Friday, May 13, 2011

Pawpaws and Pollinators

With large, tropical-looking leaves, and a dark, mysterious flower, the Pawpaw (Asimina triloba) is the kind of tree Tennessee Williams would have planted next to the magnolias, if he had been into gardening. The blooms look like a dark red bell hanging below the branch. The dark color makes them less than showy than a Flowering Dogwood, but attractive, especially when backlit by the afternoon sun. It’s the classic Missouri bottomland tree, but when I mention that it’s pollinated by flies because the bloom looks like roadkilll—well, it turns people off.

I wrote about Pawpaw’s relationships with butterflies and moths last July. Still, people imagine, as I did, a Pawpaw in bloom, swarming with big hairy flies and a putrid odor wafting on the wind. Nothing could be farther from the truth. I took the photo above on April 20, but when I downloaded it from the camera and the photo filled the screen, I saw the flies for the first time. The flower was about 2 inches across at its widest point. I would have had to wear my reading glasses to see the flies.

Pawpaws don’t make it easy on a pollinator. To bear fruit, the bloom needs pollen from an unrelated Pawpaw. What’s more, process depends on a pollinator that is believed to be inefficient, sloppy, and promiscuous—that is, it’s like to visit many things other than a bloom of a Pawpaw.
Gosh, the flower looks alarmingly fleshy in this extreme crop! Those flies look pretty scary too, but the red eyes in the photo are caused by the flash.
Ted MacRae at Beetles in the Bush wrote about Pawpaws last May. In comments,  James Trager, entomologist and restoration biologist at Shaw Nature Reserve, wrote that the tree is likely pollinated by fungus gnats. Fungus gnats are important pollinators, but the critters in my photo don’t have the comparatively long legs I associate with gnats.

I posted my photo on Bug Guide to ask for some ID help.  One person suggested that they could be lesser dung flies. I estimated that the fly was 2 mm long or less—about the right size for dung fly family (Sphaeroceridae).  The Wikipedia article states that dung fly larvae eat fungus or bacteria on rotting vegetation. Another user on Bug Guide suggested that because the blossoms are stinky, they must be some type of carrion fly. That brings us to the next question, what does a Pawpaw blossom smell like? 

I headed out to the tree again and got up close and personal with the flower. It didn’t smell like roadkill, or dung—it had just the faintest odor of yeast. I could only detect the scent when my nose touched the bloom. Of course, yeast is a fungus, so fungus gnats and lots of other decomposers should find the fragrance attractive. But why be shy about the aroma if the plant really wants to be pollinated? 

I have some ideas about that. First, just because the odor is hard for me to detect doesn’t mean that it’s hard for insects to find. Although I had been thinking that there was only one specialized organism that pollinated Pawpaws, I found this statement in a 2001 article in Ecology and Society by Carol Ann Kearns:  "Many species of flies are generalists that visit multiple plant species." There are probably a number of different fly families that offer their pollination services to Pawpaw, though the flies don't seem to get any reward. 

Second, it's possible Pawpaw is a bit conflicted about this whole fruiting process. According to California Rare Fruit Growers, because each bloom has more than one ovary, "a single flower can produce multiple fruits." Add to that this advice to growers resorting to hand pollination, from a 1990 article from 
Kentucky State University Cooperative Extension Program: "
Do not overburden the tree with fruit, as this will stress the tree, resulting in smaller than normal fruit, and may cause limbs to break under excessive weight." Perhaps half-heartedly attracting an inefficient pollinator is Pawpaw's adaptation to it's own heavy fruit.


  1. Extraordinarily lurid! I remember pawpaw from my youth, but the fruit and not the flowers. Some tropical fruit have low pollination success and large fruit that I can imagine limbs breaking off if they were more successful (mango comes to mind).

    I don't know if you remember my comment on your guest post dogbane at BITB last year, but Adrian Thysse has done some research and discovered:

    I found this at CBIF:

    “Spreading dogbane (Apocynum androsaemifolium) is a native herb found across Canada. This plant has been reported to cause serious poisoning potential in cattle, horses, and sheep after ingestion (Johnson and Archer 1922). This information was credited to a report from the Arizona Experiment Station and to an article that was erroneously stated to be about Apocynum. However, that article actually concerned Nerium (oleander) poisoning of livestock. Therefore, the various signs and symptoms attributed to dogbane poisoning since 1922 are usually based on this mistake (Kingsbury 1959).”

  2. Re the flies - my first guess would have been Drosophilidae - and fruit flies and a yeasty odour go together.

  3. Wow, Dave, I wish I'd thought of the phrase, "Extraordinarily lurid!" Great description! Thanks for your info about dogbane, which is another really interesting plant.
    Drosophila seems like an excellent guess.

  4. I didn't know about this tree before, very interesting and curiously shaped flower.

  5. Nature has amazing diversity, doesn't it?

  6. Thanks for your wondering! I too wonder. My several year old yet still smallish Paw Paw varieties (3 different types, counting one successful graft)have some flowers: one has many, the others a few. So I cut heavily flowering flowers from a nearby grove, and put them in water, at the foot of my trees. We shall see! I did notice ants, small fruit fly types of flys, and some larger flys...

    Andrew Goodheart Brown