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Washington State University

On The Controversy Over Milkweed & Monarch Butterflies

Posted by rdsayler | November 22, 2022
Photo of monarch butterfly in hand
Monarch butterfly produced in the WSU Arboretum during a 2016 breeding event. Photo by R. Sayler.


If humans are good at one thing, it’s undoubtedly controversy. Pretty much any subject that you can come up with in the human or natural world is controversial in one way or another. In fact, if we weren’t always fighting, bickering, and arguing with each other so much, it’s valid to wonder if humans might then go extinct from pure boredom! [Just kidding! We should be so lucky.]

But at the risk of being controversial, I’ll suggest that some level of controversy, or at least disagreement or uncertainty, often drives the pursuit of science and the quest to discover how the world and the universe operates.

Most people seem to like butterflies, but yes, even butterflies are controversial. Assuming you do like butterflies, you’ve undoubtedly heard of the conservation crisis for perhaps the most widely-recognized butterfly in the United States, the monarch butterfly (Danaus plexippus). The monarch is highly recognizable given its large size, distinctive orange and black markings, and its widespread distribution across the United States.

It’s also well known as a host specialist that relies exclusively on milkweed (Asclepias spp.) on which it lays its eggs and on which larvae (caterpillars) feed and grow before turning into adult butterflies. Adult monarch butterflies will feed on nectar and gain energy from a large variety of flowers, but milkweed is required to reproduce.

Major concern for the conservation status of monarchs intensified by the 1990s when the eastern population of monarchs that overwinter in the Oyamel fir (Abies religiosa) forests in the mountains of Central Mexico began rapidly declining. Similarly, the western population of monarch butterflies has declined >99% since the 1980s leading to concern that the western population also might be in danger of extinction (see: Xerces Society; Biological Conservation).

While the counts of wintering monarch butterflies currently are in progress along the west coast, some initial results from 2022 suggest that numbers of wintering western monarch butterflies might be as high or higher than last year, which saw a slight rebound from disastrously low numbers (<2,000) tallied in 2020 at traditional wintering sites in tree groves along the west coast (see: Xerces Society).

There are several potential culprits behind declining monarch populations, including widespread development of genetically-modified (GM) crops that are routinely sprayed with herbicides that reduce milkweed populations in and bordering agricultural fields. However, just as you might have guessed, this hypothesis (and others) are controversial (see: PNAS).

Additional potential contributors to declining monarch populations include general loss of habitat and floral nectar resources, climate change, pesticides, and diseases and parasites such as the protozoan parasite, Ophryocystis elektroscirrha (often called OE for brevity), that infects monarch caterpillars from spores that are transferred between milkweed plants by female monarchs when they lay eggs (see: Wikipedia).

Infection of monarchs with OE leads to higher mortality and reduced reproductive success and fitness. Infected adults are less likely to successfully complete a migration to wintering sites and non-migratory populations have higher loads of OE parasites. And that brings us to the crux of this ecological story.

Controversy Over Planting Non-Native Milkweed

One general conservation strategy proposed to help monarch populations is to encourage the widespread planting and restoration of native milkweed populations to help compensate for lost habitat. However, several species of non-native milkweed are popular in the horticultural industry and are commonly used by gardeners.

These non-native species often include tropical milkweed (Asclepias curassavica). And depending upon where you live, and therefore your definition of native vs. non-native, you may also find butterfly weed (A. tuberosa), common milkweed (A. syriaca), swamp milkweed (A. incarnata), or others being offered for sale.

The use of tropical milkweed is particularly controversial because although it is not native to the U.S. it is widely planted and in southern and warmer areas it can remain green and productive and be used by monarchs to lay eggs in winter, outside of the normal temperate-zone breeding season. The concern is that use of tropical milkweed may encourage monarchs to not migrate and stay resident in some areas, or reproduce and lay eggs when they would normally be reproductively dormant (in reproductive diapause) on wintering areas, and thereby also become susceptible to higher levels of OE infections (see: Xerces Society).

Photo of tropical milkweed
Tropical milkweed (Asclepias curassavica) with ladybug. Photo by R. Sayler.


Recently, the California Department of Agriculture reclassified tropical milkweed as a noxious weed, allowing counties to ban sales of the plant. However, several monarch butterfly experts, including our own Dr. David James, an entomologist here at Washington State University, expressed some skepticism that tropical milkweed plantings are necessarily a major problem for western monarchs (see: Marin Independent Journal).

