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.
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.
- 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.
- 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