Last week, I attended a conference aimed at creating a statewide conservation plan for monarch butterflies. The meeting was really informative and thought-provoking. I learned a great deal about the ecology and conservation needs of monarchs from Chip Taylor of Monarch Watch and others, and was part of some good discussions about potential strategies to help the species recover. I thought I’d share some of what I learned from those discussions because they helped me better understand the issues surrounding monarch conservation. Any errors in the following are a result of my misunderstanding what smart and knowledgeable people told me, and I apologize in advance.
The monarch butterfly is a migratory species, but it takes multiple generations to make the migration from parts of North America to Mexico and back. Here in Nebraska, we’re part of the Eastern Population of monarchs, which extends from roughly the east edge of the Rocky Mountains to the east coast. The butterflies in this population leave their Mexico wintering grounds in late February each year and head north. They lay eggs in the southern United States and the monarchs produced by those eggs then head north into the northern half of the U.S. and the southern edge of Canada during May and early June.
During the summer, there are a couple generations of monarchs that mature and lay eggs without migrating. However, in mid-August and September, monarch adults get the urge to migrate and start heading south. Those that survive the trip usually reach the Sierra Madre Mountains of Mexico by early November.
Threats to Monarchs
There are two categories of threats to Monarchs: 1) Factors we can control, and 2) Factors we can’t. The big factor we can’t control is weather and the way it interacts with migration timing and butterfly survival. Weather can have a tremendous impact on butterflies, and many millions of butterflies are killed by hot weather, storms, or other events. However, since we can’t control the weather, we have to focus on what we can control.
There is a long list of human-induced factors that affect monarch populations. Those include the conversion of grasslands, roadsides, and field edges to row crops (largely facilitated by government policies) as well as farming practices that have nearly eliminated milkweed from farm fields. Pesticide use is another factor, including pesticides used for farming and pesticides used for other purposes, including mosquito control. Logging of forests in the wintering grounds of Mexico is another important issue.
I’d heard that the loss of milkweed from crop fields was a big deal for monarch butterflies, but hadn’t really understood why. At the conference, we heard that research has shown that about four times as many eggs/plant are laid on milkweed plants in crop fields as on milkweed plants in other habitats. (I’m not sure anyone understands why.) In addition, while the eastern population is spread across a huge area of North America, about 50% of the butterflies that reach Mexico are born in the cornbelt of the U.S. – the intensively farmed Midwestern states. Prior to the widespread use of glyphosate-resistant crops, milkweed was a pretty common inhabitant in crop fields throughout this prime breeding area for monarchs. Now that farmers are so much more efficient at weed control, we’ve lost the most productive egg-laying habitat in the country’s most important breeding area for monarchs.
Because monarchs can only be raised on milkweed, getting more milkweed plants in the landscape, especially within the cornbelt states, is a key part of increasing the monarch population. It’s likely that more than a billion additional milkweed plants will be required to stabilize the monarch population. Increasing milkweed populations to that extent will require a wide range of strategies. In addition, protecting and restoring the wildflower-rich grasslands and other natural areas that provide food for adult monarchs, as well as for thousands of bee and other pollinator species, is also vitally important.
One clear strategy is to plant more of monarchs’ favorite milkweed species in gardens, parks, roadsides, nature centers, and many other sites. In the north-central U.S., milkweed species such as common (Asclepias syriaca), showy (A. speciosa), and swamp milkweed (A. incarnata), are known to be favorites, while green antelope horn (A. asperula) is important in more southern states. You can find sources of seeds and plants at Monarch Watch or from the Xerces Society’s Project Milkweed website. Sites like monarchgard.com can help with garden and landscape design ideas.
More milkweed in gardens and landscaping can make a big difference, but an even bigger part of monarch recovery needs to come from a change in the way milkweeds – and the weedy, edge habitats they thrive in – are perceived by the public. Elimination of milkweed from roadsides, field edges, and odd corners and margins of our landscapes happens because we are uncomfortable with the “messiness” of those areas if they aren’t frequently mowed and/or sprayed with herbicides to make them look uniform in height and composition. Allowing milkweed and other wildflowers to thrive in those odds-and-ends habitat areas can have a huge impact on monarchs and other pollinators, along with pheasants, song birds, and many other wildlife species. Reducing mowing frequency and spot-spraying truly invasive plants – instead of broadcast spraying to kill anything that’s not grass – in these habitats saves both money and time as well.
The Role of Prairies?
