Are Monarch Butterfly Populations Declining Because of Deaths During the Fall Migration?

By Sahotra Sarkar

On the fourteenth of October last year tens of thousands of monarch butterflies flew over Austin in a steady stream on their way to Mexico. Most were high, probably over a hundred meters, forming a cloud of dots against the sky. It was at least a decade since so many monarchs had passed over Austin. Though we knew that the huge number of monarchs over Austin was due to a cold front that had forced them into a narrow band, there was some reason to be optimistic that the population of monarchs had begun to recover from the lows of the recent past.

That hope was soon dashed. Winter data from Mexico showed that the number of monarchs had declined to about 150 million, less than half the size seen the previous winter (2018 -19) confirming the ongoing trend of long-term  decline. (Overwintering monarchs in Mexico numberd 400 million in the early 1990s but only 100 million sinc 2010 with a historic low of about 35 million in 2013 -14.)

What is responsible for this decline, and what can we do about it? Until recently most ecologists believed two factors to be responsible: disappearance of wintering habitat in Mexico and and breeding habitat in the north. For wintering habitat, Mexican authorities began systematic conservation efforts in 2000 and these now appear largely to have succeeded. (Victor Sànchez-Cordero from the National Autonomous University of Mexico (UNAM) and I, along with colleagues, have a piece in press in BioScience pointing out that blaming Mexico for monarch declines is not appropriate.)

For breeding habitat the perceived problem is the disappearance of milkweeds, the only food for caterpillars. It is worst in the Midwest which provides the largest fraction of monarchs later migrating to Mexico. Responsibility for milkweed decline is believed to lie in the proliferation of genetically modified (GM) glyphosate-resistant crops and use of that pesticide (which is also believed by some to be carcinogenic).

The belief that milkweed disappearance is responsible for monarch declines has spurred many conservation efforts including the creation of monarch waystations with plentiful milkweed along the migratory route. However, a slew of recent results question whether milkweed declines are drastic enough to cause the declines. Rather, many researchers have shifted their attention to an entirely different possibility: mortality of monarchs in Texas and northern Mexico during the southbound migration.

This “migration mortality hypothesis” assumes that environental factors in these regions have created the monarch declines. In Texas, the most likely culprit is drought conditions since the mid 2000s. Southbound monarchs do not need milkweed because they don’t reproduce and have caterpillars. Rather, they need nectar from flowers for fuel (and limited to milkweed). Mortality increases if flowers are insufficient. It also increases if drought limits roosting sites.

The migration mortality hypothesis is unproven. To prove it will require a lot of citizen science this upcoming Fall. In a later post I will outline what my Mexican collaborators and my group in Austin hope to achieve. If birders occasionally shift their attention downwards, every time they see a monarch at a plant, they will be able to use a new app to photograph both insect and plant and the information will automatically be transmitted to researchers. We have a lot of exciting possibilities for monarch research.

Pictured above: Monarchs in the biosphere reserve in Mexico, winter 2019. Image copyright Francisco Botello Lopez.