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Planting the Wrong Kind of Milkweed May Harm Monarch Butterflies

Photo by Alexander Wild.

The monarch butterfly is the Bambi of the insect world in North America, a favorite among children and adults because of its colorful wings and its migratory pattern, which takes it as far north as Canada and south into Mexico and California where it overwinters.

For decades, monarch populations have fluctuated, as this graph from the Word Wildlife Fund shows. Reasons for the fluctuation include weather patterns — extremely cold and wet winters are especially harmful — as well as deforestation in the areas where they gather to overwinter, and loss of habitat in the U.S.

Monarch caterpillars feed on one plant only — the milkweed. However, there’s been noticeably less milkweed in corn and soybean fields recently as farmers have used modern techniques to keep these and other weeds out of their fields. In order to counteract this trend, many conservationist groups have encouraged people to grow milkweed in their backyards and gardens, and they have even distributed milkweed seeds for this purpose.

Unfortunately, these well-intentioned efforts may have backfired, according to a new article by scientists from the University of Georgia published in the Proceedings of the Royal Society B. There are more than 100 different species of milkweed, and they are not all created equally. The species that was distributed most by conservationists — tropical milkweed (Asclepias curassavica) — is one that does not deteriorate in cold weather. Instead, it continues to thrive, producing new leaves and flowers in the fall and winter, which is causing monarchs to stay where the food is instead of continuing their migration south for the winter.

“Tropical milkweed provides monarch larval food throughout the year, and reports of monarchs breeding during the winter — rather than migrating or overwintering — have become common in the southern U.S.,” the authors wrote. “These behaviors are almost exclusively restricted to sites where tropical milkweed is present.”

To make matters worse, the researchers found that monarchs that did not migrate were more likely to be infected by the protozoan Ophryocystis elektroscirrha (OE), which causes the butterflies to suffer from wing deformities, smaller body size, reduced flight performance, and shorter adult lifespans.

“Shifts towards year-round breeding on tropical milkweed, resulting in high rates of OE infection, could pose an additional emerging threat to the long-term viability of migratory monarchs,” they wrote. “Transitioning from migratory to non-migratory behaviors coupled with a shift to year-round breeding on introduced host plants dramatically increases the prevalence of a debilitating parasite for North American monarchs.”

Read more at:

Loss of migratory behaviour increases infection risk for a butterfly host

Canceled flights: For monarch butterflies, loss of migration means more disease


  1. Somehow this will be blamed on modern agriculture, even though it wasn’t the guy trying to manage weeds who established the wrong host plants. I can already see it–just like the sanctimonious beekeepers who were pointing fingers at imidacloprid with one hand while pouring acaricides into their hives with the other.

  2. The authors ignored the extensive published work of Dr. David James in southwestern Australia that shows evergreen tropical milkweed does not cause migratory monarchs to become sedentary. Quite the opposite – James’s work showed tropical milkweed sustains the Australian monarch migrations.

  3. All of my Tropical Milkweed died soon after the first frost…hense Tropical. Also you would have to be absolutely clueless if you think that planting a butterfly species host plant will do anything other than help that species…or perhaps you are
    on Monsanto’s payroll.

    • It is important to help the habitats, not just the monarch butterflies. Planting a nonnative milkweed doesn’t do any good to the other species of milkweeds or to the habitat at large.

    • You speak the truth!! I was shaking my head in confusion as to How someone could think this would interfere with monarchs migration. When the first frost comes not only does the milkweed die. So does the monarch. The monarchs never lay eggs on my milkweed after the first week of September. The monarchs are gone. The last set of eggs laid in September pull out of here well before our first frost. Not only does tropical MW die at frost, but most of its already starting to shut down in September here in NJ. I have a hard time keeping it alive for the last remaining cats to survive, but we always make in just in time.

      Even when I lived in Leesburg Georgia (southern Georgia) it got too cold for MW at a certain point. I wonder how much these people get paid to promote lies. So sad. Keep up the monarch MW tropical or native. There’s an army of us. I think we’ll win if we keep going. 🐛🦋🌺🌈

      • Agree. The University of Georgia scientists didn’t balance the known migratory population building benefits of evergreen tropical milkweeds vs the possible migration deminishing aspects of the plants. Example: The seasonal monarch migrations in New Zealand and southern Australia are just like our own – the butterflies become mostly non-breeding in the early fall and overwinter in clusters as shown in the photos below. These migrations did not exist until weedy evergreen tropical milkweeds from South Africa were inadvertently introduced to those countries in the mid-1800’s. Therefore we know that for the past ~160 years the migratory population building benefits of the tropical milkweeds introduced to Australia and New Zealand have far outweighed any possible migration deminishing aspects of the plants.

      • I am in Southern California, and my tropical milkweed does not die. I have native and tropical, and the tropical thrives all year long. The tropical milkweed does increase OE rates because a lot of people don’t know to cut it back in the fall and throughout its growing.

  4. Michael, please do research the connection between Monsanto and these scientists from the University of Georgia as well as the publisher of the article — the UK’s Royal Society (their national academy of science), and be sure to let us know what you find out.

    • Why do you think monsanto is against or intentionally harming monarch butterflies. Linking monarch decline to Monsanto is false and illogical. Glyphosate herbicide is off patent and their are hundreds of manufacturers. Second, milkweeds never survive in cultivated fields due to tillage, not the spraying of glyphosate. Milkweeds line local, state and federal highways. It is a weed that thrives in non cultivated fields since it is a perennial plant. As a weed scientist and agronomist, i have never seen milkweed in corn and soybean fields since my career started in the mid 80’s. Monsanto nor glyphosate is hurting the monarchs. Farming practices, used by farmers, may be declining milkweed populations, but we have been cultivating row crops long before monarchs began a decline. There is simply no evidence to support the “monsanto is killing monarchs” myth and you are a fool to believe it.

      • Tom, I see you work for Monsanto. I walked beans in the 70s ad 80 and there was lots of milkweed growing in cultivated fields.

  5. I agree with Paul Cherubini, and I also question the Georgia research projects that go on and on about o.e. only to what end? The witch hunt would indicate that anyone who handles a Monarch Butterfly is contaminating them with fungi that will cause their demise. Really? Maybe in lab breeding programs, but, in the wild, and capturing one or more individually and tagging and recording is quite different from bagging a net of 100’s at a time, which might create cross contamination, but, no one wants to discuss the cross contamination issues, rather, to point fingers at those who do good field work, to the best of their ability. We who come to these Monarchs sites all want the Monarch Butterfly to survive. None of us is out to destroy the Monarchs. Let’s get better at looking at the dimensionality of the monarch life cycle and enable them to have a better chance than our other environmental demises, due to our lack of understating.

  6. During the past 10 days there have been multiple sighting reports on Journey North of brightly colored young monarchs appearing in the central USA such as these:
    North Carolina:

    So that means these butterflies had to have grown up in the southern States or in Mexico on non-native tropical milkweed and then migrated north, thus helping to repopulate the central States. In this way we can see how planting tropical milkweed is beneficial to the migratory monarch population.

  7. I am confused about some of these posts. It is October 25th and there are still feeding Monarch caterpillars on my tropical milkweed. I was sold the red/yellow milkweed seed as common milkweed ☹️.I live in Tallahassee Florida and the migration date has passed weeks ago. So perhaps there is merit to this study.

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