Milkweed Observations from the WSU Arboretum

Here in the WSU Arboretum on the campus of Washington State University, Pullman, we restore native plants and manage several botanical gardens primarily for the benefit of pollinators and insect conservation. We use extensive plantings of our regional native milkweed, the showy milkweed (A. speciosa), along with a variety of other native plants and pollinator-friendly annual flowers to provide a foundation for pollinators in arboretum gardens.

Students looking at flowers and pollinators in the WSU Arboretum
Students looking at flowers to census pollinators in the WSU Arboretum. Photo by R. Sayler.


In 2016 we observed what felt like a magical event when monarch butterflies colonized our milkweed patches in the WSU Arboretum and laid eggs there in what might be the first time in 2-3 decades based on our casual field observations. However, that’s an interesting ecological detective story itself perhaps for another day. Instead, for now we’ll share some of our observations and thoughts about planting milkweeds in gardens in Washington.

In addition to our native showy milkweed, we’ve had a few specimen plantings of butterfly weed in arboretum gardens. In our Palouse Prairie temperate environment with cold winters and hot, dry summers, this species only formed small clumps, was relatively slow growing, and did not appear to be particularly invasive or aggressive in the garden.

Butterfly weed (Asclepias tuberosa)
[Butterfly weed (Asclepias tuberosa) in the WSU Arboretum. Photo by R. Sayler.]


We’ve only tried one small planting of swamp milkweed intended for an educational exhibit in the arboretum, but the garden site was hot and the soil dry and the planting did not persist, most likely as the common name implies, because this species does better in moist soils more common along the margins of wetland areas. Therefore, from these rather limited observations, these two species of milkweed did not appear to us to be likely to be highly invasive or aggressive in the typical home garden in eastern Washington.

We’ve not planted tropical milkweed in the WSU Arboretum, although I do have experience with this plant in the nearby and warmer environment of the Snake River Valley. There tropical milkweed makes an attractive specimen planting in large container pots, as the plant will often grow 2 -3+ feet tall, but it also grows well in dry sandy soils with occasional watering.


Photo of tropical milkweed in planter alongside salvia flowers
In large containers, tropical milkweed makes an attractive pairing for pollinators with flowers from Salvia spp. Photo by R. Sayler.

The bright red/orange/yellow flowers on this non-native milkweed are highly attractive to some species of wasps, butterflies (e.g., the Gray Hairstreak, Strymon melinus), and our common local hummingbirds in the Palouse Prairie and Snake River Valley, including the Black-chinned hummingbird (Archilochus alexandri) and the recently expanding population of the Anna’s hummingbird (Calypte anna).

However, I have seen tropical milkweed begin to spread naturally from wind-blown seeds landing on the ground near potted plants. Therefore, as an ecologist and gardener, I’m keeping my eye on my tropical milkweeds growing in large flower pots, but am not yet worried that the species might be invasive here in Palouse Prairie, especially given our cold, freezing winters.

Hummingbird on tropical milkweed
[Black-chinned Hummingbird (Archilochus alexandri) feeding on a tropical milkweed flower. Photo by R. Sayler.]


Bottom Line: Are Non-native Milkweeds a Problem In Washington State?

I can’t yet answer this question definitively for Washington State as a whole, particularly for the warmer Seattle side of the state, but my experience here in the WSU Arboretum and in the nearby Snake River Valley would suggest that gardeners in the cold, temperate climates of eastern Washington need not worry about small-scale specimen plantings of non-native milkweeds. However, we always recommend that gardeners be aware of plants that may be invasive and present problems by checking current noxious weed lists (see: WA Noxious Weed List) or by contacting county extension agents.

Such milkweed plantings die back naturally in the cold of winter and so annual garden plantings of a few tropical milkweeds would seem unlikely to inhibit southward migration of monarch butterflies from this area and would not act as a continuous source of large infestations of the protozoan parasite, Ophryocystis elektroscirrha. In fact, we may well continue to have a small display of several non-native milkweeds in the WSU Arboretum for educational purposes.

What Do We Recommend for Milkweed Plantings?

Here in the WSU Arboretum we strongly recommend and encourage people who are interested in monarch butterfly conservation, and who want to plant milkweeds, to use our native showy milkweed. Why not?

It’s a beautiful plant, obviously well-adapted to our climate, and produces dramatic flowers with a sweet, almost intoxicating scent. Once you experience the scent of this plant, you will not forget it. And while it is not likely that everyone who plants milkweeds will be rewarded with the sight of monarch butterflies laying eggs and caterpillars growing on your plants, showy milkweed is highly attractive to any number of pollinators and other insects, including native bees, wasps, moths, milkweed beetles, and especially other butterflies including the large and beautiful zebra swallowtail (see: WSU Insider).

