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#21 - Netted Pawpaw

When we talk about pollinator gardens, too often people refer to only two kinds of insects: butterflies and/or bees. The latter is furthermore mostly restricted to the ubiquitous European honeybee (Apis mellifera) which, as the name implies, is a native to Europe and was introduced to the Americas for its honey in the early 17th century. Its adaptability and undemanding character as an ecological generalist made it also very useful for modern human agriculture as the main pollinator for most of our crops. Habitat destruction, pesticides, and loss of plant diversity have made it impossible for most other insects to live around conventional farmland, leaving only one species of pollinator to ensure our food supplies.

However, a pollinator can be anything that is physically able to successfully fertilize a certain type of flower. Netted Pawpaw (Asimina recurvata), for instance, has undergone a coevolution with certain species of beetles that the plant provides nutrients and shelter for while the beetles pollinate the flowers. This deciduous shrub grows to be 4 feet tall and can almost exclusively be found in Florida in a wide variety of plant communities around scrub, pine flatwoods, and dry prairie. Even though several different species of pawpaws have long been known to Europeans (the British naturalist William Bartram described two in 1773), and the fruits were historically used as a common food source by Native Americans, the first ecological description of the plant’s relationship with beetles was by the Dutch botanist Johannes Uphof in 1933. Since then more research has revealed the complexity of the relationship between these insects and pawpaws.

The flower of Netted Pawpaw consists of three large outer, and three smaller inner petals (see photo) that arch over the sexually reproductive organs, forming a so called “pollination chamber,” in which the beetles, like the hairy scarab Trichiotinus viridian, feed on the nectar and specialized nutritious structures on the insides of the inner petals called corrugated tissue (see photo of gnaw marks on the corrugated tissue of the inner petals.) Unlike bee or butterfly pollination which occurs rather quickly, beetle pollination, also known as cantharophily, is usually a long process that can last for hours or even days. The plant reacts to the gnawing on its petals metabolically with an increase of the temperature of the flower, which causes the beetle to linger in the warmth of the flower’s interior. This presumably increases the fertilization rate as the insect repeatedly crawls over the anthers (see photo of the white round cluster of anthers) allowing pollen to stick to the scarab beetle’s ventral hairs. As the beetle finally moves on, the pollen is then carried to the stigmas (the female organ) of the next flower.

Restricting our understanding of what a pollinator is to butterflies and one species of bee can make us blind to the actual diversity and complexity of nature. Habitat simplification through human development and modern agriculture has in many cases reduced the biological diversity to a handful of plants and wildlife that are barely enough to ensure our own food supplies. But introduced species, pesticides, and climate change have put further stress on these few insects that pollinate our crops. A common proper understanding and recognition of the complexity of our natural world is therefore crucial not only for the environment, but also for our own well-being.

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