David James takes issue with the loud and persistent claim that non-native milkweeds pose serious threats to monarch butterflies and the viability of their migrations.
When asked if he thinks the technically non-native Tropical milkweed poses a dire threat to monarch butterflies, James’ answer was emphatic.
“No, I do not. Not at all in fact,” said the research scientist and agricultural entomologist at Washington State University.
Having studied monarch butterflies for more than four decades, James focuses on the monarch population of the Pacific Northwest these days. That population, much smaller, less famous and even more at risk than those east of the Rocky Mountains that migrate north from Mexico each spring, moves around various sites along the Pacific coast.
Tropical milkweed, Asclepias curassavica, has been much debated as a significant factor in monarch decline and disease for almost a decade. According to several studies, presence of the easy-to-grow, widely available perennial can encourage monarchs to break their reproductive diapause and stop migrating. The orange or yellow bloomer is so irresistible to monarchs, some research suggests, that it lures monarch females to lay eggs and start the next generation of butterflies in the fall, rather than wait until spring when conditions might be more hospitable.
Also, research suggests the plant’s resilience and appeal contribute to the build-up of the deadly, spore-driven disease, Ophryocystis elektroscirrha, known in the monarch world as OE.
But James contends that the presence of milkweed–native or non-native–is NOT a primary cue for suspending reproductive and migratory behavior. “Changing day lengths and temperatures are,” he said.
James’ study of monarchs began in the 1970s in Sydney, Australia. There, he documented winter breeding monarchs on milkweed right next to large clusters of non-breeding monarchs in nearby trees.
The juxtaposition challenged conventional monarch wisdom–that reproductive and non-reproductive monarchs can live side by side–led him to earn a PhD in entomology, and write more than a dozen papers on the migrating insects.
“The science behind assertions that Tropical milkweed can terminate migration and reproductive dormancy in eastern U.S. monarchs is unfortunately flawed,” said James. “A lot more work needs to be done to prove that Tropical milkweed by itself can terminate dormancy and reproduction in eastern U.S. monarchs,” he said, adding that data on this topic does not exist for western U.S. monarchs.
James’ early research suggests that non-reproductive and migratory monarch populations in Australia are not adversely affected by the presence of non-native and other ”tropical” milkweeds such as Gomphocarpus fruticosus, an African milkweed sometimes called Swan plant or Balloon plant.
“In fact,” said James, the presence of milkweed appears to be a prerequisite for the choice of an overwintering site by monarchs in eastern Australia. All overwintering sites (occupied by non-reproductive monarchs) are characterized by milkweed presence.”
James’ historical perspective and experience seems especially timely given recent reports of the dramatic decline of the California monarch butterfly population. This year’s Thanksgiving count of only 1,914 butterflies was hailed as a near-extinction event by pollinator advocacy organization, the Xerces Society, a claim echoed by many media outlets.
The bad news revived the contentious debate in monarch butterfly conservation circles about the causes of monarch decline, with non-native milkweeds like Tropical milkweed and Balloon plant taking much of the blame.
Scientists like Andy Davis of the University of Georgia Odum School of Ecology and editor of the Migration Studies Journal, believe non-native milkweeds and resulting diseases are primary drivers of monarch butterfly decline.
“Please don’t buy into the idea that this research on Tropical milkweed is not yet settled. It very much is,” said Davis on his Monarchscience.org website in October. “No reputable monarch scientist recommends that people plant or use Tropical milkweed in their gardens or in their homes for rearing.”
Davis firmly believes that monarchs must migrate in order to maintain a healthy population.
“From my reading of the science, it looks like winter breeding is the antithesis of a healthy migration,” Davis said recently via the DPLEX, an online discussion group that reaches about 800 monarch butterfly enthusiasts, including scientists and citizen scientists.
“In other words, you can’t have it both ways – have eggs and larvae in the winter and also a healthy migration,” said Davis. “If the goal is to promote the traditional migration, then you should not be encouraging any winter breeding, like have absolutely no milkweed present to tempt them, until the spring. If the goal is to promote or establish a year-round breeding population, then you should pretty much keep doing what you are doing.”
James sees it differently. A healthy monarch population is not an absolute proposition of the insects’ either migrating or reproducing.
“With great adaptation they can likely get more flexible–which is why you see more reproduction in the southern U.S. and now in San Francisco than we did a decade ago,” said James. He speculated that the small count of overwintering monarchs does not represent the totality of the California monarch population, probably as a consequence of a rapidly warming climate. “They’re reproducing. They’re spreading out and going to different places and you can’t count them all.”
