Blinded by Science blog 9: How are Monarch Butterflies faring as a species?

Today’s blog post deals with a topic near and dear to all our hearts . . . bugs.  I know it is a topic that should lend itself to calm and quality discussion.

Or not.

Fine, we’ll focus on a cute, non-threatening insect.

You are not any of those three things.

I was talking about monarch butterflies.  Those pretty critters we all learned about in school.

See . . . it is cute, non-threatening and an insect.

Monarch butterflies are one of the most recognizable insects in North America with a very distinct orange and black pattern.  The butterfly has a four stage life-cycle: egg, larva, pupa, and adult.  Monarchs are a migratory species of butterfly, traveling in the fall from southern Canada and the northern United States to Mexico, a trip that is roughly 1,200 to 2,800 miles.  But over the last 20 years, monarch populations have decreased by about 90 percent.  And this leads us to today’s question.

Steve P. asks, “I know Monarch Butterfly populations have taken a big hit in the last few years.  What is the latest update?  What are the theories/reasons behind their decline?  Is there any hope that a sustainable population can be maintained?”

The biggest reason for the decline in Monarch population is habitat destruction in both Mexico and the United States.  In Mexico, to combat deforestation, the government in 1980 designated 56,000 hectacres of land as a wildlife refuge.  This refuge has since been called the Monarch Butterfly Biosphere Reserve and is now a UNESCO World Heritage Site.  Unfortunately illegal logging is still a problem.  Additionally, the land is also the home of multiple indigenous populations (most of whom are very poor) who, to make a living, use the forests in ways that harm the butterflies’ habitat (agriculture or logging).  As such, conservationists have begun working with these groups to incentivize them to take care of the Monarchs’ habitat.

In the United States, habitat destruction was a byproduct of new agricultural practices.  With the advent of genetically modified crops, specifically those that were engineered to be resistant to glyphosphate, the concomitant increase in pesticide use killed off native milkweed (the plant Monarchs lay their eggs on and the caterpillars feed on).  Without these plants, the Monarch butterfly populations dropped.

In a twist, recent efforts to plant milkweed to save monarchs appears to have backfired*.  As the article illustrates, people attempted to help by planting milkweed in their gardens at home.  Unfortunately, they planted the wrong species.  They planted a tropical species of milkweed, which in theory should not have mattered, but it did.  This species of milkweed did not die off in the winter months in the south like the native species would.  This caused two problems.  First, because it did not die off, Monarchs had no reason to continue their migration to the south and they ended up spending the winter in areas that they did not normally stay (though I could not find any information on whether this is a problem in and of itself).  However, it does directly lead to the second problem, parasites.  Milkweed hosts a parasite (Ophryocystis elektroscirrha) that is very debilitating to Monarchs, sapping them of energy to the point that they are unlikely to be able to make the full migration, dying along the way.

Normally, migration would “weed out” the infected butterflies so they do not pass the parasite on to their progeny.  Additionally, infected milkweed would die off during the winter months so when Monarchs return, they have a chance to use uninfected milkweed.  But because the tropical milkweed species doesn’t die off in the winter, Monarchs are staying put and getting infected with the parasite which is devastating to the already low Monarch population.

So what can be done to help?  The article points out that those who planted the tropical milkweed could remove it and plant a native species instead.  Or, if the native species is difficult to obtain, at least cut back the tropical plant every couple of weeks during the winter to encourage Monarchs to continue their migration south.  Also, the U.S. Fish & Wildlife Service is currently reviewing whether to list the Monarch butterfly under the Endangered Species Act.

So Steve P., there is definitely hope that a sustainable population can be maintained, which is good news for all the Lepidopterists out there (or anyone that likes to see the Monarchs migrate).

naturepl.com / Ingo Arndt / WWF-Canon

*The actual article is here but it is behind a paywall.