Monarch Populations

This is a great question! I think it is one of many nails-in-the-coffin for the “monarchs are endangered” myth. Andy Davis has done some slightly deeper analysis of the iNaturalist monarch data, focusing on the estimated summer range. Here’s the blog post:

Three other big nails in that coffin are 1), monarch summer populations (the actual population) has not declined for as long as we’ve been monitoring it, 2) monarch populations exploded around 200 years ago and have not gone down recently, and 3) we’ve actually known for a long time that the estimations of declines in Mexico were subject to a lot of error, as we simply measured area occupied by monarchs and assumed that the density was constant. Even if the declines in Mexico are really that steep, there has been no correlation between “low” years of Monarchs in Mexico and “low” years of Monarchs in the Summer–it seems that Monarchs are able to hit a kind of carrying capacity every year regardless of how many of them start the northward migration in Spring.

Claiming that monarchs are endangered when they are not declining is a bad idea. It has already caused people to captive rear monarchs and plant tons of tropical milkweed, which has now led to 75%+ OE levels as far north as Chicago.


Yes, people rearing and releasing captive monarchs is a major downside to all the attention on whether they are endangered or not. Protecting wintering grounds specifically will be a great conservation measure, but aside from that my perception is that other interventions have little to no positive effects on monarchs specifically and some potentially serious negative ones.

On the other hand, I have seen some nice local projects creating “butterfly meadows” on public lands in areas that would otherwise have been mowed and been more or less barren ecologically. When they are seeded with native species, these areas should help a lot of organisms besides monarchs that would never get the media/conservation attention.


If the monarch was actually listed under the Endangered Species Act, it would likely put an end to captive rearing in the U.S. because it then becomes a regulated species. Though attention from the IUCN designation indeed has had mixed outcomes.


I agree, protecting wintering grounds and other natural habitat does make monarchs an umbrella species. I also love the idea of butterfly meadows, but I think it only works in areas with an ecologically educated population. In Mississippi, a remnant Jackson Prairie was plowed to make a zinnia field–public support for these fields comes in part from a desire to help pollinators including monarchs. More education about native ecosystems is desperately needed


Monarchs breed year round on native milkweed species where climate allows as iNaturalist more than amply shows. Davis is irrelevant. Oe is a function of climate and climate change, not the species of MW.

All monarchs in North America are part of the migratory subspecies that the IUCN used to list as endangered, Danaus plexippus plexippus. I think the reason why they are not IDed to subspecies is because most iNat users are not aware of the subspecies and most iNat identifiers don’t see a point in adding subspecies-level ID in regions where one subspecies occurs.


While I don’t know how every pop-sci article chose to present the data (and I have no doubt that many presented it wrong), Xerces and the NPS made it very clear that the 2,000 number was a snapshot quantity.


volunteers counted less than 2,000 monarchs during the annual Western Monarch Thanksgiving Count.


Data from the volunteer-driven Western Monarch Thanksgiving Count indicate less than 2,000 western monarchs overwintering in California in 2020.

Neither source states that the monarch population in the west was a stable 2,000 for the entire year.


For a comprehensive review spanning decades of the Western North American population of Danaus plexippus plexippus, Washington State University biologist Dr. David James has made his recent paper in the journal Insects available for free to the whole world. He also ties in some monarch data from his own native Australia, whose Mediterranean desert climate and conditions mirror those of California:


Davis is irrelvant

Strange backlash.

…where climate allows…

Key fact is “where climate allows.” Very few regions of the US allow for year-round breeding on native milkweeds. Tropical milkweed allows for year-round breeding across much of North America. I have annotated hundreds of observations of larval overwintering monarchs in the southeast, and they very very rarely occur on native milkweeds, and in each of those cases it’s on cultivated native milkweeds in pots. It is extremely common to see monarch caterpillars in Asclepias curassavica in winter, and every year there are lots of Facebook posts of poor people whose tropical milkweed-eating monarchs got turned to black mush by an inevitable freeze.

OE is absolutely a function of species as well as climate, though you are correct in the sense that in areas with very high OE levels no milkweeds should be planted, native or not. Besides, there are half a dozen other legitimate problems with Tropical Milkweed–its phenology is wrong for most of the US, it is projected to become a trap for monarchs in moderate climate change predictions (the toxin contents get higher with temperature than native milkweeds), it’s an invasive species in the South, it “medicates” monarchs, allowing them to spread OE farther, it pulls monarchs out of migration, and toxin contents of tropical milkweed get too high for monarchs when Aphis nerii gets it, compared to native milkweeds which do not share the same effect. I can provide sources for all of this.