When he documented the winter breeding monarchs on non-native milkweeds in the company of large clusters of non-breeding monarchs in nearby trees in Sydney, Australia, in the 1970s, “This intrigued me,” he said recently, adding that conventional wisdom dictated that monarchs were not supposed to overwinter next to breeding populations.
“Fast forward to California, 2021, and I’m feeling a great sense of déjà vu,” said James, adding that winter breeding by monarchs is on the rise in California and non-breeding populations are on the decline. “The same happened in Sydney in the 1970s,” he said.
The comparisons are remarkable. Overwintering colonies in Sydney, Australia, in the early 1960s went from 20-40,000 monarchs per site to 1-3,000 per site in the late 1970s–more than a 90% decline. According to James, the Australian population has remained relatively steady since.
“Sound familiar?” said James, referring to this year’s historic low count in California. He pointed out that Sydney sits at 33.51 degrees latitude south of the equator, about the same distance that California roosting sites sit north of the equator.
“There was no winter breeding in Sydney in the early 1960s yet by the late 1970s it was common. All of this is documented in scientific journals,” he said.
James believes the role of Tropical milkweed or other non-native milkweeds in monarch butterfly decline is vastly overblown.
He is not alone.
Chip Taylor, founder of Monarch Watch, the citizen science tagging organization based at the University of Kansas at Lawrence, said he’s staying out of the Tropical milkweed debate “because there are far more important things to to worry about–like climate change.”
Jeffrey Glassberg, President of the North American Butterfly Association, has been steadfast in his view that monarch butterflies are highly adaptable and that non-native milkweeds serve a role in sustaining a changing population. In an article published on the NABA website in 2015, he called Tropical milkweed a “life buoy” for monarchs.
His position has not changed.
“At least some, perhaps many, of the monarchs moving northward in the southeastern states appear to be coming from southern Florida, where the population uses Tropical milkweed,” said Glassberg recently via email. “And what about the native milkweeds in southeastern Arizona….Why shouldn’t those be destroyed?” he asked, referring to milkweed species that continue to offer foliage throughout winter. “And what about the fact that the overwintering monarchs in California depend upon non-native Eucalpytus?”
James concedes that OE on year-round milkweeds can build up and cause problems. “But it’s not the end of the world,” he said. He strongly suggests using native milkweeds whenever possible.
“My research on Australian monarchs in the 1970s/80s taught me one overriding thing: the monarch is an extremely adaptable creature.”
TOP PHOTO: Monarch butterfly on Tropical milkweed. Photo by Monika Maeckle
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The camp of people who believe: “if fall migrants come into contact with tropical milkweed (or even just smell it), then this will tempt them to drop out of the migration and start breeding” have been unable to shoot videos showing that’s what actually happens in real life field situations. But the camp of people who have witnessed fall migrants pausing to nectar on the flowers of tropical milkweed have been able to shoot videos showing both sexes show no interest mating or laying eggs while feeding and will continue migrating after they’ve finished nectaring: https://youtu.be/hyFwKYyPkUo
Similarly, the camp of people who believe that if tropical milkweed is planted next to the overwintering cluster trees the butterflies will “drop out of the reproductive diapause and start breeding” have been unable to shoot videos showing that’s what actually happens in real life field situations. But the camp of people who don’t believe that’s what really happens have have been able to shoot videos in December showing the overwintering clustering butterflies will ignore large quantities of tropical milkweeds that have been planted close to the cluster trees: https://youtu.be/YCw7Ty9TBiU
Hey Paul. I know this is an old post, but I have some updated (albeit anecdotal) info. I live in southeast NC – just south of Wilmington. I had tropical milkweed blooming in October/November, and at one point counted 20 monarchs in that flower bed. They went on to lay somewhere in the neighborhood of 60 eggs in late October. I then brought all of those caterpillars inside because the temps dropped. The temperatures rose again, and I’ve released 15 to 20. The rest of the cocoons were not viable. Or did not hatch. The parents did move on, so it may be possible that two things are happening – 1). the monarchs are dropping out of migration to feed and b). they are then mating, laying eggs, and then returning to their migration, leaving behind a population of monarchs that probably won’t make it.
I mixed my letters and numbers, but you get the point.
