Milkweed, a genus of plants known as Asclepias, is a common wildflower often found in North American gardens and fields. Gardeners frequently seek out plants that can withstand the browsing pressure from white-tailed deer, a common challenge in many landscapes. Milkweed is widely regarded as a deer-resistant plant, making it a popular choice for areas with high deer populations. This resistance requires looking closely at the plant’s internal chemistry, which acts as a deterrent against consumption by large mammalian herbivores.
The Chemical Defense Mechanism
The primary reason milkweed is avoided by deer lies in the sticky, white fluid, or latex, that flows through the plant’s tissues. This milky sap contains compounds known as cardiac glycosides, often referred to as cardenolides. These compounds serve as a chemical defense against most vertebrates, including deer, by directly affecting their physiology.
Cardenolides are steroid molecules that specifically target and disrupt the function of the sodium-potassium pump, an enzyme found in the cell membranes of animal tissues, including heart muscle cells. Ingestion of these compounds can lead to severe gastrointestinal distress and life-threatening cardiac symptoms in mammals. The plant’s immediate defense is the bitter taste of the cardenolides, which causes deer to reject the plant instantly after taking a bite, preventing a toxic dose.
Practical Reliability and Exceptions
While milkweed is generally classified as deer-resistant, this term does not mean it is completely deer-proof. The designation of resistance is based on the plant not being a preferred food source, but exceptions can occur based on environmental factors and individual animal behavior. Deer may browse milkweed during periods of scarcity, such as severe drought or a harsh winter when more palatable food sources are unavailable. In these situations, the deer’s immediate need for calories can temporarily outweigh the deterrent effect of the plant’s bitter taste and mild toxicity.
Young, newly emerged milkweed shoots may also be more susceptible to browsing before the concentration of cardenolides has fully developed. There is variation in cardenolide concentration across different species of Asclepias. Some native varieties, like Common Milkweed (Asclepias syriaca), are rich in these bitter compounds, whereas others, like Butterfly Weed (Asclepias tuberosa), have less of the milky sap. Milkweed is rated by agricultural extensions as “Seldom Severely Damaged,” indicating that while browsing is possible, it is not a frequent or highly destructive occurrence.
Milkweed’s Role in Ecosystems
The chemical defense mechanism that deters deer is the very trait that makes milkweed ecologically unique. Milkweed plants are the exclusive host plants for the larvae of the Monarch butterfly (Danaus plexippus). The female Monarch will only lay her eggs on Asclepias species because the caterpillars are specially adapted to consume the leaves and tolerate the cardenolides.
Instead of being poisoned, the caterpillars sequester the toxic compounds in their own bodies, retaining them through metamorphosis into the adult butterfly. This process makes the Monarchs toxic and distasteful to their predators, such as birds. The bright coloration of the caterpillar and the adult butterfly acts as an aposematic, or warning, signal to predators that they are chemically defended. Planting milkweed is an important conservation action, providing the sole food source for the next generation of Monarchs.