Threats to Monarchs from Parasites
Parasites are small organisms that live inside their hosts, taking nutrients and other resources from them. Perhaps the most harmful parasite for monarchs is OE (Osphryocystis elektroschirra). When an OE-infected monarch lays eggs, she scatters dormant parasite spores on the eggs and on the surrounding milkweed. When the caterpillars hatch, they eat their egg shell and the nearby milkweed — along with the OE spores. The spores infest and multiply in the caterpillar’s gut and in the pupa, where they continue to increase in number.
Caterpillars with a high dose of OE parasites are likely to die before reaching the pupa state. Surviving infected adults often have difficulty emerging from their pupal cases and they are generally smaller and shorter-lived than uninfected adults.
OE was first observed infecting monarch butterflies in the 1960s, and it has since been observed in all monarch populations worldwide. Around 30% of western monarchs are heavily infected with OE.
Native milkweed dies in the fall, taking the OE on its leaves down with it. However, since tropical milkweed does not die back, the OE remains and multiplies, infecting multiple generations of monarchs. This is one reason why it tropical milkweed (Asclepias currasavica) is not considered a good option for monarch habitat. To learn more, read this article from the Xerces Society for Invertebrate Conservation as well as this one from Monarch Joint Venture.