New study: nectar plants more important than milkweed for Monarch butterfly migration

A research paper published April 27 in the journal Oikos is causing a stir in the Monarch butterfly world.  The paper challenges the conventional wisdom that the major cause of the decline of the Monarch butterfly migration is a lack of milkweed tied to the increased adoption of herbicide tolerant crops in the butterflies’ midwestern breeding grounds.

Anurag Agrawal and his team of researchers at Cornell University tackle this widely held assumption in their recent paper, “Linking the continental migratory cycle of the monarch butterfly to understand its population decline.”  Agrawal suggests that the emphasis on milkweed may be misplaced and that solutions addressing habitat fragmentation, and most interesting and relevant to Texas, late season nectar plants should receive more attention.

“Planting milkweed is probably not a bad thing to do but it’s not going to increase their population or save them from some demise,” said Agrawal in a nine-minute video titled “Beyond milkweed:  Monarchs face habitat, nectar threats.” The video, below, was released in conjunction with the paper.

“Milkweed is probably not the limiting thing for the Monarch population,” he said.  “Perhaps planting milkweed will not improve or increase the populations as much as we thought in the past.”

Calling the decline of the Monarch butterfly migration a “very very gnarly problem,”  Agrawal gave kudos to citizen scientists for their involvement in the study and said more than once that  we must “get the science right.”

“We must identify the key weak points. It is absolutely critical.  If we don’t, we could be investing a lot of effort into conserving a portion of the migration but it may have ultimately no impact conserving the species overall,” he said.

Agrawal and his team matched Monarch butterfly counts made by citizen scientists and others to the different stages of the migration.   After doing so, they found that counts were not down until the end of the migratory cycle, when the butterflies started heading south.

“By the time they get to Mexico, their numbers are plummeting,” said Agrawal, “but at the end of the summer, when they start that migration, their numbers are not down.”

The research underscores the different needs of the species at the different stages of their life cycle.

In early spring, as Monarchs move north through the Texas funnel, they need milkweed on which to lay the first generation of eggs.  They continue to need milkweed through the summer as subsequent generations reproduce as they move north.

But in the fall when the Monarchs are heading south, they don’t need milkweed since they enter their reproductive diapause–that is, a temporary stage in which they postpone sexual activities and save their energy to migrate to Mexico.  At this point, they need water, ample nectar sources, trees or other protection at night for roosting, and connected habitats.

Agrawal’s point is well taken.  Immense focus has been devoted on increasing the Monarch butterfly host plant, milkweed.  Milkweed, anything in the Asclepias family, is necessary in the early stages of the migration when the Monarchs are reproductive.  But what about later in the game, when Monarchs are NOT reproductive?  They require nectar to fuel their flight.  Agrawal and his colleagues suggest that late season nectar plants and perhaps interconnected habitats–ie, pollinator corridors–would make their continuous route south more doable.

University of Georgia ecologist Andy Davis made a similar point last year when he spoke to the Washington Post. Davis challenged the idea of basing the Monarch butterfly population census on counting the number of hectares in Mexico occupied by the migrating creatures each winter.

“If you were charged with figuring out how many people are participating in the Boston Marathon each year, you wouldn’t count the number of people who cross the finish line,” said Davis.  “For many years we’ve been counting the finishing Monarchs in Mexico….We’ve been doing it backwards.”

What Agrawal says about nectar plants makes sense, especially for Texas.

Think about it:  as Monarchs migrate south in the fall through the Texas funnel, they are  not reproductive. Their goal during this part of the migratory cycle is to fuel up on late season nectar plants and build up their body fat so they can make it to Mexico and survive the winter.  There, in early March of the following spring, they will leave their mountain roosts to mate, lay eggs on milkweed, and start the cycle all over again.

That suggests that in the fall in Texas, there really is no need for milkweed for migratory Monarchs.  In fact, one could argue (and many scientists have) that having milkweed available late in the season will encourage Monarchs to break their diapause and reproduce, thus never arriving in Mexico.  This also suggests what Monarchs DO need in the fall: late season nectar sources.

The point is important and timely, given that millions of dollars in conservation funding and grants are being awarded as we speak. Should those grants and initiatives be focused on cultivating more milkweed or developing more sources and locations for late season nectar plants?

In much of Texas, called “the most important state” to the migration by Chip Taylor of Monarch Watch and others, the fall garden trumps the abbreviated spring garden season.  Temperatures climb into triple digits so early here that it’s challenging to grow many fruits, vegetables and flowers until the fall when things cool off.  In such a context, and given the timing of the Monarchs arrival here in the fall, late season perennial blooming plants should be encouraged.   We are well-suited to provide them.

Need ideas?  Try Goldenrod, Frostweed, Autumn sage, Late flowering boneset, Asters, Cowpen daisy in your fall garden if you want to help Monarchs.  In my downtown San Antonio butterfly garden last year, Duranta, sometimes called Brazilian skyflower, was a big draw for Monarchs and other pollinators.  It’s not native, but a well adapted flowering bush–and highly appropriate, fantastic plant.

Monarch Watch offers this super useful list of appropriate nectar plants by season. The Xerces Society also has a guide for useful pollinator friendly plants.  And the Pollinator Partnership offers plant guides for every region in the country.

Scientists will continue this debate. Chip Taylor recently took Agrawal’s paper to task in a rebuttal paper, “Conclusion of No Decline in Summer Monarch Population Not Supported” and Davis rebutted the rebuttal on his highly educational Monarch science blog.

Meanwhile, those of us in Texas should plan and plant late season bloomers.  Doing so will offer the bonus of providing fuel and sustenance not only for Monarchs but other pollinators.