Monarch Butterflies Shrink, Get Paler After Skipping a Meal

Brilliantly colored monarch butterflies literally are what they eat—and missing even one meal can be harmful, a new study says.

New experiments show that the insects‘ vibrant colors are the result of a good diet as larvae—and that just 24 hours without food can significantly dull a butterfly’s colors, which can range from yellow to nearly red, said study leader Andrew Davis, an ecologist at the University of Georgia in Athens.

The effects aren’t just aesthetic, either. Food deprivation as larvae also leads to smaller wings, which means the butterflies take longer to make their 3,000-mile (4,800-kilometer) migration, according to laboratory experiments published April 2 in the journal PLOS ONE.

This comparison image shows the difference in size of the monarch butterfly due to lack of milkweed to eat
The top monarch had access to food. The bottom butterfly, which skipped a meal, is smaller and slightly faded. Photograph by Andrew Davis

Every autumn, millions of monarchs fly south and west from southern Canada and the United States to the forests of the Mexican states of Michoacan and Mexico, stopping at sites along the way to breed and feed—a process that spans five generations. After spending the winter in Mexico, a new generation of insects begins the long journey northward toward the U.S.-Canada border.

But this famous trek is in “grave danger,” according to a recent report showing that monarch colonies in Mexico now occupy the smallest area since records began in 1993. That’s in part due to the widespread loss of a plant called milkweed, which was once ubiquitous in North America but is declining due to agriculture and development. (Read about the discovery of the monarchs’ winter home in a 1976 issue of National Geographic magazine.)

Adult butterflies lay their eggs only on milkweed, and once the caterpillar emerges, it feeds on the milkweed plant until it becomes a butterfly. But if the larvae don’t have enough milkweed to eat, that’s when the problems begin.  (Watch video: “Monarchs and Milkweed.”)

Sylvia Fallon, a senior scientist at the National Resources Defense Council in Washington, D.C., who was not involved with the study said, “Although the researchers did the study in the lab and not in the wild, it appears possible that the loss of milkweed could affect their ability to migrate.”

On the Wing

Davis and colleagues knew that restricting food during the larval stage affected both color and size in other butterfly species.

To find out if monarchs experience similar impacts, the team split monarch larvae into three different groups in the lab. One group had no access to food for 24 hours, one lacked food access for 48 hours, and a control group had access to food. The larvae were otherwise allowed to develop into adult butterflies. (Watch a video of monarch butterflies.)

A monarch feeds from a flower in Navarre, Florida, in 2013. Photograph by Nick Tomecek/Northwest Florida Daily News/AP

The results revealed the larvae that went without food for 24 hours developed into butterflies with smaller wings than the insects in the control group. Predictably, the group deprived of food for 48 hours had even smaller wings than the 24-hour group.

Just a small decrease in wing size can add up to a major delay during a journey of thousands of miles, Davis noted. (Read more about great migrations.)

For instance, the butterflies with the largest wings tend to lead the way in the fall migration southward, and the first butterflies to arrive in Mexico have wings that are one to two percent longer than the wings of the rest of the migrators, research has shown.

Beyond the Pale

Davis and colleagues also analyzed the butterflies’ wing color by scanning their wings and analyzing color hues and saturation in Photoshop.

The monarchs that went without food for 24 hours became a paler orange than those in the other two groups, a decrease in color that the authors said was statistically significant. (See more monarch pictures.)

However, those butterflies that went hungry for two days did not lose color, which Davis and his team plan to investigate.

“Our guess is that the butterflies took the energy they normally put into growing their wings and used it to make color. We’re currently testing this idea,” Davis said.

Follow Carrie Arnold on Twitter and Google+.

Carrie is a freelance science writer living in Virginia. When she's not writing about cool critters, she's spending time outside, drinking coffee, or knitting. You can visit her website at http://www.carriearnold.com
  • Paul Cherubini

    Andy K Davis’s statement that: “Just a small decrease in wing size can add up to a major delay during a journey of thousands of miles” is speculative and unsupported by the fact that spring migrant monarchs born in northern Mexico and Texas in April and May are smaller in size than late summer fall migrants and yet they fly much faster (averaging 50 miles a day vs. 35 for fall migrants)

  • Nura haruna

    Why wings differ in color in relation to environment

  • Ichabod Cheap

    Paul Cherubini fails to recognize there are different selection pressures operating on the fall migration population. Distance being a primary one. Amongst many other considerations larger size may not be energy cost efficient for populations making shorter migratory trips. You can be assured these variables have been considered. Furthermore, the US midwest has seen marked reductions in milkweed resources.

  • Cathie

    Some of the decrease in the population of the milkweed is due to the Japanese beetle that feeds upon the flower of the milkweed.

  • LM

    I’m surprised by this study. I looked after monarchs for 3 years for different graduate studies, seeing them daily and sometimes twice a day. My job was to feed them, monitor them, and I took it upon myself to note changes or observations where applicable.
    We fed them some specialized butterfly food, then sugar water, then Gatorade. I don’t think there was much of a difference with the butterflies not enjoying any of them, and Gatorade didn’t spoil nearly as fast so it was an easy solution. I don’t presume to understand butterfly psychology, but I did know some butterflies just refused to eat, leading to, or being caused by, being a couple days before death. I didn’t notice any colour variation when this happened, though. I also do know some butterflies are simply darker or lighter to begin with – this would make sense that they had different larvae eating habits (though half of each series of butterflies were lab-grown, the other half wild).
    One other interesting observation was that butterflies – while I love them dearly – are pretty much jerks! If they shared a food dish with another butterfly (male or female), they’d often walk to the other side of the dish, step ON the other butterfly, causing their wings to touch the liquid sugar, and eventually weaken their wings so pieces would either tear off or the wings would stick together – both cases leaving the other butterfly essentially handicapped. Meanies. But I still love them 🙂

  • Patricia F. Neyman

    In a magazine like National Geographic which prides itself on being based on fact, references should be given for data referred to. Likewise, people writing in and referring to supposedly proven information should give references. Not only does that show the author knows what s/he is talking about, but it makes it possible for me to find that data myself and read it, and then to make my own decisions as to whether the data has a firm foundation, and whether the citer of that work interpreted the results correctly.

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