Fabulous article, Monika! Love all the research from David James. And a great comment, Paul!
It is absolutely fascinating to me that each type of butterfly has its own life-path and cannot be swayed from it. Wow.
Keep up the good work, everyone. I love all this science stuff!
Must admit I was leaning toward the school that leaving my milkweed up in the fall did not hurt the population much and even helped. This year I had a constant stream of Caterpillars that eventually became butterfly’s, but I have noticed a down side. Here in Houston the weather can drop below freezing and if they are in the crystallize state it kills them, also those that make it to butterflies are often left without flowers to feed on. Then last week we had this unprecedented freeze that killed all Monarchs at all stages – from now on I will cut my milkweed down to the roots in the fall they would have been better to migrating.
Keith, perhaps someone will do a study, gathering all that data on those that do remain behind and freeze. It would be interesting to see if those numbers significantly affect the monarch population as a whole.
They’re going to be extinct. The Monarch wing has a different blade shapes and it’s because of where it migrates from. To accommodate the distance traveling to Mexico. It is plausible to extrapolate the variation of this very important plant will and contribute to its extinction. The very nature of survival is adaptation because of what’s in the environment. We should really be talking about how a nature preserves should have plantings of milkweed throughout the whole United States and especially hot zones where they lay their eggs so that they will be able to be studied and figure out if the tropical version of this plant is harmful to them. The private sector and government should make milkweed farms not soybean only where the aim is for the butterfly to migrate to lay eggs and reward the act of saving this species with grants and subsidies. Thanks for the discussion!
It’s best not to get the government involved in anything important! Let’s go Brandon go
A webinar I attended a few weeks ago said to cut back the tropical milkweeds way back from
October to March to avoid any disease -currently a Protozoa parasite-weakening butterflies.. Native California plants , native to to this area and type of soil germinate easily and die back naturally in the fall. They said the butterflies need other companion nectar plants near the milkweed-preferably some California natives. They need a variety of flowers and nectar. They said we’ve gone from millions of monarchs, I think in Southern California, in 1990’s to less than 2000 documented last year. For this area they recommend for native California milkweed. Some that occurs around Daley Ranch and Escondido are Asclepias Eriocarpa. They also suggested Asclepias Erosa, fascicularis (This from Central Calif I think., Local seeds germinate better)and I can’t remember all, I took notes as fast as I could. It was startling to me. Same with the great loss now of Ash trees due to an insect that made it over here, with no natural enemies. The California Native Plant Society is in the process of creating a Pollinator toolkit.Fish and Wildlife will work with private landowners to establish native gardens.
I am with my fellow Houstonian. The last two years we had a sudden freeze around Thanksgiving and maybe earlier the year before and both times there were monarchs and caterpillars all over my tropical milkweed. I am moving and will not plant tropical at my new house.
Unfortunately this February I had pollinator plants all over the place again along with Monarchs but no caterpillars then we had a hard freeze and everything was dead so the monarchs that were getting ready to continue north had no food.
For the record today I have a monarch in my yard seems to have just hatched but not sure – this would seem impossible cant possible see how it survived the freeze we had all last week in Houston. Unfortunately there are no plants of any type around everything seems dead to the roots – Would love to know how this butterfly survived.
I appreciate getting more information about the Western Monarchs, as I live in Northern CA and most info seems to pertain to the eastern crew. There’s a group of us who have become involved and as we encourage others, it’s hard to know what to say. I started with tropical and last year went all native. On warm winter days, I see a few monarchs floating around. Many have old warn wings, so I have to think they stayed for the winter. My narrow leaf is just now showing signs of tiny green leaves popping out. I only kept tropical in pots for use as a food source last year and then got rid of them. Others around me have stayed with the tropical and cut it back to break the OE cycle. We are planting our own seeds now, as pesticide-free plants can be hard to find. I bought tropical at Home Depot two years ago, put 12 caterpillars on the plant and all were dead within 24 hours. Pesticide-free pollinator plants and milkweed almost seems like the biggest challenge right now.
I had exactly the same Home Depot experience last year on the Calif Central Coast (SLO County)…bought two well-established A. curassavica in 2-gallon pots at HD after asking the Garden Section mgr: Does HD treat their milkweed with any type of insecticide? I need to know because I want transfer some Monarch caterpillars to them.
HD Mgr Answer: “No, we don’t treat the milkweed plants with any insecticide.”
