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A Closer Look at Monarchs & Milkweeds: Latest Information from Xerces

Milkweeds (Asclepias species) are getting a lot of attention these days since they are the only larval host plants for the threatened Monarch Butterfly.   As you may know, butterfly caterpillars have a different diet than their adult counterparts.   Most caterpillars eat plant parts, usually leaves.  Monarch caterpillars only eat the leaves of milkweeds.

The Xerces Society, a non-profit leader in protecting invertebrates and their habitat, recently published a very useful resource, Milkweeds: A Conservation Practitioner’s Guide.  This 156 page work by Brianna Borders and Eric Lee-Mader, examines the biology and ecology of milkweeds, the insects they support, information on propagation and seed production, advice for including milkweeds in habitat restoration, and more.

The authors reinforce the now commonly held view that the loss of milkweeds across North America is a major factor in the decline of the population of Monarch butterflies.  Milkweeds in agricultural fields have been historically important for Monarchs.  Now, farmers are increasingly choosing to plant “RoundUp-ready” crops – usually modified corn and soybean seeds whose plants can tolerate being sprayed with the herbicide glyphosate (RoundUp is a named brand with glyphosate as the key ingredient).  This enables farmers to spray entire fields, killing all the “weeds,” including Milkweeds, while sparing their crops.

Milkweeds: A Conservation Practitioner’s Guide cites several disturbing studies.  A study by Pleasants and Oberhouser in 2012 estimated a 58% loss in milkweed density in the Midwest between 1999 and 2010, and a corresponding 81% decline in monarch reproduction.  As we lose more milkweeds, we lose more Monarchs.  Conservation and restoration of milkweed populations are essential to ensuring the survival of the Monarch.

While it is important to plant regionally appropriate milkweed species, especially where milkweed is scarce, Borders and Mader explain that commercial sources of milkweed seed are scarce in many areas of the U.S.  The Xerces Society launched Project Milkweed in 2010 to help address this problem.

The more we know, the more effective we can be in saving milkweeds and Monarchs. Here are some very helpful facts presented in Milkweeds: A Conservation Practitioner’s Guide.

Milkweed Distribution and Diversity

  • There are 72 species of milkweed native to the U. S. and Canada.
  • All of the lower 48 states have native milkweed species.
  • Texas has the greatest diversity of native milkweeds with 37 species.
  • Milkweed diversity is lowest in some Northern states – Washington, Oregon, Idaho, Montana, North Dakota, and all New England states have 10 species of milkweeds or less.
  • Arizona, California, Florida, Texas, and Utah all have milkweed species that are native to that state, and not found anywhere else.

Is Milkweed a Weed?

In the U.S. no milkweeds are classified as noxious weeds.

Habitats

Milkweeds occur in many different habitats in North America, including prairies, plains, deserts, open woods, pine barrens, canyons, arroyos, bogs, marshes, and wet meadows.

Plant Toxicity & Sap

  • Milkweeds are named for the milky sap in their stems, leaves and pods.  This sap contains latex and complex chemicals called cardenolides.
  • Of the 72 native milkweeds, only Butterflyweed (Asclepias tuberosa) lacks this milky sap.
  • All milkweed plant parts have cardenolides that make them unpalatable to creatures other than milkweed specialists.
  • Every milkweed species has multiple types of carenolides which vary in their chemical structure and the way they are metabolized by herbivores.
  • Cardenolide analysis has shown that 85% to 92% of Monarchs overwintering in Mexico had eaten Common Milkweed (Asclepias syriaca) as caterpillars.
  • Cardenolides can be potentially toxic or lethal to livestock such as sheep, cattle, horses, goats, and even chickens and turkeys.

Plant Characteristics

  • Flower color varies considerably among milkweeds and include: white, yellow, green, purple, pink, orange, and red.
  • Milkweed pods come in many shapes, sizes, and textures. Some pods are covered in soft hairs, while other have no hairs.
  • Milkweeds vary greatly in their growth habits – some are short and sprawling while others have tall erect stems up to 6 feet tall, like Common Milkweed (Asclepias syriaca).
  • For any given Milkweed species, plant height can vary greatly based on local climate and plant genetics.
  • While growing from seed, some milkweeds also grow vegetatively, by producing new shoots from buds on their roots.
  • Naturally occurring hybrids are rare with milkweeds.  The most frequently found example is between Common Milkweeed (Asclepias syriaca) and Showy Milkweed (Asclepias speciosa).

