By Sam Hoadley
From 2020 through 2023, the Trial Garden in late summer was dominated by the towering purple inflorescences of the ironweeds and the scores of pollinators that visited their flowers. The ironweed (Vernonia) trial was replaced with mountain mints (Pycnanthemum) which also attract an impressive volume and diversity of insects, but one regular visitor to the ironweed trial is conspicuously missing. That insect is the denticulate longhorn bee, or Melissodes denticulatus, which is an ironweed specialist. These bees rely exclusively on the snowy white pollen of ironweeds to feed their larval offspring which are located in subterranean burrows. Interestingly, adult Melissodes denticulatus are less picky about nectar sources and can be seen visiting other native plants in the aster family such as members of the genus
Helianthus, but without ironweed pollen, denticulate longhorn bees would not be able to sustain their species.
In 2022 and 2023, the Pollinator Watch Team collected data to try to better understand which ironweeds would be the most appealing to this specialist bee. With 45 different ironweeds on the menu, theoretically there would be no shortage of choices. The data collected during those two years revealed a discrete window of time when the most Melissodes were observed. Consistently, denticulate longhorn bee activity started in late July and concluded in late August, which happens to coincide with the bloom period of our most abundant native species of ironweed in northern Delaware, New York ironweed (Vernonia noveboracensis). Based on this pattern, we hypothesized that the most desirable host plant for Melissodes in the Trial Garden would be New York ironweed. However, although several examples of Vernonia noveboracensis originating from the mid-Atlantic region were included in the trial, these actually had very low pollinator and Melissodes counts. We suspected the reason was that nearly all of those plants were plagued by diseases such as powdery mildew and rust. In the wild, Vernonia noveboracensis grows in wet meadows, flood plains, and even roadside ditches with moist to wet soils but the Trial Garden soil is considerably drier. The stress of being grown in sub-optimal garden conditions resulted in a large incidence of disease which significantly impacted not just their floral display and ornamental value overall, but also their ability to attract and support pollinators.
So where did we find the Melissodes in the trial? Since the New York ironweed plants were unhealthy and had low flower numbers, these specialist bees found an alternative pollen source and flocked to western ironweed, Vernonia baldwinii. Vernonia baldwinii is not locally native and instead occurs in the central to western United States as the common name would suggest. Western ironweed proved to be a suitable source for Vernonia pollen because it was better adapted to the drier soil conditions in the Trial Garden than New York ironweed and it bloomed during that same window of late July through August when the local populations of Melissodes denticulatus are most active. If you have drier soil and are interested in attracting denticulate longhorn bees to your home garden, western ironweed could be a viable option. Simply put, healthy plants equate to better and more beautiful floral displays and importantly more value for pollinators.
With the arrival of late summer and the start of ironweed and Melissodes season, one of the best places to see the denticulate longhorn bees at Mt. Cuba Center is in the wet meadows of our Natural Lands. These places not only provide an opportunity to observe specialized pollinator relationships forged over millennia, but they are also a place to observe natural plant communities that can serve as a source of inspiration for naturalistic garden designs and restoration projects. Some of the other notable native plants that can be observed in these same habitats include species featured in other trials like Solidago gigantea that produces golden yellow inflorescences timed perfectly to compliment the flowers of New York ironweed along with the bubblegum pink flowers of our local variety of swamp milkweed, Asclepias incarnata var. pulchra. Although there are many other fabulous and fascinating native plants that can be found in these communities, what garden would be complete without some sedges like the stately Scirpus cyperinus adorned with nodding wooly inflorescences that dance in the slightest breeze.
In addition to a heathy population of Melissodes denticulatus, these wet meadows in our natural lands also host Vernonia stem borer moths. These moths are another component of the complex web of life that exists in our natural lands thanks to healthy native populations of plants like New York ironweed. If you have a wet spot in your home garden, consider planting New York ironweed and some of its natural companions found in our own wet meadows and get out to your local natural areas to see some wild ironweed and the ever-present denticulate longhorn bees in all of their late summer glory.
Sam Hoadley is the Manager of Horticultural Research at Mt. Cuba Center where he evaluates native plant species, old and new cultivars, and hybrids in the Trial Garden. Sam earned his degree in Sustainable Landscape Horticulture from the University of Vermont. This article on Ironweed (Vernonia) and more information on the Vernonia trials can also found on the Mt Cuba Website.