Current Nature Report

In ditches and roadsides where the umber ground is cracked and fissured despite precocious fall storms, and blackened, half-decayed organics form a purgatory film, awaiting the necessary conditions for microbial rebirth, a surprisingly diverse floral ecology exists as if spring hadn’t turned to summer or summer to fall. The concomitant insect life is just as diverse and colorful, as these late-season flowering plants support a wide variety of native pollinators at a time when the number and frequency of flowers is steadily decreasing throughout the region.


This native bee is Halictus ligatus, a species of sweat bee in the Family Halictidae clinging to a flower of Willamette Valley gumweed (Grindelia integrifolia) along the Lower Entrance Road at Mount Pisgah Arboretum. This family of bees dates back to the early Eocene, or approximately 40 million years ago and can be found on almost every continent.


A different species of sweat bee in the Genus Agapostemon (Family Halictidae) feeds on the disk flowers of the Rough-leaved aster (Aster radulinus)–one of few flowers that is likely to continue blooming well into October.


Here a much smaller Carpenter bee in the Genus Ceratina feeds on the tiny disk flowers of another aster species, the Few-flowered aster (Aster modestus), which can be commonly found along rivers and stream banks. In the Arboretum it is found scattered along the Coast Fork of the Willamette River, and in rare instances along ditches. Carpenter bees mostly nest in wood, and are frequently solitary, with a few species being unique among bees in the female’s ability to reproduce without a male.


This Mylitta crescent (Phyciodes mylitta) was feeding on Aster modestus before perching on a piece of grass. The Mylitta crescent can be found throughout western Oregon in fields and empty lots—it is one butterfly that is not particularly picky about habitat, and feeds extensively on some invasive species. A much larger and more striking butterfly, the California sister (Adelpha californica) can be seen flying in the Water Garden and oak woodlands of the Arboretum likely well into October.


This tiny Syrphid fly of the Genus Sphaerophoria alights on Cluster tarweed (Madia glomerata). The Family Syrphidae is a large and important group of pollinating flies known for their brightly-colored markings and mimicry of wasps and bees. They are frequently considered to be second only to bees as the most important pollinating insects, yet comparatively little is known about their biology. Syrphid flies, ranging in size from about 5mm like the specimen above to nearly an inch, can be found all over the Arboretum, and are some of the only pollinating insects flying in late winter.

Most of our late-blooming plants thrive in areas of disturbance, where water collects later into the season, and where late-germinating species do not have to compete with the higher biomass potential of established plant communities. Late-season flowers once widespread in the Willamette Valley, such as Checkermallows, Willowherbs, Tarweeds, and Gumweeds, are now becoming less common despite a significant amount of human-caused disturbance. Many invasive species also rely heavily on disturbance, and control methods such as roadside mowing and herbicide spraying are negatively impacting the native species which thrive in these same environments. As a result, native pollinating insects are declining in numbers as well. It was recently announced to much fanfare that a population of the locally rare Western bumblebee (Bombus occidentalis) was just discovered on Mount Hood. This same bee was commonly found in the Willamette Valley as recently as 15 years ago, and while the disappearance of the Western bumblebee serves as a potent warning, the recent rediscovery provides a glimmer of hope.  Though the current focus is on the decline of European honeybees, the fate of native pollinators merits just as much, if not more attention.


August Jackson

Assistant Site Manager