Flora Fact: Pick Your Poison
Antelope horns are toxic to many animals but essential for monarch survival.
By Karen Clary
While a cow or deer will turn its nose up at antelope horn milkweed, monarchs depend on this common pasture plant for survival.
Like most members of the milkweed family, antelope horn is highly toxic to livestock and wildlife. The sticky, milky sap in the stems and leaves contains poisonous cardiac glycosides.
But it’s this very toxicity that makes antelope horns and other species of milkweed important host plants for monarch butterflies. Monarch caterpillars are immune to the poison, which they absorb as they feed on milkweeds, making the insects unpalatable and poisonous to predators.
In Texas, milkweeds are crucial to monarch survival because they are host plants for migrating generations coming up in the spring from wintering grounds in Mexico. Unlike the generation before them, which made a one-generation journey south, successive generations make the journey north, and most of the first new generation starts in Texas.
Monarch caterpillar on antelope horn milkweed.
As rangeland gives way to development, the milkweeds that sustain the monarchs are dwindling. Federal and state land management agencies and the North American Pollinator Protection Campaign are taking an active role in public outreach, education and conservation by creating pollinator gardens that provide habitat for the monarch butterfly.
They are encouraging gardeners who live along the migratory path of monarchs to plant milkweeds in their butterfly gardens.
Carl Linnaeus named the genus after Asclepius, the Greek god of healing, because of the many folk-medicinal uses for the milkweed plants, primarily the roots, which were used to treat pleurisy. The subspecies name capricornu is Latin for antelope horn.
The antelope horn flower is not a typical flower. It has extra parts; the most conspicuous part, the star-shaped ring of “horns,” is made of highly modified stamens. It is one of the few plants that package pollen into microscopic saddlebags called pollinia. The usually easy-to-find stigma, where pollen is deposited by insects, is tucked deep inside the flower.
Sugar-laden nectaries at the base of the flower attract visiting bees, wasps and butterflies. As they make their way to the nectary, their feet or mouthparts snag the pollinia, pulling them free when the pollinator flies off.
Pollination occurs in reverse when the pollinator drops the pollinia off on the stigma as it roots around the next flower in search of nectar.