Monarch Information

These butterflies start out as tiny eggs—very tiny, around one millimeter long, or about the size of a pencil tip. Female monarchs only lay eggs on milkweed leaves, and they attach the eggs with a glue-like substance. After a few days to a week, a larva, or caterpillar, emerges from the egg. Monarch caterpillars sport white, black, and bright yellow stripes. As it grows, the monarch larva sheds its skin. After around 12 to 14 days the caterpillar hangs upside down and sheds its skin to reveal a harder casing—the chrysalis—underneath. It’s in this pupal stage that metamorphosis occurs, when the ensconced caterpillar finishes its transformation into an adult. A butterfly emerges after about a week or two, with its recognizable orange and black markings. 

Watch a Monarch Butterfly Egg Hatching (Video)

Monarch Caterpillar Changing to Chrysalis (Video)

Monarch Butterfly Emerging Timelapse (Video)

The Monarch butterflies are the only butterfly to undergo a migration of this kind. There are 4-5 generations of monarchs born each year, and every generation furthers the migration. 

The monarchs that overwinter here at Natural Bridges will fly to the bay area where they will breed, and then die. Monarchs born in the bay area will migrate to the central valley where they will breed and then die. Monarchs from the central valley will migrate to the Sierra Nevada Mountains where they will breed and then die. Monarchs born in the Sierra Nevada Mountains will migrate to the Rocky Mountains where they will breed and die. That last generation born in the Rocky Mountains is what is sometimes referred to as the "super generation", though biologically, they are no different from the other generations. 

The last generation, in our case, the ones born in the Rocky Mountains, will undergo the longest migration of all the generations. As the temperatures in the mountains cool, the monarch's undergo a hormone change, called reproductive diopause. This reproductive diopause redirects all energy away from reproducing, and toward baseline survival. It also triggers the monarchs to migrate from the Rocky Mountains all the way back to California to overwinter. While overwintering, the monarchs will not breed at all. Once the weather warms up, the monarchs break from their hibernation and migrate away from us to breed elsewhere. And thus, the cycle starts anew. 

Upon first arrival, the monarchs drink as much nectar as they can to build up fat reserves to survive the winter. Once the weather cools, they will remain in the trees, only leaving to feed when absolutely necessary. 

Monarchs are picky about the groves they will overwinter in. First of all, they are gregarious hibernators, meaning they will not stay somewhere that has too few butterflies. The condition of the grove is also extremely important. The butterflies cannot survive freezing, so monarchs in California will almost exclusively overwinter near the coast. They also need sturdy protection from the elements and predators. The Natural Bridges Monarch Preserve is an ideal overwintering site. It is ring, or doughnut shaped; this means it had wind protection from all sides, but the hole in the center ensures warm sunlight can filter through during the height of the day. 

This western monarch migration is different from the eastern, where the monarchs overwinter in Mexico. For the western monarch population, California is their Mexico. 

Monarch Migration Article by Monarch Joint Venture

In recent years, new research has emerged regarding the Monarch parasite Ophryocystis elektroscirrha, or O.e. for short. This is a parasite that has always existed in monarch butterfly populations in low levels. It is only found on the Monarch butterfly, and the other two butterflies of the same genus: the Soldier and the Queen butterflies.

This parasite is a protozoan, meaning is has a single celled phase, and a spore phase. When a caterpillar east an O.e. spore, it becomes infected. As the caterpillar grows and eventually becomes a chrysalis, the parasite multiplies. When in the chrysalis phase, the parasite can hinder the process of cell rearrangement, and can lead to deformed wings or even death before emerging from the chrysalis. When the adult butterfly emerges, the O.e. spores are coating the outside of the monarch's body. As this butterfly flies and interacts with other monarchs, spores fall off of its body. Adult monarchs with this parasite are weakened, even if some may not be deformed and may be able to survive to reproduce. If an infected female monarch lays eggs, she will shower spores over the entire milkweed plant, and on her egg. When the caterpillar emerges and begins to eat the plant (and the spores along with it) the cycle begins again. 

In a normal population, these parasites are kept in very low levels via natural selection. The monarch migration keeps their populations healthy because those individuals that are sick, weak or have bad genes will not survive to reproduce at the next stop of the migration. This includes those infected with the O.e. parasite. Monarchs with this parasite will often be too weak to fly to the next stop and will not reproduce, therefore not passing the parasite onto the next generation. A very small number may survive to reproduce, and will pass the parasite on, but these individuals will have very low levels of infection. In a wild population, less than 10% will have O.e. infections.

Humans have inadvertently changed monarch butterfly population dynamics, raising O.e. levels. Many people have begun to plant milkweed to aid the monarchs. This is a wonderful idea, almost everywhere. However, when we plant milkweed near overwintering sites, we encourage monarchs to not migrate. This is not only unnatural, but actively harmful. The migration keeps the monarch population healthy by removing sick or weak individuals from the gene pool. When we remove the migration, those sick and weak butterflies are able to reproduce and pass on their sickness or weakness. This is especially important with the O.e. parasite. The weak butterflies will not migrate away because they have local milkweed they can survive on. They will reproduce, creating more infected butterflies who do not migrate. This population of non-migratory butterflies is called a residential population. These residential populations tend to have extremely high levels of O.e. (about 90% of butterflies we tested during the summer of 2022 had infections). 

What is OE? Article by Project Monarch Watch

Keep Monarchs Wild: Why Captive Rearing Isn’t The Way To Help Monarchs Article by The Xerces Society

Milkweed FAQs Resources by The Xerces Society

Tropical Milkweed—A No-Grow Article by The Xerces Society

Calscape - Website from the Native Plant society, good for where to grow what milkweeds, nectar plants, and host plants for other butterflies