Welcome to The STEM Sessions Podcast. I am Jarl Cody, your host and narrator.
Four or five years ago, I noticed a monarch butterfly feeding on flowers in my yard. Wanting to encourage more, I researched the specific plants that would attract them. Every article said adult monarchs can drink nectar from many different flowers, but they will only lay their eggs on milkweed which is the host plant for their caterpillars.
Further emphasizing the importance of milkweed, the articles attributed the drastic decrease in the monarch population to milkweed habitat loss. To help the monarchs, the articles continued, we must plant milkweed at our homes.
So, in the following years, I planted a lot of milkweed, resulting in an ever increasing number of adult monarchs and caterpillars. I even began placing the caterpillars in netted enclosures to protect them until they formed their chrysalis and emerged as adults. In 2020, I successfully released 34 monarchs.
I had been planting tropical milkweed, because it is readily available at local nurseries. But after recently learning about probable negative impacts of tropical milkweed on monarchs, I’m modifying my plan for 2021. And because there is still time for you to implement a similar plan, I thought I’d share the data.
Apologies in advance for butchering the scientific names.
This is The STEM Sessions Podcast – Episode Five. Not All Milkweed is Created Equal
The monarch butterfly (Danaus plexippus) is the iconic North American butterfly. It is easily recognized by its color of saturated orange with bold, black outlines and veining, and a wingspan of approximately 4 inches. It is a model of migration, with successive generations completing a journey of hundreds, if not thousands of miles.
There are two populations separated by the Rocky Mountains. The Eastern Monarchs migrate between northern Mexico in the winter and the midwest and East coast of the U.S. in the summer. The Western Monarchs migrate between coastal California in the winter and inland states like Utah and Nevada in the summer. Both populations have reached dangerously low levels.
Over the last 35 years, the population of Western Monarch butterflies has decreased by over 99% based on counts taken at over-wintering sites. In the mid-1990s, the over-wintering population in California was estimated to be over one million. In recent surveys, the number has plummeted to 30,000, and the preliminary data for the 2020-2021 winter is even worse.
Short term fluctuations in population may be attributed to drought, late rains, and forest fires; all of which disrupt the monarchs’ breeding and migration schedule. But long term population decrease is most certainly attributed to the loss of milkweed plants, the host plant for monarchs, at inland breeding sites. While adults can feed on nectar from many flowering species, caterpillars have evolved to only feed on milkweed. So if there is no milkweed, there are no caterpillars.
Almost every “save the monarchs” campaign has centered around planting more milkweed, whether in home gardens, office complexes, or public spaces. So that is what I and thousands, if not hundreds of thousands, of others have done. We go to our local plant nursery, buy established milkweeds, and transplant them in our gardens or in containers. Each plant produces hundreds of seeds, which disperse on the wind, hopefully to take root elsewhere. But now, data is emerging that the milkweed stocked in the vast majority of nurseries may actually be harming the monarchs.
Tropical milkweed (Asclepias curassavica) is native to the American tropics such as Central America. It has multiple clusters of bright yellow, orange, and red flowers with large dark green leaves, making it attractive to monarchs as food and to humans as an ornamental plant. It is easy to propagate and grows quickly making it a profitable choice for nurseries to keep in stock. However, unlike milkweeds native to the United States, it does not go dormant in the winter when planted in temperate climates, and that’s a big problem for monarchs.
Monarch butterflies have co-evolved with a protozoan parasite called Ophryocystis elektroscirrha or OE. Adult monarchs tolerate OE to a certain level, but above that threshold, adults can experience reduced migration success and physical impairments such as decreased body mass, flight ability, lifespan, and mating success. High levels of OE can also cause monarchs to fail to emerge from the chrysalis, because they are unable to fully expand their wings.
When adults visit a plant to feed or lay eggs, OE is inadvertently deposited on the leaves. In the case of native milkweeds which die back after blooming, the parasite dies, too. This means next year’s growth will be parasite free, and the monarch population will feed on clean leaves. But in the case of tropical milkweed, the parasite doesn’t die because the plant does not experience any die back. This allows levels of OE to increase over time, and as more generations of monarchs visit the plant, the higher the risk caterpillars will be exposed to dangerous levels of OE.
In addition to increased levels of parasites, there is research suggesting tropical milkweed itself may become toxic to caterpillars in elevated temperatures. Milkweed produces a chemical called cardenolide, which the caterpillars ingest, making them unappealing to potential predators. It’s the reason they eat only milkweed in the first place. But even the caterpillars have an upper limit to their tolerance. The cardenolides can reach toxic levels in elevated temperatures, whereas native milkweeds do not show the same increase.
The resilience of tropical milkweed can also interfere with the timing of monarch reproduction and migration. Tropical milkweed grows and flowers nearly year-round in every climate zone, while native milkweeds are seasonal. In the interior pacific states, the presence of tropical milkweed can delay migration to the coast or cause monarchs to mate instead of migrate entirely. Similarly, tropical milkweed planted on the California coast near over-wintering sites can encourage monarchs to reproduce when they should be conserving energy by over-wintering.
Disruptions to migration and breeding patterns have even resulted in the establishment of permanent monarch populations in areas with tropical milkweed. Unfortunately, these populations have been shown to have high levels of the parasite, putting the visiting migrating monarchs even more at risk of parasites.
So what can we, as private citizens, do to help stabilize the monarch population if the readily available milkweed species is doing more harm than good? The answer is still plant milkweed, but select species native to your area. For the Western Monarch population, that includes milkweed species such as woollypod (A. eriocarpa), California (A. californica), heartleaf (A. cordifolia), narrowleaf (A. fascicularis), showy (A. speciosa), rush (A. subulata), and desert (A. erosa).
It’s highly unlikely, you’ll find anything that isn’t Tropical milkweed in your local nursery, so you’ll need to look to the internet for the native types. Fortunately, the Xerces Society for Invertebrate Conservation has a seed and plant finder, searchable by area and species. I ordered my plants through JoyfulButterfly.com, which offers milkweed species from much of North America. Speaking of my plants, I ordered the narrowleaf, showy, and heartleaf milkweed species, and they should be delivered two or three days after this episode is published.
If tropical milkweed is all you can find, you can still help mitigate its negative effects. The Xerces Society and other experts recommend cutting it back after every growth cycle. While not perfect, this approximates seasonal growth and die-off, which will reduce the impact to migration and breeding, and minimize the build-up of parasites on the plants.
Thank you for listening to this episode of The STEM Sessions podcast. I do my best to always provide accurate information, but, unfortunately, I’m fallible like everyone else. So I encourage you to do your own research on the topic we discussed. Corrections and new information are always welcome.
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