A monarch-killing parasite may be spreading because of us

Red Stem Staff | 07 March, 2022

            A monarch-killing parasite may be spreading because of <i>us</i>

As one of the most charismatic and beautiful species in North America, monarch butterflies have captured the imagination of a continent. There is good reason for this! These delicate yet tough insects travel thousands of miles every single year, and everywhere they visit holds an opportunity to champion conservation. They lay their eggs only on milkweed species and need their host plant to be abundant along their whole migration route. In addition, they need to drink lots of nectar to power their long-distance flight, which means lots and lots of flowers, preferably native ones. In the southernmost point of their migration, they hibernate for several months in the mountains of Michoacan, Mexico, where they need healthy conifer forests to roost in. At every point along their migration, there have been decades of concerted international efforts to protect and nourish these beautiful insects.

So why is it that monarch numbers keep falling? While climate change, pesticide use, and habitat loss are likely all contributing factors, a new study points to our conservation efforts themselves as a potential cause for monarch decline; our well-intentioned actions may be spreading a parasite that reduces the monarchs' stamina and thus their ability to migrate long distances. Ophryocystis elektroscirrha (OE) is a protozoan parasite that infects monarch butterflies through spores shed by an infected butterfly as she is laying her eggs. Caterpillars ingest the spores, which multiply in the chrysalis, covering the wings and body of the developing butterfly. This butterfly then continues the spread of the parasite as it visits flowers and lays eggs. A sharp rise in the prevalence of this parasite coincides with the rise of the monarch conservation movement over the last twenty years, leading to tens of millions fewer monarchs with the stamina they need to reach Mexico.




So why is this happening? As of now there is no direct evidence for what exactly is causing this spike. However, there are three possible culprits that point to actionable steps we can take to limit the spread of OE.

  1. Commercial butterfly-rearing operations. Some companies rear monarchs and sell their chrysalises to nurseries and other locations. Research shows that the parasite is extremely prevalent in these populations. Do not purchase these butterflies! If you grow a flower garden, they will come to you!
  2. Home rearing of butterflies as an educational project or to protect them from predators. This step is likely to be the most relevant to Red Stem readers. It appears that letting the butterflies grow in their natural environment is the best for them. If you do rear monarchs, take steps to reduce risk of contamination, such as only rearing them one at a time and sterilizing your enclosure. Signs of infection may be visible in the pupa as dark spots or streaks a few days before emergence, and infected butterflies should not be released. 
  3. Non-native milkweeds grown as ornamentals. While tropical milkweeds are sometimes grown as annuals in Chicago, they are mainly a problem in warmer areas where they do not die back in the winter. This allows the parasite spores to persist from one year to the next and infect even more butterflies.

The upshot of this research is that the best way to support wildlife is almost always to simply create and nurture healthy, well-connected habitat and let the ecosystem members go about their business.

Further Reading:

What is OE? How to identify OE at various life stages

Monarchs have a growing parasite problem, and it's not from natural causes A deep dive into monarch-parasite dynamics and our potential involvement

Parasite dynamics in North American monarchs predicted by host density and seasonal migratory culling The research behind it