Hybridization with other Asimina species The common pawpaw is the largest and most well known of the 13 species of the
Asimina genus in North America. Of those 13, 11 prefer very warm weather and have ranges rarely extending northward of Florida or coastal Alabama. Their ranges do not overlap with
Asimina triloba. One southern U.S. species,
Asimina parviflora, does overlap in range with pawpaw. This species is smaller than pawpaw in both its flower and its woody growth.
A. parviflora is more shrublike, rarely growing even a third as tall as pawpaw. Genomically verified hybrids of
A. triloba and
A. parviflora have been classified as
Asimina piedmontana.
Pollination Pawpaw are
self-incompatible, meaning pollen cannot fertilize flowers on the same plant. Other floral features of pawpaw indicative of beetle pollination include petals that curve over the downward-pointing flower center, along with food-rich fleshy bases of the inner whorl of petals. A "pollination chamber" is thereby created at a depth that only small beetles can access during the initial female-receptive stage of floral bloom. As with other well-studied species of Annonaceae, the delay in the shift from female to male floral stage offers beetles a secure, and possibly
thermogenic, residence in which not only to feed but also to mate. Receptive
stigmas at their arrival, followed by pollen-shedding
stamens during pollinator departure, are regarded as an early form of
mutualism evolved between plants and insects that is still dominant in the most ancient lineages of flowering plants, including the
Magnoliids (of which
Annonaceae is the most species-rich taxonomic family). Beetles are the dominant form of pollinator ascribed for genera and species within the Annonaceae family. However, two species of genus
Asimina (
Asimina triloba and
Asimina parviflora) bear a floral character that has given rise to an alternative hypothesis that carrion or dung flies are their effective pollinators. That floral characteristic is the dark maroon color of the petals. Hence, while no scholarly papers have documented carrion or dung flies as effective pollinators in field observations, the strength of this hypothesis has led some horticultural growers to place carrion in pawpaw orchards during the bloom time. Professional papers on genus
Asimina and its species have warned of the difficulties in discerning whether insects observed on or collected from flowers are effective pollinators or merely casual and thus opportunistic visitors. A
citizen science project in southern
Michigan utilized
natural history forms of observation, along with video and photo documentation, during a "pawpaw pollinator watch" in May 2021. Larvae and adult beetle stages of
Glischrochilus quadrisignatus were also documented by the citizen project on the ground-level side of rotting fruit in a pawpaw orchard in Michigan following the fruit harvest. The fruit of pawpaw is thus recognized as having
coevolved with large mammals serving as long-distance seed dispersers. The
megafaunal dispersal syndrome is a common feature of some plants native to the Western Hemisphere, where a large proportion of
megafauna went extinct near the end of the glacial episodes. Such fruits are now regarded as
evolutionary anachronisms. Another indicator of dispersal adaptation for megafauna is that pawpaw fruit (wild types and most cultivars) tend to remain green or become blotched with brown when at peak ripeness. Mammals (other than
primates) rely on
olfactory rather than visual clues for discerning ripe fruit, so fruit color is no signal of ripeness for large mammals. An advantage of maintaining green fruit skin throughout the ripening process is that
photosynthesis can continue during this time. Following the
extinction of much of the ice age megafauna,
bears would have continued dispersing pawpaw seeds in their dung. Hand carrying of fruit and seeds by humans expanding from Asia into North America would have extended the range of long-distance seed dispersal. Pennsylvania, and Michigan. Small
mammals, including
raccoons,
gray foxes,
opossums, and
squirrels, assist in local movements of seeds.
Interactions with animals Pawpaw defends against
herbivory by producing strong-smelling natural
toxins known as
acetogenins. Pawpaw leaves, twigs, and bark are therefore seldom consumed by
rabbits,
deer, or goats, nor by insects. and
mules have been seen eating the leaves in
Maryland. The fruit pulp is readily consumed by mammals, including
opossums,
foxes,
squirrels and
raccoons. Larvae of the
zebra swallowtail (
Eurytides marcellus), a
butterfly, feed exclusively on young leaves of
A. triloba and various other pawpaw (
Asimina) species, but do not occur in great numbers on the plants. A partially citizen-led project in
Pittsburgh, Pennsylvania is working on restoring pawpaw for the purpose of encouraging return of the zebra swallowtail as well as the general restoration of riparian areas. In June 2024 a zebra swallowtail was documented laying eggs on a backyard pawpaw plant in Pittsburgh. This was the first time that this butterfly species was seen there since industrialization and river slope destruction had decimated its plant host. A local news editorial wrote, "The return of the zebra swallowtail, after 87 years, is a huge success for naturalists, conservationists and native tree lovers.... Organizations like Grow Pittsburgh, Tree Pittsburgh, the Pittsburgh Parks Conservancy and dedicated chat groups for Pittsburgh conservationists made the zebra swallowtail butterfly's restoration possible." Other insects that have evolved the ability to consume pawpaws include
Talponia plummeriana, the pawpaw peduncle borer, whose larvae can be found in flowers, and
Omphalocera munroei, the asimina webworm, whose larvae mostly feed upon leaves.
Drosophila suzukii, the spotted wing drosophila, and
Zaprionus indianus, the African fig fly, have been recorded developing within pawpaw fruit collected in Connecticut, United States, marking the first known use of a member of the Annonaceae family by
D. suzukii and an expansion of the known host range of
Z. indianus.
Patch-forming clonal growth Pawpaw is well suited to life as an
understory tree. Its large seed enables significant below-ground growth before the above-ground growth needs to access sunlight for photosynthesis. As well, the species is so shade-adapted that propagation of seedlings in nursery and landscape settings may fail if the emerging plants are not protected from direct sunlight. Pawpaws are not the first to colonize a disturbed site, but because they are capable of growing in deep shade, they can establish from seed beneath mature deciduous trees and then spread into a subcanopy patch. They may even become dominant through time by depriving native
canopy trees from re-establishing via seed in a
treefall gap, owing to the dense shade within a pawpaw patch. Under such circumstances, the pawpaw subcanopy becomes the forest canopy, albeit at a height half as high as the usual canopy of native trees. Accessing full sunlight, the patch is then capable of producing more fruit. Upslope expansion has been attributed to a lessening of human-set fires within forested habitats of the eastern United States. Unlike common canopy trees such as
oaks and
pines, pawpaw has no resistance against ground fires. So a reduction in fires has enabled pawpaw, as well as other shade-adapted native trees (including
American beech and
striped maple), to become more common. Pawpaw exhibits a high tolerance for intense shading, even compared with other shade-adapted species such as striped maple. Pawpaw leaves cast very heavy shade, and this chokes out seedlings and saplings of most canopy species (though not native spring herbs that benefit from the late-leafing habit of pawpaw). This helps pawpaw outcompete rival species and is a contributor to its increased abundance in forests across its range. ==Conservation==