Ask Mr. Smarty Plants

Butterfly weed, milkweed, chigger-plant these are all common names for the plants of the genus Aesclepias which includes A. incarnata  which you ask about. In short, yes, the genus is poisonous. All parts of the plant are poisonous.

According to many sources including North Carolina State University, the University of Pennsylvania School of Veterinary Medicine, the ASPCA, and the Wildflower Center, all parts of the plant are poisonous in “large quantities.” It appears that livestock are most likely to ingest the plant.

Clinical signs of poisoning include profuse salivation, incoordination, and violent seizures. Early signs of poisoning are followed by bradycardia or tachycardia, arrhythmias, hypotension and hypothermia.  Death may occur from 1-3 days after ingestion.

However, the plants are premier food sources for butterflies, especially for Monarch butterflies. In the mid 1800s, naturalists observed that birds avoided eating butterflies whose larvae fed primarily on milkweed. It was later shown that the feeding larvae accumulated emetic cardiac glycosides that were retained and even concentrated in adult butterflies. Birds that ate the butterflies containing these glycosides vomited shortly after feeding and learned to avoid butterflies having the pattern typical of Monarch butterflies. Viceroy butterflies have evolved the ability to mimic the monarch’s appearance thus avoiding predation by birds who mistake the mimic for the distasteful monarch.