Monarch butterflies are already under threat by human action. A combination of agriculture, pesticide use, and increasingly frequent natural disasters has caused habitat loss for both their summer and winter breeding grounds, with future conservation for both sites remaining unclear. Recently, continuing climate change has been found to cause additional hazards to the species, now in the form of changes to one of their staple foods, milkweed.
Milkweed: The Preferred Food of Monarchs
Milkweed contains a steroid called cardenolides. Cardenolides are toxic, impacting the sodium-potassium pump of the heart muscle, and in high enough concentrations can be fatal to humans. The toxin makes milkweed unpalatable to most insects and herbivorous species. Monarchs are the exception; monarch butterflies lay their eggs on milkweed leaves, and monarch caterpillars feed exclusively on milkweed after hatching. The inedible nature of the plant protects the eggs from being accidentally eaten, and the caterpillars have evolved to absorb small concentrations of the cardenolides in the leaves, making them poisonous to birds and other would-be predators.
Threats to Milkweed
The relationship has until this point served the monarch well. However, changes in climate and CO2 concentration could cause trouble for the butterflies. One lesser-known advantage of feeding off milkweed is that it helps prevent parasitic infection, such as with Ophryocystis elektroscirrha, a protozoan parasite obligate to monarchs that can persist into their adult stages, in monarch caterpillars. A 2018 study published in the Ecology Letters journal grew milkweed plants under four variations of CO2 concentration, including that of the modern-day atmosphere and that of scientists’ predictions for the next 150 years, and observed the monarch caterpillars feeding on the plants for changes to parasite susceptibility. The experiment found that caterpillars feeding on milkweed grown under high CO2 concentrations are significantly more vulnerable to parasitic infections and are often overwhelmed by them, dying significantly earlier than caterpillars fed low-CO2-grown milkweed.
On the other hand, changing climate conditions can have the opposite effect on milkweed- increasing instead of decreasing cartenolide concentration- which proves to be no better for the monarchs. Though monarch caterpillars have evolved a tolerance to the milkweed plant’s toxins, too much can still prove deadly for them. A study from Louisiana State University found that, as temperatures rise, certain milkweed species- especially Asclepias curassavica, or tropical milkweed- produce more cartenolides, to the point of poisoning the caterpillars feeding off of them.
What can be done to help combat this problem? One method is to examine the various species of milkweed used by the monarchs for food. A. curassavica, considered an invasive species in North America, naturally has a slightly higher cartenolide concentration than the native species, Asclepias incarnata. Monarch caterpillars feeding off of the invasive plant would therefore have a slight advantage off of those feeding off of the native plant, as they’re considered even more unpalatable to predators and have increased resistance to parasites. However, the Louisiana State University study found that the cartenolide levels in native milkweed don’t change as drastically in response to higher temperatures as the invasive milkweed, meaning that if future trends continue, native milkweed would provide the advantage instead. As A. curassavica is currently the more popular garden flower, an effort towards planting more A. incarnata instead could prove beneficial to monarch butterflies.
Additonal Reading
Article by Kayla Windelspecht
Photo Credits: 5BlueMedia … used by permission
