Common characteristics of milkweed plants

Milkweed plants develop tall, freestanding stems with oval-shaped leaves that taper to a point, and which protrude opposite one another directly from the stem. In summer, large numbers of small flowers grow on stalks that emerge from a common central point (known as umbels), forming rounded clusters near the top of the plant. The flowers typically have a pleasant smell and attract a wide variety of insects. After blooming, green seed pods develop that in late summer or autumn dry and crack open to release small flat seeds attached to silver-white silky-hairs. The seeds are dispersed by wind. When broken, the stems and leaves of most milkweeds release a milky-white sap. Milkweeds are perennials, and propagate by seed or rhizome. Milkweeds are mildly toxic and should not be consumed by humans, pets, or livestock.

The easiest way of distinguishing various milkweed species is by province, the characteristics of the site where the plant is found, the width and length of the leaf, the seed pod texture, and flower colours.

When inspecting the underside of milkweed leaves for monarch eggs or chrysalises, be careful, for both are easily overlooked and easily damaged. Monarch eggs are no larger than a pinhead, and chrysalises, which dangle from a thin thread of woven silk, are leafy green coloured (see below).

Common questions about milkweed

Are milkweed plants poisonous?

Milkweed plants contain a toxic substance called cardenolide which, if ingested in large quantities, can cause cardiac arrest in humans, livestock, and other animals. Milkweeds taste very disagreeable, and most animals that lack the monarch’s resistance to cardenolide will avoid eating it. There is a widely-held fear that cattle, sheep, and other livestock will accidentally poison themselves by eating milkweed growing wild in farmers’ pasture and on rangeland. However, grazing animals will only eat milkweed if there is nothing else available to eat, such as when animals are overcrowded onto poorly managed pastures. Accidental poisoning of grazing livestock by milkweed is therefore rare on well managed farms. Farmers who grow and cut hay to feed livestock must take care to ensure it does not contain large amounts of milkweed, and for this reason will use herbicides to control milkweed in their hay fields. Historically, agriculture officials in many jurisdictions classified milkweed as a noxious weed, but this is changing.

Can I safely grow milkweed in my yard?

Yes. Milkweed is a very attractive plant, and monarchs will find it in your garden. Should your pets try chewing on a milkweed plant, they will find the taste disagreeable, and will usually stop eating it before they suffer any harm. The five species of milkweed tracked in Milkweed Watch are all native to Canada; select the species that is native to your province and is best suited to soil conditions on your property.

How else can I help?

A key factor in the instability of monarch populations is a shortage of flowering plants, caused by the expansion of cities, suburbs, and agricultural fields. If you live in the city, consider replacing some of your lawn with flower gardens. If you live in the country, allow wildflowers to grow on unused land, and avoid spraying herbicides unnecessarily. This will help not only monarchs, but other butterflies, bees, and other pollinators whose populations are also in decline.

About monarchs

The monarch (Danaus plexippus) is native to North and South America. Monarchs cannot live in freezing temperatures, and so migrate to warmer climes in autumn. Two distinct monarch populations spend their summers in Canada. Monarchs that summer in western Canada migrate to California in the winter, whilst eastern monarchs spend their winters in Mexico.

Although the milkweeds on which they eat make the monarch caterpillars and adult butterflies slightly toxic and unpalatable to many predators, certain species of wasps, spiders and birds have adapted and are able to prey upon monarchs. Monarchs are also susceptible to naturally-occurring parasites. However, human activities pose the greatest threat to monarch populations today, including the destruction of habitat in the overwintering grounds, growing use of agricultural chemicals, and a decline in the availability in milkweed across much of the species range.

Monarch lifecycle

The monarch lifecycle consists of four distinct phases: egg, larva (or caterpillar), pupa (or chrysalis) and adult butterfly. It takes approximately one month to complete the progression from egg to adult. The adult will for between two and six weeks if born in the summer; if born in the late summer and destined to migrate south, the adult will live for between six and nine months.

  • Phase 1: Egg

    Female monarch butterflies will lay a single, oblong-shaped, yellowy white egg on a given milkweed plant, typically on the underside of a leaf near the top of the plant. Egg is approximately 1mm in diameter and has tiny ridges along the outside. A tiny, pale green larva, only a few mm in length, hatches within 3 to 8 days, depending on air temperatures. The adult butterfly will typically lay several hundred eggs during her lifespan.

     
  • Phase 2: Larva (Caterpillar)
    After hatching, the larva eats the egg casing and then feeds on the surrounding milkweed leaves. The larva grows quickly and sheds its skin four times, and develops black, gold and white stripes that become increasingly distinctive with each molt. The larval stage lasts between ten days and two weeks.
     
  • Phase 3: Pupa (Chrysalis)

    The mature caterpillar may leave its host milkweed plant in search of a convenient place to pupate. When it is ready to begin pupating, the caterpillar hangs from hindmost legs on the underside of a leaf or stem, attached via a silk pad which it spins. Over the course of the next 8-15 days, the caterpillar undergoes a metamorphosis to the adult butterfly. The chrysalis is green coloured to camouflage the immobile pupa.

     

  • Phase 4: Adult butterfly
    Male monarch butterflies are distinguished from the females by a black spot that the males have on each hindwing. Butterflies will begin mating within 3-8 days after emerging from the chrysalis. If born in early- or mid-summer, the butterfly will live for between two and six weeks, focusing on reproduction while feeding from the nectar of blossoming flowers. Adults that emerge late in summer will live considerably longer – between 6 and 9 months – as they must migrate to wintering grounds in Mexico and California.

For more information on monarchs, or to participate in the monitoring of monarch populations, visit one of the following sites:

References

  • Canada Species at Risk Public Registry http://www.registrelep-sararegistry.gc.ca/species/speciesDetails_e.cfm?sid=294
  • Flockhart, D. T. T., Pichancourt, J.-B., Norris, D. R., & Martin, T. G. (2015). Unravelling the annual cycle in a migratory animal: breeding-season habitat loss drives population declines of monarch butterflies. Journal of Animal Ecology, 84(1), 155–165.
  • Inamine, H., Ellner, S. P., Springer, J. P., & Agrawal, A. A. (2016). Linking the continental migratory cycle of the monarch butterfly to understand its population decline. Oikos, 125(8), 1081–1091.
  • Pleasants, J. M., & Oberhauser, K. S. (2013). Milkweed loss in agricultural fields because of herbicide use: effect on the monarch butterfly population. Insect Conservation and Diversity, 6(2), 135–144.
  • Ries, L., & Oberhauser, K. (2015). A Citizen Army for Science: Quantifying the Contributions of Citizen Scientists to our Understanding of Monarch Butterfly Biology. BioScience, 65(4), 419–430.
  • US Department of Agriculture Plants Database. https://plants.usda.gov

Photo credits

(in order of appearance on this page):

  • Monarch on common milkweed flower = purchased stock image
  • Monarch on swamp milkweed near shore of Georgian Bay = Liz Wylie
  • Monarch caterpillar on milkweed leaf = Ontario Nature
  • Chrysalis = Trina King-Tassone

    • Adult butterfly = Trina King-Tassone