So, after transferring about 10 caterpillars to these milkweed plants inside a large butterfly habitat, I was extremely disappointed to find all of the transfers dead on the floor of the habitat less than 48 hrs later. Went back to HD, found a different person—one of their non-managerial garden staff—and told him about the disastrous results. He looked sorry and offered, “It’s true, the section mgr was basically correct, we did not treat the milkweed you purchased from HD with any insecticides. But, he failed to tell you, or maybe didn’t know, that our independent distributor does spray them with Sevin before we get them. “
Sevin is one of synthetic pyrethroids that was structurally optimized for a long half-life and high toxicity, as compared to the very short T1/2, modest toxicity, pyrethrin chrysanthemum constituents it was based on.
You are correct, HD doesn’t spray and chemicals……… and would be clueless to say whether their supplier does.
I’m planting milkweed in nj. I heard the monarch population here is depleted. Can anyone confirm?
Yes…last year at work, dill weed in 6″ pit had @ 7 or 9 small catapillars, Monarchs, just about gone, brought them home to transfer to my asclepias, they refused to feed on it and all died…
If they were eating dill, then they were Swallowtail caterpillars, which don’t eat milkweed.
However, the title is misleading.
I think the last sentence should have been the title instead.
” James concedes that OE on year-round milkweeds can build up and cause problems. “But it’s not the end of the world,” he said. **He strongly suggests using native milkweeds whenever possible.**”
Research seems to support that exotic milkweeds increase OE infection. Many home gardeners are trying to help the monarchs, but are we “loving them to death?”
Anecdotally: my garden, about 5 miles from Pismo Beach Monarch Overwintering site, in the past, had tropical milkweed. Did not notice much out of season breeding,
but did have high failure rate on healthy adult monarchs hatching, (because of OE? only 3 of 8 observed chrysalises hatched successfully).
Last couple years, only had native milkweed, and still problems: too many larva, not enough plants. (probably wrong to do, but I culled small larva, leaving only one or two per my few plants.)
Different topic: I think gardens too near overwintering sites should NOT grow any milkweed. (How far is far enough away?)
Article is good reminder that much research is lacking.
Time will tell if CA overwintering sites rebound from the dismal 100’s only counts this year, or will follow what happened in AU, with continued small counts, (because of climate?)
If you are needing to cull due to lack of resources, I know many stands of uneaten native milkweed nearby that could host them. Please reach out.
Edith Smith explains tropical milkweed and monarchs very well. HERE’S WHY TO GROW TROPICAL MILKWEED !
This can also be found in the files section of the Facebook group “How to Raise Monarch Butterflies”
But why take a risk of spreading more parasites or halting the migration???
OBVIOUSLY, native varieties would be a better option because it’s what grows best in that region and what the Monarchs and other pollinators in that area are used to. If it’s native to your region, it will respond to your climate, age and die back at the appropriate time so that we don’t have to worry about females being encouraged into egg-laying instead of migrating or an increase in the spread of the OE parasite?
If there are studies showing how growing non-native varieties could possibly halt migration behavior or increase the spread of parasites, I wouldn’t take the risk of growing non-native varieties. Because if it all turns about to be true, the damage would already have been done! Either females have deposited eggs already on the plant or possible parasites have already escaped the yard the moment certain caterpillars became adults and flew off.
The short version of this very detailed (with links to multiple sources) article is: just say no to tropical milkweed.
The collective research on tropical milkweed is – it’s bad for migration. 1) It leads to less-optimal wing development for long-distance flight; 2) it makes monarchs want to become reproductive, even if they are migrants; 3) it increases local and population levels of OE, which we already know hinders migration potential; and 4) it actually reduces their flying ability.
Note, notice none of the professionals in this field plant tropical milkweed, they advocate for natives. None of them trying to save monarchs are planting it. And finally, they all are advocating against pesticides like the ones found on plants sold in most nurseries and in the big box stores. Neonicotinoids are so invasive it will even make your soil toxic.
Don’t believe me, do your own research! There is a lot of new info and it’s worse than I thought. If you want to help kill monarchs, buy a tropical milkweed plant at a big box store and feed pesticides/spread OE to them.
As to the Californian above asking how far you should be from the coast before planting milkweed, what the pros here in the Bay Area teach us locals is no milkweed with 5 miles of the coast or over wintering sites. If I was in Pismo, I would only put in nectar plants.