Milkweeds and Insects

  • Most milkweed species are mostly or completely self-incompatible and need the help of an insect pollinator to transfer pollen from one unrelated plant to another.
  • Milkweed flowers offer abundant, high quality nectar, which attracts a great variety of insects.
  • Many beneficial insects use milkweed nectar as an alternate food source during one or more of their life stages.
  • Although many different insects visit milkweeds, large bees, wasps, and butterflies are thought to be the most important pollinators of milkweeds.  This varies among milkweed species.
  • Bumble bees, Eastern carpenter bees and honey bees are all effective pollinators of some milkweed species.
  • Milkweeds can be quite valuable to honey bees, resulting in high quality honey.  A large honey bee colony can gather 13 – 17 pounds of nectar from Common Milkweed in one day.
  • The International Bee Research Association classifies milkweeds highly for honey production.
  • Common Milkweed (Asclepias syriaca) produces nectar during the day and night – quite valuable for many moths. The nectar production of other milkweed species is not well known.
  • Butterflies are not as efficient in transferring milkweed pollen, as bees and wasps are.
  • Swallowtail butterflies are some of the most important butterfly pollinators of milkweed.
  • Surprisingly, Monarch butterflies do not often pollinate milkweeds effectively!

Milkweeds as Host Plants

  • Milkweeds are larval host plants for several other butterflies and moths including:  Queen Butterfly, Dogbane Tiger Moth, Unexpected Cycnia and Milkweed Tussock Moth.
  • Less than half of the 72 native milkweed species are documented as host plants for monarchs. Due to limited distribution or other factors, many milkweed species have not been studied as host plants.
  • Research from Zalucki et al (2010) suggests that there is reduced survivorship of Monarch caterpillars when they feed on milkweeds with high latex flow and high levels of cardenolides.

Milkweed Reproduction

  • Some milkweeds will flower and produce seed in the first year after planting.  Some milkweeds require several years to reach reproductive maturity, including:  Asclepias incarnata, Asclepias syriaca, Asclepias tuberosa and Asclepias viridis.

To see Milkweeds: A Conservation Practitioner’s Guide and for more helpful information, visit the Xerces Society website.

You can be part of the effort to save Milkweeds and Monarchs, directly through planting and elimination of pesticides, and indirectly by supporting organizations like Xerces and using your voice and your votes to pressure big agriculture to change its ways.   One thing is for sure – inaction won’t save the beautiful Monarch.

From Kim Eierman at EcoBeneficial!

Photo: Monarch caterpillar on Milkweed leaf
Photo credit: Michael R. Perry

Comments

  1. What is the best way to introduce Milkweed in the garden if I don’t have any? Can it be purchased at a garden store as a plant or must it be sewn from seed?

    • Sorry for the late reply. Your terrific question was hiding in the spam folder. You can purchase milkweeds in containers, as you would any other perennial. You can also plant milkweeds as seeds. The challenge can be in finding the plants and/or seeds which are native to your area. Here is a link to some suppliers from Monarch Watch The good news is that more and more growers are growing milkweed since the demand is escalating with the Monarch crisis. If you have not already done so, join your local native plant society which can be a great information resource, including suggestions of where to buy locally grown native milkweed. Good luck and thanks for your effort to support Monarchs!

  2. Thanks Kim for this very helpful info about milkweed. I have seen a huge diminishing in numbers of butterflies in the 12 years my front yard has been planted in common milkweed. Now.. almost no caterpillars.

    • Great to hear that you have been on the front lines planting this critical plant for Monarchs. We all need to step up our planting in this way with milkweed for Monarchs and many other host plants for other caterpillars as well. The added bonus – you are also attracting and supporting a ton of other pollinators. Commmon Milkweed (Asclepias syriaca) has lots of nectar for many insects, including challenged honey bees and native bees. I read that it makes great honey! Common Milkweed produces nectar during the day and also at night – supporting many nocturnal moths with nectar after the sun goes down. Thanks for your comment!

  3. California monarchs fly to Hawaii to lay eggs. I have seen dozens of monarch caterpillars on oleander hedges (maybe another plant; I haven’t lived in Hawaii for a long time).

    • It’s fascinating to learn how far Monarchs have traveled. Here is what the National Wildlife Federation has to say about Monarchs in Hawaii: “Monarchs that were released or lost their way from California have found success year-round on the Hawaiian Islands.” There is some interesting research from Emory University described in Science Daily about their genetic differences: “The researchers used 11 genetic markers to compare the genetic structures of eastern and western monarchs, as well as non-migratory monarch populations in Hawaii and New Zealand. The results showed extensive gene flow between the eastern and western monarchs, and a genetic divergence between these North American butterflies and those from Hawaii and New Zealand.” Thanks for your comment!