Thursday, June 4, 2020

Pawpaws, flies, and Jurassic pollinators

Pawpaw flower with many flies, C. Kent CC-by-SA 3.0

Last time I blogged about a non-traditional pollinator, beetles. Today it's the turn of another unusual plant, the Pawpaw, which is pollinated by flies, beetles, and scorpionflies.  The picture above is from my garden today. I was out with an artist's paintbrush transferring pollen between pawpaw flowers on two trees, when for the first time I noticed swarms of small flies in and on the flowers. Some are probably fruit flies, but there are some larger flies too.

Pawpaw (Asimina triloba) is a native Carolinian Canada tree with delicious fruit. Pawpaw leaves are the food for Zebra Swallowtail caterpillars; I blogged about this recently here and here. Like the topic of yesterday's blog, the Spicebush Calycanthus, pawpaws are part of the magnoliid group - relatives of magnolias. Pawpaws are the only northern member of the custard apple or soursop family, Annonaceae. Most family members are from the tropics. They include beloved fruits such as custard apple, soursop, cherimoya, sweetsop, ilama, soncoya, atemoya, and biriba, as well as sweetly scented flowers such as ylang-ylang that are used in perfumes.

bank of Potomac river, Selden Island, Maryland
Like other magnoliids, Pawpaws have non-traditional pollinators. The mature flowers have a slightly sweetish decaying fruit smell and attract flies, as seen above. But there's a fascinating extra group of pawpaw pollinators, the scorpionflies (Mecoptera). Both pawpaws and scorpionflies like moist areas, which is where I found them together in May 2013 on the banks of the Potomac river near Ashburn, Va. Large stands of pawpaws were growing and blooming near the riverbank. I tipped one flower to the side and here's what I saw:
Pawpaw and scorpionfly, head in, feeding.

Whose were those striped wings and odd abdomen? Later I found one resting inside the flower:
Scorpionfly (Panorpa) in pawpaw flower
Although if may look a bit like a moth, the pollinator is actually a scorpionfly. This ancient insect group predates flies and butterflies. Here's a fossil scorpionfly from the Jurassic period - yes the time of the dinosaurs in Jurassic Park!
fossil scorpionfly Jurassipanorpa sticta from China - He Ding et al 2014

One of the more interesting and somewhat controversial theories in pollination science is that ancient scorpionflies were in fact the original pollinators of Jurassic flowering plants. The scientists focused on the long, beak-like proboscis or feeding tube of the scorpionflies, which you can see in the picture I took in Virginia:
Scorpionfly with long proboscis.

There is an excellent summary of this work on the Smithsonian website.

Present day scorpionflies not only visit pawpaws and other fly-pollinated flowers, they are also major scavengers of dead insects and other carrion. These harmless (they don't bite!), ancient pollinators are fun if you can find them. I'd suggest looking in the woods near the streams and ponds of High Park or near the Humber river if you are in western Toronto, or similar stream-side habitats wherever you live.

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