Monarch butterflies lay their eggs on just one type of plant, and that’s milkweed (genus Asclepias). Monarch caterpillars only eat milkweed. If monarch eggs are laid on other plants, the caterpillars cannot survive and will ultimately starve to death.
In recent years, awareness has risen around the importance of milkweed (as well as other native plants) for pollinator survival. Although you’ll find ‘weed’ in the name, milkweed is a beneficial wildflower, not a noxious weed. Planting the right species of milkweed for your area can be a huge help to monarchs and a number of other species. Planting species that are specific to your region will ensure it won’t take over your yard. To see which species are appropriate for the Northeast, download the Milkweed Information Sheet from monarchjointventure.org by clicking here.
When choosing where to plant your milkweed, keep in mind that it can be toxic to house pets and livestock. Dogbane, though lesser-known, is a milkweed relative that is also toxic. Both plants belong to the family Apocynaceae. Dogbane looks similar to milkweed and is often difficult to distinguish from milkweed. The toxic elements in both milkweed and dogbane are cardiac glycosides. These are chemicals that inhibit function of the heart muscle. The lethal dose of milkweed or dogbane, for most animals, is about 0.05% of body weight. For a cow or a horse, that’s about half a pound of plant material; for a sheep or goat, it’s just a few ounces. Fresh leaves are the most toxic, but dried leaves also contain the toxic compounds. Death from poisoning can occur within 12 to 24 hours of ingestion.
The truth is that most animals won’t eat milkweed and dogbane because they do NOT taste good. The risk for poisoning increases when fields are overgrazed or dormant, such as during mid-summer. Perhaps the highest risk for livestock poisoning is from contaminated hay simply because it can be difficult to tell that toxic plants are present. Livestock owners who feed hay should ensure they buy from a reputable dealer and learn how to select quality hay. It’s also generally advisable to ask your hay dealer what weeds are problematic in their fields and what measures are taken to control them.
Sources: U.S. Fish and Wildlife Service, University of Maryland Extension