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Additional Information from Technical Papers
Prior to the Spring 2021 observation, Connie Barlow researched online the technical papers on likely beetle pollinators. Overall, effective pollinators for plants with flowers similar in structure to pawpaw are those beetles with hair-like appendages on their sides and underside. They are called "flower pollinating beetles" precisely for this reason. (See below photos.)
Photos LEFT and MIDDLE from Maryland Biodiversity Checklist
• RIGHT from Matt Buckinham, Flickr 2015
ABOVE: Published scientific papers have reported two genera of scarab beetles as effective pollinators of more southerly species of genus Asimina (whose flowers are smaller than the northern species, but have similar structure): Trichiotinus (left) and, to a lesser extent, Euphoria inda (middle and right). Notice that the photo in middle shows Euphoria in the context of leaf litter. Flower pollinating beetles generally are neither carrion beetles nor dung beetles. That is, they lay their eggs not in rotting meat or dung but into rotting wood or thick leaf mulch, where larval grubs feed and eventually pupate. Key papers on Asimina species that identify beetle pollinators:
• "Effects of Fire and Pollinator Visitation on the Reproductive Success of Asimina reticulata (Annonaceae), the Netted Pawpaw", by Louise K. Barton and Eric S. Menges, 2018, Castanea. (Note: Trichiotinus viridans, the "hairy flower scarab" is documented as the most prevalent visitor, and "low fruit set may be due to the lack of an effective pollinator, resulting in inbreeding depression.... Our study confirms that A. reticulata is self-compatible only if pollen is able to move among flowers, and we found that, in most cases, fruit was aborted prematurely and was found littering the ground beneath the plants.")
• "How diverse are Annonaceae with regard to pollination?", by Gerhard Gottsberger, 2012, Botanical Journal of the Linnean Society. (Note: In this paper two Florida species, Asimina obovata and Asimina pygmaea are linked with pollinating beetle species in Trichiotinus and Euphoria genera. These flowers are categorized as "diurnal without thermogenesis.")
Photos from Maryland Biodiversity Checklist, Fritz Flohr Reynolds.
ABOVE: The American carrion beetle, Necrophilia americana, requires carrion for egg-laying and its larval stage, but as an adult it will happily feed on mushrooms — and fermenting pawpaw fruit. Note the lack of "hairs" on this beetle and thus its inability to serve as a pollinator.
A 2006 paper by Katherine R. Goodrich et al., "When Flowers Smell Fermented: The Chemistry and Ontogeny of Yeasty Floral scent in Pawpaw (Asimina triloba)", was published in International Journal of Plant Science. It offers evidence that carrion beetles are not involved in pawpaw pollination: "We did not detect the dimethyl oligosulfides that characterize the microbial decomposition of meat and universally constitute the odors of carrion-mimicking flowers." Instead, the authors reported a "yeastlike fragrance" in pawpaw flowers. However, "fermentation volatiles were largely absent from female-stage androgynoecia". (Many technical details in this paper may be useful for project participants.)
RECOMMENDED READING: In addition to various technical papers cited and linked here, a 296-page book on pawpaw is highly recommended: Pawpaw: In Search of America's Forgotten Fruit, by Andrew Moore, 2015, Chelsea Green.
• "The diversity and evolution of pollination systems in Annonaceae", by Richard M.K. Saunders, 2012, Botanical Journal of the Linnean Society:
... There are two distinct floral forms within genus Asimina. One group (consisting of A. parviflora and A. triloba) possesses small maroon flowers that emit a foetid aroma; and the other group (consisting of all other species) possesses large, white, pink or yellow flowers that emit a more pleasant fragrance (Kral, 1960; Goodrich & Raguso, 2009). These differences clearly represent different pollination syndromes. Observations of floral visitors confirm that the foetid-smelling species are visited by small flies and/or small beetles (Willson & Schemske, 1980; Norman etal., 1992; Rogstad, 1993), whereas the fragrant species are typically visited by large beetles (Norman & Clayton, 1986; Norman et al., 1992) or rarely by flies (Uphof, 1933; Norman et al., 1992).
... Annonaceae flowers are visited by a taxonomically diverse range of insects: predominantly beetles (Coleoptera), but also thrips (Thysanoptera), flies (Diptera) and, rarely, bees (Hymenoptera) and cockroaches (Dictyoptera). Species are typically pollinated by only one of these groups, although there are several examples of species that are pollinated by more than one group, including: .... Asimina parviflora (Norman et al., 1992), A. pygmaea (Uphof, 1933; Norman & Clayton, 1986; Norman et al., 1992) and A. triloba (Willson & Schemske, 1980; Johnson & Willson, cited in Norman et al., 1992) are visited by flies and beetles.
... Despite the vast diversity of dipterans and their evident importance in pollination, there are few reports of fly pollination in Annonaceae. Flower visits by flies have only been reported for Annona (Webber, 1981b; as secondary floral visitors only), Asimina (Norman et al., 1992), Monodora (Gottsberger, 1985; Gottsberger et al., 2011), Pseuduvaria (Morawetz, 1988; Silberbauer-Gottsberger et al., 2003; Su et al., 2005) and Uvariopsis (Gottsberger et al., 2011). It should be noted, however, that Norman et al. (1992) did not observe any pollen attached to the common drosophilid flies that visited Asimina flowers (pollination resulted from visits by nitidulid beetles and rarer calliphorid flies), and that pollen transfer between flowers was not demonstrated in any of the other studies. ... To avoid confusion of stimuli, Calliphoridae (blowflies), which lay eggs in dung and carrion, favour yellow colours in the presence of sweet scents, but brown-purple colours in the presence of foetid scents (Kugler, 1956). The latter pollination system is observed in Asimina parviflora (Norman et al., 1992) and operates by DECEIT as the flies visit the flowers in order to lay eggs. Sapromyiophilous flowers, furthermore, often have partially enclosed pollination chambers that require the flies to crawl in through narrow openings (Faegri & van der Pijl, 1979).
• "Phylogenetic Analysis of the North American Beetle Genus Trichiotinus (Coleoptera: Scarabaeidae: Trichiinae)", by T. Keith Philips et. al, 2016, Psyche: A Journal of Entomology.
... Adults are good fliers and forage on a variety of flowers while larvae are known to feed on various species of decaying hardwoods.... Trichiotinus is dependent upon decaying hardwoods as larval food including oak [6]. Jackson et al. [39] present evidence for a split in distribution of oaks (Quercus spp.) during the most recent glacial maximum on either side of the Mississippi drainage and may be indicative of the effects of earlier glacial maxima as well. P. A. Delcourt and H. R. Delcourt [40, 41] also postulated the presence of spruce (Picea glauca) forests in the Lower Mississippi Valley. This extension south of these more cool adapted forests all the way to the gulf coast that divided the hardwood forests into eastern and western blocks was thought to be due to glacial meltwater creating a cooler climate locally [42].
• "Monographic Revision of the American Genus Euphoria Burmeister, 1842 (Coleoptera: Scarabaeidae: Cetoniinae)", by Jesus Orozco, 2012, BioOne Complete.
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