My 6th grade teacher really liked monarch butterflies. Like, a lot. We grew monarch butterflies in our classroom: there was a big butterfly net where we watched caterpillars grow, make a chrysalis, and hatch into butterflies over the span of just a month. At one point I think we let the monarchs fly around in free in the classroom (I remember a friend running one over with her chair and getting in trouble). But eventually, we released the monarchs outside and let them fend for themselves.
Looking back on it, it’s not too surprising that my teacher loved monarch butterflies. Between their large, striped caterpillar bodies and slowly flapping orange wings as adults, monarch butterflies are easily recognized across America. And because they grow so quickly and obviously, they’re great for teaching students about metamorphosis; many schools and science museums grow monarchs as a science lesson. Given how many of us have probably seen a monarch butterfly at some point, let’s take a closer look at these insects.
There are a two main populations of monarchs in North America that are separated by the Rocky Mountains: the eastern population and the western population. Both populations are migratory (more on that later); the only difference is where they overwinter. The eastern population overwinters in Mexico, and the western population overwinters in California. There are also populations of monarch butterflies that don’t migrate at all; these populations live in year-round warm areas like Florida. But whatever the population, monarch butterflies are pretty amazing.
- Why the name monarch?
- Legend says that early European settlers in North America named the butterfly after Prince William (later King William III). Why? Well, Prince William was the Prince of Orange, a location in southern France. So settlers saw the orange color of the monarch butterfly and named it after their prince. Their scientific name, Danaus plexippus, means “sleepy transformation;” this could be a reference to their winter hibernation and the fact that they undergo metamorphosis.
- Monarch butterflies lay their eggs exclusively on milkweed, a plant that tries to avoid predation by producing bad tasting glycoside toxins. Monarch caterpillars don’t care about this toxin, though. As soon as they hatch, the caterpillars will begin eating the milkweed that their egg was on. And they eat A LOT of milkweed – one estimate is that monarch caterpillars eat between 175 and 200 milkweed leaves before becoming chrysalises. As they eat, the caterpillars store the milkweed’s glycoside toxins in their body, making the caterpillars taste bad to predators. These toxins remain in their bodies even after metamorphosis and continue to protect the monarchs. Unlike the caterpillars, the adult monarch butterflies feed on nectar from a variety of flowers (but still including milkweed!).
- Müllerian mimicry
- One species of butterfly, the viceroy, looks very similar to monarchs. Researchers for a long time thought that this was an example of Batesian mimicry, a type of mimicry where a palatable or non-toxic species resembles an unpalatable or toxic species. This protects the non-toxic species, as predators will confuse it with the toxic species and thus avoid eating it. More recent research, however, suggests that monarchs and viceroys are instead an example of Müllerian mimicry. Unlike Batesian mimicry, where one species is harmless, in Müllerian mimicry, both species are toxic or unpalatable. The idea here is that both species benefit from looking like another unpalatable species – there are more chances for predators to learn that they are toxic! And in fact, both monarchs and viceroys are toxic. As mentioned before, monarch caterpillars feed on toxic milkweed; in a similar vein, viceroy caterpillars eat willow and poplar leaves. This diet produces bitter-tasting compounds, so predators avoid eating them.
- Monarch butterflies are known for their impressive, multi-generational migration. They are, in fact, one of the few migratory insects! I’m going to explain the migration pattern of the eastern population here, but it’s basically the same for the western population (they just start in California).
- Let’s start the migration in Mexico, where the eastern population of monarch butterflies spends the winter. In these overwintering sites, monarchs flock together in huge numbers at the oyamel fir forest: tens of thousands of monarchs can sometimes be found on a single tree! Once it begins to warm up in March, these butterflies start flying north. March and the start of northward migration is also when this generation of butterflies sexually matures and mates. They fly a little bit north, mate, lay eggs, and then die. The northward migration is then continued by their offspring, and the cycle of “fly a little north, mate, and die” continues. It takes from three to five generations of monarch butterflies to finally reach southern Canada. It’s the final generation that makes the journey back south for the winter. This generation hatches in late summer, and, instead of sexually maturing, they fly all the way back to Mexico where they will stay for the winter.
- How does the late summer generation know where to fly to in Mexico? After all, it’s been generations since they’ve been there. Scientists aren’t totally sure yet, but monarchs seem to be using a combination of directional aids like the Earth’s magnetic pull, the position of the sun, and ultraviolet light. As they migrate, they will stop at roost sites like pine and fir trees to rest for the night. The thick canopies of these trees help keep the temperature of the roost site warm overnight; the monarchs sun themselves in the morning to warm up before continuing their flight.
The migration of the monarch butterfly is amazing, but also threatened. Remember, monarch caterpillars depend on milkweed; they won’t eat anything else. As land development occurs and the landscape changes, the milkweed population is becoming fragmented. This ultimately impacts the ability of monarch butterflies to successfully lay their eggs and puts stress on the species as a whole.
But you can help! An easy way to aid monarch butterflies is to simply plant milkweed. And as long as you are planting milkweed, why don’t you plant some other native plants as well? Native flowers give insects and other pollinators a natural source of food. You don’t need to plant much; according to the World Wildlife Fund, a single square foot of native wildflowers can help rebuild and connect habitats for pollinators like monarchs. You can check out which wildflowers are native to your region here, and even get a packet of free native seeds to plant here. It’s a win-win: grow some pretty flowers and help animals in the process.