If you’ve spent any time at Hitchcock Nature Center (HNC) recently, you may have noticed that monarch butterflies are already beginning to visit the park. And as the goldenrod comes into full bloom, those of us at HNC are anxious to notice the contrast of monarch orange on tufts of goldenrod yellow. Each September we host our annual Monarch Tagging program to provide members of the public the opportunity to catch, tag, and release a monarch butterfly (or more) while aiding researchers’ understanding of the monarchs’ breeding, migration, and wintering habits. In addition, tagging events also help provide answers into the stark decline of monarchs in recent years. While there’s still much to learn, researchers are confident that a few key factors are contributing to the decline of one of the world’s most iconic and beautiful insects: a reduction in monarch breeding and feeding habitat due to its conversion to herbicide-resistant agricultural land, the destruction and deforestation of overwintering sites, and climate change.
The State of Iowa largely only provides breeding and feeding habitat for monarch butterflies as they migrate; Iowa’s cold winters largely preclude any overwintering. But as they migrate through HNC in the fall, adult monarchs can refuel for their journey south on a variety of nectar producing plants, and throughout the summer their larvae have the chance to feed on several species of milkweed (Asclepias sp.).
When talking monarchs, milkweed has a tendency to grab most of the headlines, while most other migration fuel sources are often left out of the conversation. However, these fuel sources play an important role in facilitating the monarchs’ ability to complete their migration event. Unfortunately, more and more monarch nectar sources are becoming collateral damage of agricultural expansion (with the U.S. adding nearly 100 million acres since 2006) and its associated herbicide drift. When considering the aforementioned threats to monarchs, it’s alarming (or at least it should be) to hear that the nectar sources critical for fueling the monarchs’ migration are being so rapidly lost. What’s more troubling, is that in some cases, the law requires it.
Many people may be surprised to find that an important fuel source for monarchs (among other fauna) is thistles, a group of plants for which mainstream society and state laws have long shown an aversion. The USDA even lists three species of thistle in the genus Cirsium as possessing “high” or “very high” value for monarchs. But given thistles’ spiny and prickly form, society’s distaste for thistles is understandable, with a few thistles clearly displaying “weedy” or invasive tendencies, encouraging this distaste (think Canada thistle).
This distaste is nothing new. Ancient Greek, Norse, and even biblical texts portray the thistle in a negative light, often referencing them as a curse on humans or the result of extreme despair or grief. More recently in this country, aggressive non-native thistles were noted as early as 1795 by the State of Vermont as noxious weeds. By 1868 in Iowa, landowners were threatened by the General Assembly of Iowa with a five dollar fine if they allowed Canada thistle (C. arvense) to blossom or mature on their land. Today, the Iowa Weed Law mandates that all thistles in the genera Cirsium and Carduus be considered noxious weeds, and therefore eradicated wherever possible.
But several species of thistle, many in the genus Cirsium, are native to Iowa, and rarely pose threats to native vegetation or agricultural land. Some species, including wavyleaf thistle (C. undulatum), are actually considered conservative species, found most commonly in high quality, undisturbed native prairie. Recently, Chad Graeve, Pottawattamie County’s Natural Resource Specialist, found a single wavyleaf thistle in bloom in one of HNC’s higher quality remnant prairies, a sight not often seen, even by those who pay attention.
Conservative thistles, including wavyleaf thistle, aren’t invasive plants, and in fact, can be just the opposite, easily pushed aside by disturbance or other more aggressive vegetation. Most (but not all) native thistles live in relative harmony with a balanced and healthy ecosystem and provide sound ecological benefits to their neighbors and visitors; monarch butterflies are just one of the many species of insects that will greedily take advantage of a thistle’s nectar.
The spotted blooming wavyleaf thistle, in combination with the arrival of monarch butterflies in the park, sparked a conversation among Pottawattamie County Conservation staff when it comes to thistles: Are a few bad apples spoiling the bunch? Yes, some “bad apples” deserve to be kept in-check or even eradicated, but should all the “apples” suffer the same fate? Iowa’s weed law is particularly aggressive when it comes to thistles; most Midwestern states’ noxious weed laws list specific species of thistles, but not entire genera. Iowa’s weed law even hints at its aggressive posture and willing inclusion of the “good apples” in the law, referring to them as “thistles of lesser importance”. Ironically, the Iowa Department of Natural Resources lists three species of Cirsium as “Species of Special Concern”, only a step away from being considered “Threatened” by the state. For us at Pottawattamie County Conservation it begs the question: Why include them all?
While there may be many reasons for including all thistles in the two genera, one reason may involve a lack of education. For many people, one species of thistle looks the same as the next, or at least so similar that they won’t take the time to key it out; maybe they don’t even know what “to key something out” means. In that case, perhaps the law subscribes to an Arnaud Amalric-like philosophy, at least when it comes to thistles. A quick search online provides reliable sources for identifying one thistle from the next, and with a little practice, most people can learn to key out a variety of plants quickly, a skill more of us should possess. Regardless, given the role the “good apples” play for the monarchs and so many other species in our roadsides and natural areas, it may be time for us to start taking a closer look at thistles. And maybe the weed law too.
Originally Posted September 6, 2016
2018 Update to Iowa Noxious Weed Law
Iowa extension says: ”Only thistles in the Carduus genus are listed as Class B Noxious Weeds. This results in a 'delisting' of Iowa's native thistles: tall thistle (Cirsium altissimum) and field thistle (Cirsium discolor).”
1) Class A noxious weeds for eradication – weeds determined to be the highest priority
for eradication of existing infestations and prevention of new infestations.
2) Class B noxious weeds for control – weeds determined to be a priority for preventing
new infestations and stopping the spread of the species.”
(https://crops.extension.iastate.edu/blog/bob-hartzler/changes-iowas-noxious-weed-law)
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