pleasing array of images of Mentzelia involucrata, including floral details, the Xeralictus bees, and habitat shots. Link here:

http://faculty.ucr.edu/~chappell/INW/plants/sandblazingstar.shtml

Here is another link, also to the University of California Riverside, discussing the way the male Xeralictus sweat bees are tricked into pollinating the ghost flowers. This was the main page that led me to create this blog post, using (mostly) my own photos. But credit does go to UCR for inspiring me. 🙂

https://entmuseum.ucr.edu/bug_spotlight/posted%20Images-pages/33.htm