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What Time is it in Nature: Monarch Butterfly

August 2, 2014

This year has been an odd year for butterflies at Prairie Ridge.  Several species that are usually very common on the grounds, such as the Pipevine Swallowtail or the Eastern Tiger Swallowtail, have been far less abundant this year.  We’ve also had population booms in some species that don’t normally reach large numbers.  One of these oddities is one of the most widely recognized and beloved North American butterfly species, the Monarch Butterfly (Danaus plexippus).

Monarch adult

Many people know what Monarchs look like, but let’s cover their appearance for those who aren’t sure.  Monarchs are relatively large butterflies with a vibrant orange coloration.  They are easily recognized by their wide black wing borders, black markings along the wing veins, and white spots along the wing margins, thorax, and upper section of the forewing.  It’s easy to tell males and females apart by sight.  Males have a dark scent gland near the center of each hind wing, giving them a sort of oval bulge along one of the black wing veins.  Females, as pictured above, do not have the scent gland.

Monarchs are milkweed specialists and feed on a variety of milkweed species, including Common Milkweed and Butterflyweed.  After a pair mates, a female will find a milkweed plant and lay her eggs singly on the underside of the leaves.  After hatching, the caterpillars will feed on the milkweed plants.  The caterpillars are distinctive:

Monarch caterpillar

They exhibit warning coloration, advertising to birds and other animals that they contain nasty chemicals that will make whatever eats them sick.  In the case of Monarchs, the chemicals they advertise are toxins that come from the milkweed they eat and then store in their bodies.  These potent toxins are carried into adulthood as well, a fact which the Monarch’s stark orange and black wing pattern boldly advertises.

Monarchs have been popular insects for many years, a sort of iconic American butterfly.  They are also one of the few insects known to migrate annually.  Each fall, the Monarchs in the eastern US and Canada begin to sense changes in the weather and start moving south, following landmarks such as the huge Missouri and Mississippi Rivers or the Atlantic coast.  They move from the area where they’ve spent the summer feeding to their overwintering sites in the mountains of Mexico, as much as 3000 miles.  Nearly the entire population of eastern North American Monarchs flies to just a few mountaintops in Mexico, and they will rest together through the winter before making the journey north once again.  Once the Monarchs reach the southern US, they will lay eggs and die and their children will complete the northward migration and lay their own eggs, spreading across the northeastern US and eastern Canada.  After 3-4 generations of Monarchs are produced in the summer, the temperatures begin to drop and the great-great-grandchildren of the migrating Monarchs from the year before will make the long trip back to Mexico in the fall.

Monarchs have received a lot of press recently because their numbers have declined sharply over the last few decades.  Last year’s overwintering population was one of the smallest on record since the Monarch overwintering grounds were first discovered in the mid-1970’s.  The Monarch overwintering grounds in Mexico are at risk as people in the area cut down the trees the Monarchs depend on.  However, Monarchs do most of their feeding and reproduction in the US and Canada, so what we do here has a massive impact on their population.   Land use changes have eliminated a lot of milkweed habitat across the eastern US, making the food plants the Monarchs depend on scarce.  Recent efforts by scientists and conservationists have stressed the importance of planting milkweeds so that we can conserve this beautiful and charismatic species well into the future.  In fact, planting a patch of milkweed in your yard is probably the best thing you can do to aid in the conservation efforts.

Monarchs are rarely observed at Prairie Ridge during the summer months, but this year they arrived late and have persisted longer into the summer than usual.  Visit the milkweed patches near the Nature Neighborhood Garden and the new Prairie Ridge entrance off of the greenway along Edwards Mill Road on your next visit and you are likely to see many Monarchs feeding, laying eggs, and reproducing.  After last year’s population decline, we are very excited to see so many Monarchs on the grounds, so we invite you to come out and see them before they begin their migration to Mexico next month!

What Time is it in Nature is a weekly feature highlighting the current plants, animals, and other wildlife at the Musuem’s public outdoor facility, Prairie Ridge Ecostation. Find out more about the natural happenings at Prairie Ridge at our What Time is it in Nature Archive!

Photos by Chris Goforth

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