Milkweed Conundrums by Lisa Lofland Gould
AS YOU probably know, NCNPS maintains a list of North Carolina nurseries that carry native plants and don’t sell plants listed on our invasive species list as “Rank 1—Severe Threat”. Both lists are works-in-progress, with new nurseries being added regularly, and on-going assessment of the invasives list, which is now 11 years old and needs updating badly.
Recently NCNPS ran into a conundrum: a nursery asked to be added to our list, but offered two non-native milkweeds, Balloon Cottonbush (Gomphocarpus physocarpus, sometimes listed as Asclepias physocarpa), and Tropical Milkweed (Asclepias curassavica), both of which are being promoted to feed Monarch butterfly caterpillars. Neither plant is on our invasives list or known to be naturalized in NC, but both come with serious concerns for the health of Monarch butterflies and the Southeast’s ecosystems.
Balloon Cottonbush (also known as Hairy Balls Milkweed, among many other common names) is a shrubby perennial native to southeast Africa; it has been planted widely and has naturalized in many parts of the world. Because Monarch butterflies are able to lay their eggs on Balloon Cottonbush and the caterpillars can survive, it is being touted as a great plant for Monarchs. This plant thrives in grasslands, pastures and some agricultural fields, roadsides, and other disturbed waste places, but can also grow in wetlands. It is fast growing, drought tolerant, and like other milkweeds, produces wind-borne seeds, so it is not surprising that it is now considered invasive in many countries, including Australia, China, Hawaii, the Canary Islands, and parts of Polynesia, and is being assessed for invasiveness in India, Cuba, Jamaica, and southern Europe. A close relative, Gomphocarpus fruticosus, is also from Africa and planted for Monarchs; it often hybridizes with G. physocarpus.
Tropical Milkweed also supports Monarch caterpillars and is widely advertised as a great butterfly plant. One concern with this plant is that it is a host for the protozoan parasite OE (Ophryocystis elektroscirrha), which can impair metamorphosis, causing the wings to fail to open completely or weakening the adult butterfly. Both Tropical Milkweed and Balloon Cottonbush have different phenologies from our native milkweeds, so another concern is that these plants may stay green later into the fall than the natives, fooling Monarchs into laying eggs at a time when they should migrating, and resulting in caterpillars that may then perish when freezing weather comes.
Scientists agree that more research is needed to understand the impact of these non-native milkweeds on Monarch butterfly populations. The Virginia Native Plant Society has a thoughtful discussion on this: https://vnps.org/non-native-milkweed-helpful-harmful/. The Xerces Society does not recommend Tropical Milkweed; for more information see https://xerces.org/blog/tropical-milkweed-a-no-grow.
For decades, the USDA Soil Conservation Service promoted Bicolor Lespedeza (Lespedeza bicolor) for erosion control and wildlife habitat (especially habitat and food for Northern Bobwhite). In the 1950s, I remember my Girl Scout leader (a tree hugger for sure, but what can I say: she was also my mother) touting the benefits of Bicolor Lespedeza, with no idea of what a pest it would eventually become; it is now Rank 2—Significant Threat on the NCNPS invasives list.
Today, however, ignorance of the impact of widely introducing non-native species should be no excuse: we KNOW a great deal about which plants are likely to become invasive, and we have myriad examples all around us of the economic and ecological impact of their spread. One recent article estimated the global 2017 cost of invasive species at $162.7 billion; such figures do not account for biodiversity loss due to invasive species.
North Carolina is home to 17 species of native milkweed, a wealth of beauty for us and nourishment for many species of insects. It is my hope that in our desire to save single species, such as the Monarch butterfly, we will not lose focus on the bigger picture: maintaining healthy ecosystems so that the full panoply of organisms can be supported.
As always, GO NATIVE!
Chlorofiends! is a regular column in Native Plant News. If you have information or comments on invasive species in North Carolina, please share them with Lisa Gould (firstname.lastname@example.org).
*Thanks to Jim Butcher’s The Dresden Files for the column