Monarch butterflies are prone to a parasite that renders the pupa unable to emerge and reduces the adults ability to fly. To overcome this attack, female monarchs choose where to lay their eggs specifically to prevent the parasite from doing damage.
The protozoan ophryocystis elektroscirrha parasite resides in the abdomen of the butterfly. Mothers pass it on to their offspring. However, to ward off the tummy bugs, mama butterflies look for a specific milkweed plant upon which their larvae, the caterpillar, can feed. The tropical, or curassavica, species of milkweed, out of several hundred species that butterflies feed upon is the medicine of choice for an infected mother.
The caterpillar stage of the monarch is colored yellow, black, and white, which in the animal kingdom spells poisonous-to-eat. As an adult, the lovely orange, black, and white we so admire, is also a warning that keeps predators at bay. The larvae ingests cardenolides from its herbivore – discouraging food source. This dinner makes the larvae itself toxic, no longer a tasty morsel for ants, nor as an adult as a snack for birds or mice. Blue jays quickly barf up a milkweed-fed monarch.
However, black-beaked orioles and black headed grosbeaks are common predators that can tolerate cardenolides. Orioles winter in Mexico along with the butterflies and find that the monarch-infested trees present an opportunity for feasting. The monarchs overwinter in dense clusters on the boughs and trunks of oyamel firs in central Mexico. These cloudbelts protect the butterflies from freezing on cold nights. However, logging of these monarch motels is an additional threat to the butterflies who go south for the winter.
Jaap de Roode, an evolutionary biologist at Emory University works in the lab bearing his name, one of a few in the world studying monarchs. He focuses on why parasites harm their hosts. His team determined the monarch’s preference for plants with highly concentrated cardenolides.