Monarch butterflies are fascinating organisms, their migratory nature connects North and South America in a shared conservation goal. While small Monarch populations exist in Southeast Asia, Australia, and some isolated areas, the populations in North and South American exhibit surprising ingenuity, vacationing during the cold winter months in the warm forests of Mexico, and then summering in North America.
This migratory strategy has an ingenious benefit of saving monarchs from the lethal cold, and also helping to protect them from a dangerous pathogen, Ophryocystis elektroscirrha, or OE for short. How exactly does this work? The only plant that monarch caterpillars eat and even need to survive is milkweed, because they use the cardiac glycoside toxins from the leaves to make themselves poisonous to predators. In fact, the bright and striking warm colors on monarch caterpillars and butterflies are actually warning signals to predators of their poisonous nature. This defense mechanism is called, aposematic coloring, which means “warning sign” coloration, because it acts as a caution signal of an organism’s toxicity. This idea of bright reds, orange, and yellows mixed with a strongly contrasting white or black as being toxic and attention grabbing is so ingrained in nature that even road signs are designed from this principle. The bright red and white of a stop sign or the striking orange and black of construction signs take advantage of an innate evolved predisposition to pay attention to these colors as warnings.
The OE pathogen takes advantage of this connection between monarchs and milkweed because the disease is usually transmitted from the mother to her eggs and the milkweed leaves on which she lays them. As the summer progresses, more and more of this pathogen builds up on milkweed plants, increasing and intensifying the infection in monarch caterpillars. Though this disease is found in many monarch populations, the pathogen does not usually occur in high enough numbers to cause detrimental effects to the monarchs. This is because the milkweed plants and the OE pathogens die off every winter, while the monarch butterflies fly off to their vacation destination in the Mexico forests where they spend the winter drinking tropical nectar from flowers and enjoying the warm South American sun. They do not begin to reproduce and raise new caterpillars again until the following summer as they travel north to raise their young in the safe and nutritious North American fields of milkweed. And the newly blooming milkweed is free from last season’s OE infection, providing a healthy home for the caterpillars to develop.
This is why planting milkweed is such a fantastic contribution to monarch butterfly conservation. However, in people’s enthusiasm to help the butterflies, many have actually created a hazard for the monarchs by planting the non-native tropical milkweed, Asclepias curassavica, which naturally occurs only as far north as Mexico where there are never monarch caterpillars. Tropical milkweed when planted in the USA continues blooming year-round in USDA hardiness zones 8-11, and farther north where tropical milkweed cannot overwinter, many people bring the plant inside and keep it alive to replant in the spring. In both cases the result is milkweed plants that do not die off in the winter and create a serious health risk to monarch caterpillars, accomplishing the exact opposite goal of helping the monarch butterflies. You can find more information about the negative effects of tropical milkweed and which milkweed species to plant for monarch conservation in this publication from the Monarch Joint Venture.
Thus, monarch’s transient lifestyle naturally separates their caterpillars from dangerous levels of the OE pathogen and teaches us that we must be mindful of respecting the native biodiversity of each region. Laws do not always reflect the science, and if people are allowed to bring this plant that can be harmful to the already dwindling monarch butterfly populations, then it’s up to scientist like myself and others to share the information and empower the general public so that we can take action to save our environment together. It’s important to remember that all of us as individuals have an amazing ability to make positive environmental change through small actions in a way that lawmakers never could. Because a well informed citizen with a peaceful heart to truly make the world better can do incredible things. Remember the lesson from these migratory butterflies who teach us how local environmental action can have far-reaching affects.