Last week’s meeting also encouraged me to focus even harder on an additional aspect conservation I’ve already been working on – improving the contributions of native prairies and rangeland to pollinators. The experts at our meeting, along with other sources of information on monarchs, seem to be focused largely on milkweed and the kinds of farm fields, edge habitats and landscaping mentioned above. While all those are very important, I can’t help but think about the value of native prairies – especially in places like Nebraska where we still have millions of acres of grassland.
A healthy prairie with a diverse wildflower community is invaluable to bees and other pollinators, and also provides nectar resources needed by monarch butterfly adults. If that prairie contains vibrant populations of milkweed species that provide egg-laying habitat to monarchs, that’s even better. Many prairies don’t currently have strong milkweed populations. Some milkweed species are not strong competitors in a tight-knit plant community, and certain grazing and other management practices tend to further discourage milkweeds. Over the next several years, I am hoping to learn more about how to make prairies support stronger milkweed/monarch populations. Hopefully, we and others can help make North American prairies even better contributors to the survival of monarch butterflies.
I think we heard some quasi educational guesses as to why monarchs prefer milkweed in farm fields — less threat of predators being one. I mean,if you have no other plants / flowers attracting beneficial predators, isn’t it less likely they’ll make a special trip to a clump of milkweed among corn (corn riddled with who knows what systemic anti-insect goodies). I also wonder about how much easier it is for monarchs to see a clump of lone milkweed among a monoculture at a greater distance; we know clumps / masses of some plants in bloom create a stronger beacon. But do monarchs “see” plants or smell them? We know they taste with their feet. And finally, we need more prairie islands with some level of connectivity. What if at least every section had 20-40A of pollinator habitat? Would that even be enough to support some level of biodiversity?
Todays post is a very important one. Thank you for sharing all the information. There is an effort in Portland and all through the valley for gardners to include milkweed in their plantings. Some Monarchs were seen last summer — I am hoping to see some on my milkweed plants next summer. Have childhood memories of milkweed and monarchs (Sandhills area) The variety here is different than in the prairie. May we all see more of this beautiful butterfly in the future.
Chris, I’ve often wondered if there is any component of the various generations of the monarchs that migrate to Mexico and back that stay put and don’t migrate (either in Nebraska or further south)? Given the enormous risk and cost involved in migration as a life strategy, it would seem like there would be considerable selection pressure favoring any that might not migrate and instead were able to survive locally. And if not, has anyone ever tried to overwinter some (as eggs pupae, or even adults), just to see if they can survive winters somewhere north of the border?
Peter, I’m the wrong person to ask. I understand the mid summer generations are non migratory but I’m not aware of any that overwinter, at least this far north.
There are overwintering populations along the Gulf Coast where they won’t freeze to death.
Chris, you mention several species of Asclepias as being favored host plants. Are all species equally favored? Is Asclepias syriaca more valuable? It is typically considered more weedy and often other species are selected for landscape settings, and I wonder if other species provide equal benefit.
Good question. Dan provides good comments below. I was told that for Nebraska, syriaca, speciosa, and incarnata are the most often used by caterpillars but I think opinion varies. We’ll be looking at this during the next couple years…
It might be that they are the most often used by monarchs simply because they are the the most common on today’s landscape. Most of the other species are rare enough (purple milkweed is endangered and Sullivant’s is threatened where I live) that it would be a logistical challenge to locate enough plants to study. I’m speaking from experience in my home garden. I get caterpillars on all of the species I grow. I read that tropical milkweed (Asclepias curavassica), which is often grown as an annual in the north and is available at a lot of the big box garden centers, can cause monarchs to be vulnerable to a parasite; it’s orange like butterfly milkweed, so people should probably just grow butterfly milkweed (Asclepias tuberosa) instead.
A. tuberosa is the least favored milkweed. Monarchs will certainly use what’s available, but there have been studies that show it’s the level of cardenolides in various milkweed species (as well as regional preferences). Also, a milkweed recently wounded (leaf torn, stem broken) will put out a rush of cardenolides making it potentially more attractive to egg laying.
That’s interesting. I grow almost every milkweed possible in my region, and monarchs are always on my A. tuberosa in good numbers, but that could just be because I have a very large number of A. tuberosa and other milkweeds in lesser numbers. Maybe the concentrated display of many plants is attractive? Or maybe I’m just not paying close enough attention to what’s going on with my other milkweeds.
