Monarch Butterfly

This post is about an insect that smells things with its antennae, tastes things with its feet, and has no ears, but can hear ultrasound.

Female monarch butterfly (Kenneth Dwain Harrelson) CC BY-SA 3.0

The Monarch Butterfly (Danaus plexippus) is a fascinating uber traveler who is struggling in our modern age. While our very own Santa Cruz provides an overwintering site for many western variety Monarchs, the Eastern Monarch summer home is over 3,000 miles away from its winter home in Mexico. Each Monarch travels 50 to 100 miles a day, with one overachiever recorded flying 265 miles in one day. The entire annual cycle takes four generations. Monarchs have even been bred on the International Space Station!

Often mistaken for the Viceroy Butterfly, the iconic Monarch butterfly has a 3.5-4" wingspan with orange panels and black and white markings. The Viceroy has an additional black line across the rear wings. Much like salmon, Monarch wings change shape and color at the start of migration. Like other butterflies, the Monarch pollinates your garden, flitting from flower to flower in search of nectar.

Viceroy butterfly (Benny Mazur) CC BY 2.0

Monarch populations

Traditionally, easter Monarch Butterfly populations have been measured by how many acres of land they cover when they overwinter in Mexico. In 1996-97, Monarchs were estimated to cover over 18 acres (approximately 1 billion butterflies). In 2014, that number had dropped to only slightly more than one-and-a-half acres. In simple terms, this indicates a 96% drop in population. Before we jump to any conclusions, it is important to understand that there have not been comparable drops in summer populations and scientists do not know why. 


Monarch butterfly feeding and breeding grounds​

Monarch Butterflies lay their eggs on certain types of milkweed. This is because those are the only plants the larval/caterpillar stage can eat. Popular rhetoric blames herbicides for killing these particular types of milkweed, but many regular Monarch habitats are flush with milkweed - and no Monarchs. There are mixed opinions on why this is and what it means.


As Monarch numbers initially dropped, people became worried and started planting milkweed and nectar plants to provide global corridors of food and habitat for the lovely Monarch butterfly. In less than one year, it was claimed that Monarch populations had risen from 1.6 acres to 2.8 acres as a direct result of these actions. This would be a great story, but it is not that simple. More Monarch populations are being found in previously unused areas. Some Monarchs are not migrating at all, staying where they are and feeding on popular tropical milkweeds that grow year round. Sounds great, right? Well, it is, and it isn’t.


In “normal” eastern Monarch cycles, winters are spent in a state called “reproductive diapause”, in which means they are not sexually active. This is caused by hormonal changes that are believed to be related to daylight hours. Since native milkweed plants are dormant in the winter, everything was in equilibrium. The tropical milkweed (Asclepias curassavica) does not go dormant in winter, so Monarchs are able to stay in the same place year round and forego the migration, similar to the Santa Cruz populations. The only problem is, all that continuous feeding has led to the emergence of microbial parasite, called OE for short, that has the weaken significant numbers of an already stressed species…


What is a gardener to do?
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If you would like to help the Monarch Butterfly on its road to recovery, you can plant milkweed and nectar plants in your yard or on your balcony. Cosmos, marigolds and zinnias are all very easy to grow flowers that provide nectar, but you will still need to provide the proper type of milkweed to support monarch caterpillars. When shopping, look for native milkweed plants, not the tropical variety. If you already have tropical milkweed plants, you have two choices: rip them out and replace them, or trim them back 3 or 4 times a year, to interrupt the parasite’s lifecycle.


Have you seen any Monarchs lately? Please share your photos!

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UPDATE (3/6/2016): Due to conservation efforts, the 2015-16 winter count of monarch butterflies reached 10 acres.

​UPDATE (11/20/2016): I added milkweed to my garden and now have half a dozen Monarch butterfly caterpillars munching away on the leaves and what looks like a few more eggs getting ready to hatch - and all that is on a single plant!

UPDATE (1/3/2017): Frost has killed the aboveground portion of my milkweed and there is no sign of Monarch caterpillars or chrysalis.

UPDATE (4/13/2019): The milkweed is coming in strong!

Photo of Monarch butterfly larva on milkweed by Kate Russell