Western tiger swallowtail butterfly
Western tiger swallowtail butterfly (Papilio rutulus) feeding on nectar from flowers of showy milkweed. Photo by R. Sayler.


Tips for Growing Milkweed

Over the years we’ve learned a few things about growing our native showy milkweed. Indeed, we’ll likely produce a future special report about milkweed and our observations about monarchs breeding in the WSU Arboretum, but in reality, the plants are easy to grow.

Showy milkweed plants are quite large when mature, nearly hitting 4-5 feet in height when grown under ideal conditions in the home garden, although a more typical height might be 3+ feet. Mature plants have large, broad leaves and multiple stems that fan out and resemble an upside down umbrella. Consequently, showy milkweed is best for the back of the garden or as specimen clump with adequate room for display.

Given the hot, dry summers here in Palouse Prairie, showy milkweed does not provide good flowers and foliage for monarchs by late summer or early fall. However, we’ve learned in our large beds of milkweed that mature plants can be cut or mowed to remove above-ground stems (of course checking for eggs and caterpillars first) and induce rapid regrowth to produce flowers and leaves later in the season than normal. Thus, portions of our large beds of showy milkweed can be staggered for a longer period of floral display and availability to pollinators.

The Future of the Western Monarch Butterfly

The future of the monarch butterfly in the western U.S. is highly uncertain, as it is for a great many species around the world. The forces of habitat loss, habitat degradation, and climate change (e.g., heat, drought, fires) cloud the future of many species. However, we have been intrigued by the work of our WSU colleague, Dr. David G. James and his analyses and thoughts in a recent paper on monarch butterflies in the open-access journal, Animal Migration (see: Commentary).

Monarch caterpillar on showy milkweed
[Caterpillar of monarch butterfly feeding on native showy milkweed in the WSU Arboretum. Photo by R. Sayler.]


Dr. James provides observations of winter breeding of monarchs in the San Francisco bay area and their use of A. curassavica and other non-native milkweeds. One of the relevant questions is whether the occurrence of winter breeding in California, coupled with infection levels of the protozoan parasite OE, will reduce survival and typical migration behavior and success.

He suggests that adults produced from winter breeding may not necessarily lose their migratory behavior and could continue to populate northern interior areas of the Pacific Northwest during spring and summer. Based upon observations of monarchs in Australia, he suggests that it is possible that in the future western monarchs possibly could be comprised of a mixture of winter-breeding adults along with the typical migratory, non-breeding adults that continue to overwinter in coastal areas as they have been doing. What is uncertain is what population level may persist.

Given the adaptability of monarch butterflies, even under more worst-case scenarios, they may not go completely extinct in the western U.S., if nothing else, because of developing resident non-migratory breeding behavior and the occasional influx of eastern monarchs into the western states. In this hypothetical situation, the western population might not go completely extinct, but what could become extinct might be the normally extensive migratory behavior of historical populations that once numbered about 10 million.

Monarch butterfly in hand
Monarch butterfly emerging in the WSU Arboretum, 2016. Photo by R. Sayler.


Ultimately, it is highly uncertain what population levels of monarch butterflies may survive in the future in the western U.S. However, Dr. James has hope that the western monarch will adapt to a changing climate and persist, although possibly at lower population levels (see: WSU Insider).

It will undoubtedly take additional studies and time to more conclusively determine the relative impacts of the warming temperatures of climate change, use of non-native milkweeds for winter breeding, and resulting levels of OE infections on the migratory behavior and population dynamics of western monarch butterflies. However, here at the WSU Arboretum we can predict with 100% certainty that the ecological issues will remain controversial and unsettled for a while longer. However, that’s a good thing because these issues certainly are worthy of continued study to better understand the rapidly-changing ecology of our future world.

In the meantime, we suggest that you plant native milkweeds and treasure any observations of breeding or migrating monarch butterflies. You may be witness to the beauty of evolutionary history in progress right before your eyes.


Dr. Rod Sayler
WSU Arboretum & Wildlife Conservation Center
Washington State University

Suggested Additional Monarch Resources:

Western Monarch Mystery Challenge
Xerces Society – Monarch Butterfly Conservation
Xerces Society – Western Monarchs Call to Action
Xerces Society – Managing for Monarchs in the West
U.S. Fish & Wildlife Service – Monarchs

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