Butterfly milkweed (Asclepias tuberosa), Sullivant’s milkweed (Asclepias sullivantii), purple milkweed (Asclepias purpuracens), and poke milkweed (Asclepias exaltata, part/light shade) are milkweeds more amenable to Midwestern home gardens that will also bring a lot of monarch caterpillars to maturity. These are also less disturbance tolerant species that were largely lost early on in our conversion of prairies and savannas to row crops or to degradation from overgrazing. I would argue that they deserve to be grown for their own sake. There is really a milkweed or two or three for every garden or restoration situation. If you can get your hands on seed, they are all easy to germinate after thirty days in a refrigerated back of moist sand or perlite (usually some will germinate even without the cool period), and most reach flowering size in two or three years in the garden (butterfly milkweed sometimes flowers in year 1).
“At the conference, we heard that research has shown that about four times as many eggs/plant are laid on milkweed plants in crop fields as on milkweed plants in other habitats. (I’m not sure anyone understands why.)” An obvious answer would be that the number of eggs laid on cropland milkweed is greater because the acreage of cropland is so huge. But comparing acre to acre, egg laying in cropland milkweed may be greater because it is more similar to the way Monarchs historically laid eggs in the landscape – by finding milkweed among large open areas of other plants.
For the last four seasons, I have been collecting Monarch eggs & larva from rural roadsides here in east central Illinois during my morning run, since they are subject to mowing at more or less random times. If the summer is dry, the milkweed does not grow well and the population of preditors seems to be up.
It has been my observation that Monarch juvenile are much more common on the sparse plants along the roads than in heavyier established stands of milkweed in hay fields and prairies . One possibility is that preditors are more effective if the milkweed is concentrated.
There does not seem to be any cropland milkweed left in this part of the world..
When we assess native prairie remnants or plan prairie restorations, what is the species diversity of milkweeds and what would be the “ideal” plant population? Butterfly weed (Asclepias tuberosa) is very common in tallgrass prairies, however, due to its leaf area, I wonder if it is the “preferred” host for the monarch?
Along with the milkweed, which is the only host for the monarch, what nectar plant species are the most attractive for them?
It varies. I worked on dry, sandy prairie remnant in W. Iowa that had many milkweeds (A. tuberosa, A. amplexicaulis, A. stenophylla, A. viridiflora, A. verticillata, and A. syriaca), but most sites in the Midwest and eastern Great Plains could be expected to have at least two or three species, if in good shape. Monarchs use a lot of species for nectar, including milkweeds. Having a few species flowering at any given time during the growing season is what’s most important. In late summer and fall, blazingstars, goldenrods, Joe-pye weed, and asters are especially important. Meadow blazingstar (Liatris ligulistylis), rough blazingstar (Liatris aspera), New England aster (Symphyotrichum novae-angliae), and stiff goldenrod (Oligoneuron rigidum syn. Solidago rigida) seem to be especially favored among late-season species in my neck of the woods (Wisconsin).
There is a bit of confusion about monarch oviposition (egg-laying) and feeding preferences. Common milkweed is the primary food of Midwestern monarch caterpillars because it is by far the most abundant and evenly distributed species, not because it is the most favored by the insect. Apparently, monarchs will lay eggs on just about any milkweed (genera Asclepias and Cynanchum), but in side by side comparisons of the most most common species in central USA, they prefer swamp milkweed (A. incarnata) above all, then common milkweed (A. syriaca) = showy milkweed (A. speciosa, of the western half of the region) > orange milkweed (A. tuberosa) > vine milkweed (C. laeve). Egg-laying females are highly attracted to tropical milkweed (A. curassavica), native to the American Tropics, and often planted in gardens, but this highly toxic milkweed actually stunts monarchs’ growth some (lower body weight and smaller relative wingspan compared to monarchs reared on other milkweeds), and there are indications there may be other detrimental effects to caterpillars growing on dense plantings of thise milkweed, such as increased likelihood of disease transmission among the high populations of caterpillars that can form on them.
And what about the other species that use milkweed as a host? And what about the larger community that native milkweeds foster?
Great, Chris. I¹d like to post to my fb page using the link https://prairieecologist.com/2016/03/01/monarch-conservation-strategies/ And I have a friend from Arkansas who is involved with Monarch support there, and she¹ll share my link I¹m sure.
Is that OK with you?
Of course, Kim, thanks.