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I’ll admit the case in the Western US is quite different from the case in the Eastern US, but we have to remember these are the same specific entity, and the Western population is an infinitesimal fraction of the overall population. I’m not saying we should ignore the Western population, but we certainly shouldn’t try to apply Dr. James’ research to the whole population or use it as evidence that tropical milkweed is problem-free in the Eastern US.

It is interesting to me that Dr. James basically ignores OE and any studies from the Eastern population, while he has no qualms drawing conclusions from the opposite side of the world in Australia, where Monarchs aren’t even native. He does make some solid points about the potential benefits of tropical milkweed as a lifeline. I do think that removing tropical milkweed from the Western US would be a bad move, as it may cut that lifeline. I think at the same time we can recognize that OE is a problem and it only became a problem because of Tropical Milkweed. Hindsight is great, and rather than turning back time we can focus on avoiding similar mistakes in the future.

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James doesn’t ignore Oe. He has a whole section on it and says we don’t have a full picture on it yet specific to the Western population which tends to migrate shorter distances, and I believe him.

California Oe has always been high than Eastern due to climate, before the adoption of Asclepias curassavica.

Apparently forum policy forbids linking to individual observations, but iNaturalist contains numerous observations of late, straggling monarch larvae in October in Minnesota, Indiana etc. on native species such as swamp and showy milkweed.

(pardon me–I do not how to do a multiquote, yet)

It’s interesting that these professed traits have not negatively impacted the monarch in the native permanent range of tropical milkweed, i.e. central Mexico southward, whose conditions are like those of Florida on steroids.


You’re right, “ignore” isn’t right. After reading that, I still think he downplays it. " Major conclusions are that heavily infected individuals have higher mortality during development, are smaller at eclosion, are more prone to unsuccessful eclosion, live shorter lives as adults, and have lower fecundity [177,178]. Infected monarchs also appear to fly shorter distances and at slower speeds than healthy individuals." He then cites a study of the Western population he conducted, which used “only a small dataset,” and used the results of that study to suggest that OE’s harm to Western Monarchs is unknown. I think that counts as downplaying.

As for the Oe level being high prior to the adoption of Tropical Milkweed, I can’t find any exact dates of introduction, I can find at least one record of it naturalizing by 1973. Since plants usually do not naturalize until after they’ve been cultivated pretty extensively, I think it’s fair to say it was in horticultural use before that time. There’s also a record of it being grown at an academic greenhouse in the 1960s in the link I shared. You just have to scroll to the bottom.

I’m not denying that there are stragglers, I even referenced that phenomenon in my comment. I think a bigger question is the population dynamics issue. Tropical milkweed makes up a tiny amount of biomass compared to native milkweed, yet a vast majority of winter caterpillars in the eastern US are on tropical milkweed, especially in southeastern areas like MS. The images below come from this data set:

The monarchs that live south of central Mexico are not normally migratory and may have adaptations to OE and the other negative effects of tropical milkweed. To my knowledge that is completely unstudied.

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It’s vastly easier to look at monarch caterpillars in gardens than it is in the wild. Most people aren’t exhaustively combing the fields day in and day out; they are sticking around town or not going far from the edges of it (I know I do).

My experience in urbanized southern California is that the majority of December/January monarch eggs and larvae have been on native fascicularis which is usable for about eleven months of the year in ideal conditions, going decades back.

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I definitely appreciate your observations. My understanding from my observations is that the main reason monarchs aren’t found on Eastern native milkweeds often is because they stop putting out new growth in August-November (depending on location) and usually lose green leaves entirely by October-December. Of course there are always exceptions, but the exceptions seem to mostly be due to microclimatic issues and atypical phenology, which primarily occur in gardens. I know that I and others in MS do explore grasslands and roadsides in late Fall, Winter, and early Spring, and if we saw green milkweeds and/or monarchs we would upload them to iNaturalist. I do spend most of my time around towns but I try to spend as much time as I can in remote woods and fields any time of year, and I just can’t find green milkweeds after around September in MS. I’ve gone through every iNat observation of milkweeds in MS and the trend holds true. Of course, I can’t generalize that to Western North America, but I feel confident I can generalize that to the Southeast and the rest of the Eastern US.

I split this discussion into its own topic, but at this point it’s mostly a back and forth between two people. Please take a moment to let others join the discussion, or perhaps consider moving it to a private message.


That only applies if you are discussing bad behavior related to an observation.

I found this blog which blames the increase in OE on the triumvirate of commercial rearing, home rearing, and the planting of tropical milkweed. Monarchs have a growing parasite problem, and it’s not from natural causes ( This is, of course, the same Andy Davis that you referred to.

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Fair point. I do think that widespread commercial and home rearing would not have caught on if not for tropical milkweed, but that’s moot. I should not have claimed that OE only became a problem because of Tropical Milkweed unless I could prove it.

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You think so? Monarch rearing is very popular here in Wisconsin but Common Milkweed is always the host species used.