|Male monarch basking in Eucalyptus. Note the androconium - the black dots on the hindwing - these are the male's pheromone glands and the key feature to distinguish the sexes.|
The monarch butterfly (Danaus plexxipus) is well-known, being the longest migrating Lepidoptera in the world. Of the population in North America, some individuals may travel over 3,000 miles to complete their life cycle. Others may simply hop across the Bay or go for a 200-mile jaunt down the road. Why the difference? It helps to know a little bit about the Monarch’s life cycle to answer that question.
Like all Lepidoptera, the monarch is dependant upon a host plant – a plant that it can’t complete its life cycle without, a plant that it has evolved (or co-evolved) with to shelter its eggs and accommodate its larvae. Those plants are the milkweeds – perennial herbaceous plants in the genus Asclepias. Formerly classified in their own family, the milkweeds are now lumped into the dogbane family (Apocynaceae) thanks to the APGIII’s complete butchering of plant taxonomy. And while there are many unusual features of the milkweeds (pollen sacs, filament covered seeds, etc.), their outstanding feature is what gives them their common name – they exude a milky fluid when damaged. This milk contains latex, alkaloids, and cardenolides – compounds that can stop the heart of vertebrates if ingested in quantity, or induce vomiting at the least. Plus they taste really terrible. A great chemical defense against herbivorous animals, you would think. But it is just this toxic soup that makes the plant so attractive to monarchs.
The adult female monarch oviposits her eggs on the underside of milkweed leaves, sheltering them from the rain and predators. After about 4 days, the tiny caterpillar emerges, devours its egg case, and begins chewing on the host plant. During the next two weeks the caterpillar will go through five instars (growth phases), molting at each one.
|Monarch egg on Asclepias fruticosa, about the size of a pinhead.|
What about all those toxins? They don’t bother the larvae one bit, but they do accumulate inside its body tissues. Thus the young monarch becomes just as toxic as the plant itself, if not more so. It takes an asinine bird only one try at eating this caterpillar, and it will never sample one again (if it survives). The bright yellow, black, and white bands serve as a reminder to all those who have tasted it before – keep away! The toxins are retained through the chrysalis and into adulthood, although the amount retained varies between individuals. In the adult butterflies, toxins are concentrated primarily in the thorax; so some unfortunate butterflies will get their heads eaten by smarter birds like the corvids.
|Larvae (caterpillar) chowing down. It didn't get the memo that it's eating a toxic plant, apparently.|
After two weeks of chomping, the caterpillar will molt into a chrysalis. This is done by creating a little silk button and hanging the cremaster (a spiny black hook) into it, suspending upside down and convulsing out of its skin. This differs from a cocoon (which moths typically make) in that a cocoon is woven silk and the chrysalis is the actual exoskeleton of the pupa. Inside the chrysalis, the caterpillar’s cells are being rearranged into the form of an adult monarch, but the insect is still alive, breathing, and metabolizing in there. As the chrysalis ‘ripens’, the walls become transparent and the adult inside becomes visible. After about two weeks, the adult will eclose from the chrysalis. It then must dry its wings and inflate them with haemolymph (aka butterfly juice). Once inflated, the wings become rigid and ready for flight. Third grade science lesson… check.
|Chrysalis. Those gold dots contain the pigment for the wing scales of the adult.|
Now then, back to that migration. The annual monarch movement across North America isn’t a complete migration, for it isn’t the same animal that is completing a round-trip journey. But like other migrations, it is based on climate and food (host plant) availability. Let’s elaborate: In the fall, a generation of monarchs eclose from their chrysalides all over North American and think “Hmm… days are short. Milkweed is dying. Flowers are dying. Better fly somewhere nice.” These butterflies are in reproductive diapause, which means that hanky panky isn’t on their mind – survival is. Monarchs, unlike other butterflies that hibernate, have no adaptive strategy to survive freezing temperatures. In fact, they need temperatures above 50°F to fly. Not to mention their host plant, a perennial, is dying back, and their food source, nectar from flowers, is disappearing. Time to move!
|Two-way migration map, from MonarchWatch.org|
So they literally up and fly away, after first eating as much nectar as they can and getting a little butterfly belly to last them through the flight and perhaps the winter. South they go, riding thermals and soaring up to 4,000 feet, maybe higher. At night they roost, clustering together in thick trees with other monarchs who are making the trip. In the rain they also roost, clinging to branches with their wings folded so as not to get too wet. And they occasionally feed, stopping for nectar when they can find it.
|Compound eye with ommatidia (eyelets) enlarged, from MonarchWatch.org. I don't have the equipment to take scanning electron microscope photos, sadly.|
How do they know where they’re going? Those compound eyes, which give them an expert view of flowers, can’t see the landscape at 4,000 feet. Experiments have shown that monarchs possess a time-compensated solar compass – meaning they follow the sun with respect to the time of day. They also have tiny amounts of cryptochromes in their antennae, which may allow them to cue in to the Earth’s electromagnetic field for navigation. These insects, small and ubiquitous, are better navigators than the world’s most famous human explorers.
|Adults clustering on Eucalyptus globulus at Ardenwood Historic Farm.|
Upon arrival at their southern destinations, they find their favorite trees and hang out with all their friends. For the western population of monarchs, that’s right here in coastal California at such sites as Pismo Beach, Pacific Grove in Monterey, Natural Bridges State Beach in Santa Cruz, Ardenwood Historic Farm and Point Pinole in the East Bay. Their trees of choice include native Monterey pine, Monterey cypress, and eucalyptus. Eucalyptus? Isn’t that a non-native tree, LCN? Yes, it is. And it is an upgrade to hotel monarch because it is a flowering tree (an angiosperm), and just so happens it flowers in the winter here in good old California. So eucalyptus provides not only shelter but food for these hungry travelers. And there they hang, enduring winter storms, occasional frosts, and countless multitudes of gawking, photographing humans (myself included) with only each other and the trees for protection.
|Large cluster of monarchs at Natural Bridges State Beach.|
Those that survive to the spring will finally, after 6 months or so, leave their reproductive diapause and begin mating. Mating is a tumultuous affair for monarchs that begins with a courtship dance and ends with the male dragging the female behind him in flight while he deposits his sperm. This may sound like a raw deal, but the female makes out on the good end - she receives not only sperm but a nutrient packet that will provide a boost of nourishment for her during oviposition. After several mating sessions, the males, ragged and ancient, expire. Their life cycle is complete. The females still have work to do, though. Inside their abdomens are 100-500 fertilized eggs that must be placed on milkweed to ensure genetic continuity. So off they fly on tattered wings toward the nearest patch of milkweed. That patch may be a neighborhood butterfly garden of tropical milkweed, in full leaf in the late winter. Or it may be one of California’s 25 native species of milkweed, just beginning to grow after the winter rains.
|Female ovipositing on Asclepias fruticosa (action shot!)|
After oviposition of those eggs the female also expires. She may have flown another hundred miles north from her overwintering site, or she may have laid her eggs and died at the base of the tree she roosted in. But she and her mate’s genes continue on, and in one month a young, fresh monarch will eclose from its chrysalis with two things on its mind: following milkweed north and mating. This is the spring 'migration,' which is really more of a dispersal - successive generations of monarchs fly north, following the growth of milkweed. It may take four more generations for this lineage to make it back to where its ancestors began last fall, if it ever does. But come next fall, they will all be heading back to the temperate climate of sunny California. Who can blame ‘em?