I’ve recently planted milkweed in my yard in hopes of attracting one of the largest and most beautiful butterflies in North America to feed and lay eggs in my garden – the monarch butterfly. Chances are you’ve seen this easily recognizable orange and black beauty floating through your yard or neighborhood as well, as monarchs are found across North America wherever suitable feeding, breeding, and overwintering habitat exists. But what you may not know is that North American monarchs are one of the few migratory insects in the world, with millions traveling up to 3,000 miles each year. This is their incredible story…
First, it’s important to make the distinction that there are two populations of monarchs in North America, one that occurs east of the Rocky Mountains (the majority of the North American monarch population), and one that occurs to the west. The monarchs’ northern migration begins around March in their overwintering grounds in Central Mexico fir forests (for the eastern population) and the central to southern California coastal region (for the western population), with their summer grounds reaching as far north as southern Canada. Monarchs become sexually mature and mate during migration, with males attracting females by releasing chemicals from scent glands on the hind wings and females searching for their host plant, milkweed, to lay eggs. After three to five days, the eggs hatch and the caterpillars begin feeding exclusively on milkweed (after devouring their empty egg case!). For about another two weeks, the caterpillars eat and grow and molt several times before forming a chrysalis and undergoing metamorphosis, after which they will emerge as an adult butterfly.
Now, more about milkweed! Milkweed is quite the special plant as it produces something called glycoside toxins, which serves to deter animals from eating the milkweed. However, monarchs have evolved immunity to the toxins and in fact store them in their bodies as caterpillars. This makes the caterpillars unpalatable, keeping them safe from predators in their vulnerable state. Adults benefit from the toxins as well as they are able to maintain them in their system through metamorphosis. The bright orange coloration of the monarch is a warning to predators that they are poisonous. Adult monarchs feed on nectar from a wide range of blooming native plants, including milkweed.
Monarchs breed throughout the spring and summer, and during this time adult butterflies mate and die (typical lifespan of reproducing adults is two to three weeks) as northward migration continues, but the monarchs born in late summer are the only ones to make a ‘round trip’ back to their wintering grounds. This is where the truly fascinating aspect of monarch migration is highlighted. By the time next year’s winter migration begins, several summer generations will have lived (and died) to repopulate the United States and southern Canada, but the final generation delays sexual maturity and makes an amazing return journey to the overwintering grounds. These great grandchildren of last year’s migrators somehow know to follow the same routes their ancestors took—taking directional cues from the sun and following an internal compass passed down to them from previous generations. This migratory generation can live up to eight months!
Monarchs also occur outside of North America in Hawaii, Puerto Rico, and some islands of the Caribbean, as well as in New Zealand, and were probably blown to these locations in storms, dispersed naturally, or were possibly introduced by humans. These populations, however, are not part of the annual migration phenomena on the North American mainland.
Unfortunately, the monarch population has declined by approximately 90 percent since the 1990s, due to habitat loss and fragmentation in the United States and Mexico. Insecticides and herbicides also threaten monarchs, by either killing the monarchs themselves or their nectar and hostplants. Climate change may also threaten monarch populations by altering weather patterns during migration and in the overwintering grounds.
My attempts to attract monarchs to my milkweed garden worked, and within days I noticed caterpillars happily chomping away on my plants! At one point I had over 10 caterpillars ranging from tiny to fat, and even watched a full-size one crawl away to explore other areas of my garden. Initially I was concerned but learned that the caterpillars will often leave their milkweed plant to form a chrysalis elsewhere, perhaps so as not to get in the way of other hungry caterpillars (this is just my theory). I saw many monarchs visit my garden and even witnessed them depositing eggs on the milkweed. Maybe these were previous generations from my garden, or maybe they had just arrived, but I was thrilled that I could provide some habitat for them to continue their incredible journey.
Brynne Mulrooney, Senior Biologist