Southern California, with Tropical milkweed, after a few seasons of using and not cutting back because we didn’t know, we had the heartbreak of several emerging with misshapen wings that could not fly. Tore out our Tropical and will not plant Tropical here again. Have photos to document the ones that could not fly.
I have 10-12 milkweed plants in my yard, a mix of tropical, narrowleaf and showy milkweed with native nectar plants too. The tropical variety is the only one which grows with any success or vigor. The narrowleaf milkweed plants are scrawny, they fall over and won’t grow more than a few inches in a season. I only managed to get ONE showy milkweed plant to sprout over several years of trying (even with cold stratifying) and it is TINY, barely 4 inches tall. I was never able to get any kotolo milkweed to sprout or grow. I water regularly, use mulch and no pesticides and supplement the soil with compost and occasional organic fertilizer. I have grown many other plants indoors and out with positive results, just not native milkweed.
My point is, all the native milkweed to my area (non coastal Southern California) barely grows at all, and are incredibly difficult to germinate. Without the more vigorous tropical milkweed, I wouldn’t be able to feed any Monarchs at all. For that reason, I think it is a net positive. I do cut the tropical milkweed down to a 2 inch stub in the winter.
Right now, mid-November 2021, I’m seeing peak migration, with 14-15 caterpillars, up to 4th instar. That previously happened in Sept of 2020, two months earlier in the year. That sounds like climate change (both long term and yearly fluctuations) more than a problem with tropical milkweed. Tropical milkweed was in my yard both years.
Also, the narrowleaf and showy plants did not drop their leaves naturally in fall either year, so that OE reasoning seems suspect to me. At this point, I will have to cut down the narrowleaf and showy plants in a few weeks to avoid OE buildup. It’s hot here (90s to 100s in the summer), and I haven’t seen any caterpillars get sick and die from too much cardenolide in the tropical milkweed.
The Monarch caterpillars were all laid and hatched on the tropical milkweed, which does support the “tropical milkweed is bad” because it’s too attractive argument.
Here in Toronto Canada I noticed pretty small monarch caterpillars on native milkweed stands in late August. It seemed too late – by mid-September the weather can be quite cold – you sometimes see a mass migration swirling over the downtown in early September. Curious why the breeding cycle was so late.
Okay, I get all the Tropical Milkweed discussion, BUT I live in Central Arkansas (zone 7b winter freeze) and my NATIVES have viable leaves (not bloom)s in the Fall just like the Tropical Milkweed. So explain to me why it is okay to have monarchs on the natives at this time, which have been just as occupied throughout the season with cats, and NOT the Tropical? How can one be okay and the other not at the same time? Leaves are leaves right? If Tropical has to be cut down in the fall then shouldn’t all the natives as well with this train of thought?
If you want tropical and already have an to be cut down the tropical when the native variety begins to go dormant
Can a transient environmental effect, tropical milkweed, distort a genetic imperative such as migration? I am inclined to side with the genetic imperative. I prefer to provide every resource possible assuming the variety will support the largest population throughout the migration.
More obvious damage to the population is the Oleander aphid. The Aphids do real damage to the food supply starting early in the cycle. Here in SE Penn. the Aphids are out while the plants are just now starting to flower. We have seen only one egg laying Monarch but I have removed Aphid colonies from three of my dozen milkweeds of differing types. I don’t know yet if the Tropical is coming back this year. Its been a warm winter but only so.
NOW is the time of year to attack the Oleander Aphid. On all your plants. In the four years I have been trying to save the world one butterfly at a time.
I have experimented with a dozen methods of killing off the critters. With no braggable results. This spring I am going back to basics. Water. A $10pump type water can from the big box hardware allows violence to be inflicted without damaging the blossoms. First lightly squish the colony to disorient the bugs. Then attack them with a PRESSURIZED stream of water while cradling the flower with your hand to prevent damage.
Theory: Aphid colonies grow to overcapacity then some grow wings and go to new feeding grounds. The ants that herd them like organization and tranquility. Disturbing the colony with water 1. reduces the imperative for growing wings and moving. 2. The ants seem to abandon the colonies leaving the aphids lost and confused. 3. Fewer aphids means less damage to the plants.
Aphids have been trying to colonized for the past week. fewer than half a dozen colonies and they are getting harder to find.
While I am sure this is no silver bullet I have no damaged flowers or stems thus providing much more food for the Monarchs, all season.
Good luck to all…
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