For Monarch butterfly moms, choosing milkweed high in cardenolides is akin to a pregnant woman choosing to eat grains high in complex carbohydrates. They do it for their kids.
According to a recent study by researchers at the University of Michigan and Emory University, those infected with the common Monarch butterfly virus OE gravitate to milkweed species with the highest levels of cardenolides, the milkweed specific chemicals that make Monarch caterpillars and butterflies taste bad to predators.
Some milkweed plants contain high levels of heart poisons called cardenolides which don’t negatively effect the caterpillars. In fact, the natural toxins work to the Monarch’s benefit by making them unsavory to predators.
If you’ve ever tasted the milky substance that oozes from milkweed leaves and stems, you’ll find it bitter and awful. Birds agree. After one taste of Monarch, they never go back. A famous photograph shows a blue jay happily munching on a Monarch caterpillar. Moments later, the bird is wretching.
“What’s very amazing about it to us is that these really simple insects, these very small animals, have the ability to make a distinction about what plants will be good for their offspring and what plants will be bad for their offspring,” said Dr. Jaap de Roode, an Emory biologist who conducted the study. An interview with Dr. De Roode is worth watching.
For those of us who garden for butterflies, we should consider planting “high cardenolide” milkweeds whenever possible.
Here in Central Texas our best bet is probably the tropical native milkweed sold as Asclepias tuberosa. This is the plant that makes up the milkweed patch down on the San Antonio Riverwalk at the Urban Reach Extension.
If you’re lucky enough to be able to grow Antelope Horns, Asclepias asperula, or have access to it in the wild, that is also a good choice. I’ve had terrible luck with Antelope Horns in my garden.
In nurseries we’ll often find Asclepias curavassica, which is called Tropical Milkweed and native to Mexico and South America. It’s easy to grow and Monarch and Queen caterpillars will eat it, but it does not offer the medicinal benefits of other species which contain the cardenolides.