We know the populations of milkweeds and monarchs have dropped in recent years. But we don’t know what they were like before the prairie was plowed under and became the corn-belt. Corn fields seemed to have been highly beneficial to milkweeds. More recently, the increased use of herbicides has caused the milkweed populations to plunge and this has had an impact on monarchs. But, do we know what monarch populations were like, say a hundred or two hundred years ago? Were they as high as in the past fifty years? I have been asking myself these questions and can only speculate. Not enough research.
When did “Common Milkweed” Become Common?
I think making road sides attractive to monarch may create an ecological trap. I think a better focus of resources would be to improve management of railroad corridors which have much less possibility of collision issues than highways.
The road departments mow to clear the road of blowing snow and to discourage animals from being on the road. I doubt they can be much more than simpathetic.
I have seen where they tend to mow a strip along the road and leave areas further from the road unmown as a compromise. Utility corridors might be a better choice than railroad corridors. The monarchs do not necessarily need continuous habitat to make their journey. Indeed, they tend to land on boats to rest as they fly across Lake Michigan. However, I am sure more safe havens and places for larvae to grow can only help.
There is solid research to suggest the opposite, that rather than acting as traps, roadsides with high quality, diverse roadside vegetation have fewer butterflies killed by cars. Similarly, roadsides with reduced mowing have lower rates of butterfly mortality due to vehicular collisions. Railroad corridors and utility corridors are also important opportunities for managing habitat to benefit monarchs and pollinators, but each come with unique constraints (as do managing roadsides).
You sold me. It makes sense that taller vegetation would make the monarchs fly higher thereby helping them avoid getting hit by cars.
Now if you could just convince those who profit from the milkweed is “bad” agenda then you would have your battle half won.
Chris, I have some common and butterfly milkweed at my place. I also have a lot of whorled milkweed. Is whorled milkweed helpful to the monarchs? Thanks, Ed >
Yes to all. From what I understand, they would rank in value with common first, whorled second, and butterfly third (for egg laying))
If you want a good challenge, look for Monarch eggs on the whorled. Hint: Look at the leaf end on new growth.
We have a small lot in north Lincoln, with very little lawn. We have a number of kinds of milkweed, including common, whorled, swamp, butterfly, and purple. I don’t check the leaves often, but do see some caterpillars each summer. I see more on the common, and not as many on the others. The most I’ve ever seen at one time, though, were on a clump of butterfly milkweed one year. As for blooming plants the butterflies feed on here, liatris seems to be the favorite, especially ligulistylis. I also see them on joe pye weed and coneflowers. We see the most in the fall, when they are getting ready to fly south. I don’t remember the highest number we’ve seen, but am thinking it was around 20. I didn’t know about the preference for farm fields. It would be a treat to see them in large numbers.
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I live in Alberta, Canada just north of Edmonton. We had hundreds (thousands?) of baby monarchs here last year (lots of thistles). I did not know about the milkweed. I will plan my garden this year to be more friendly to monarchs with the addition of milkweed; in addition to flowers liked by hummingbirds, and honey bees!
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Milkweeds are important, to nurture more Monarch larvae. But I would like to see more done to help the adult Monarchs as they migrate south in late summer. Those adults need LOTS of nectar energy in order to make that long journey. Our planted 6 acre prairie has good stands of Meadow Blazingstar and other native flowers, and we get hundreds of Monarchs feeding each day in late August.
It seems to me that we need to support all the life phases of Monarchs. And that includes feeding migrating Monarchs by providing late summer nectar.
Would you comment on the desirability of adding a recommendation for nectaring plants to all gardens that plan on using milkweeds for monarch conservation. Too often, I hear people say they’re planting milkweeds, without paying attention to the needs of the adults to have these nectaring plants.
You’re absolutely right. Milkweed gets monarchs to adulthood but wildflowers get them to Mexico.
I would like to see a alternate law or hire fines for protecting all gardeners and thier projects from people who don’t like or wanting to block them from doing personal home garden or prarie or city wildlife habbitat projects maybe simaulr to the hunters new law.
These registerd and certified advocate or other agency wildlife land is for that reason.. to help wildlife while on that land. Please Don’t disturb ant ything, get perssmission if doing work or wanting to pass through, do not tamper with, damage or threaten anyone or the wildlife that lives on that land, the wildlife will thank you for that and the onwers will also!
Flower & Gardens and land wildlife habitat protective law?
Protection laws for habitat will help the wildlife not protected… untill totally protected and giving all a chance to live freely!