Monarch Butterflies' Declining Reign

  • Monarch butterflies make an appearance in Pennsylvania by late June. They mate, reproduce, and then a few generations later, migrate back to their wintering grounds in Mexico. Photo: Kara Holsopple

  • The success of Monarchs is tied to milkweed plants, like this Common Milkweed. Monarch butterflies only lay their eggs on milkweed leaves, and the plant's sticky sap is the Monarch caterpillars' only food source. Photo: Kara Holsopple

July 6, 2013

You don’t have to be a young, or even an older scientist, to recognize a Monarch butterfly.  Their bright color and stained glass-like pattern make them popular from Maine to California.  This much-loved species arrives in Pennsylvania, via their spring migration north, sometime between May and the end of June.  But residents may be seeing less of them in coming years.   

But Scott Creary, Phipps Conservatory and Botanical Gardens' resident entomologist, says Monarch sightings may be become scarcer in the future.

"Within the last three years we have seen a marked decline and just talking to people, other people have seen the same thing. We were first saying it was deforestation in Mexico that was causing the huge decline and as it turns out we can’t necessarily blame that on deforestation in Mexico, anymore," he says.

East coast Monarchs migrate south to fir forests in Mexico for the winter.  Illegal logging has destroyed some of their habitat there.  However, part of the reason for the decline may lie closer to home with a native plant species called milkweed.

Milkweed’s the only plant Monarchs will lay their eggs on. Gabi Hughes, an environmental educator with the Audubon Society of Western Pennsylvania, says the Monarch is choosy about which particular milkweed plant becomes the maternity ward for her eggs.

"Usually she’ll curl her abdomen under the leaf and deposit one single egg," Hughes says. 

The egg is only one millimeter high.  In just a week and a half the caterpillar will grow to the size of a human finger, with only milkweed as its food.  Hughes says they’re the true picky eaters of the insect world.  It's also a survival tactic.  Milkweed is toxic.  It makes the caterpillars and adult butterflies taste bad to birds and other predators.

Common Milkweed, Swamp Milkweed and Butterfly Plant, species of milkweed which grow in Pennsylvania, live in open, sunny meadows. These are the kinds of places that are being lost to development.  In agricultural fields, spraying keeps milkweed from thriving.  Plus an invasive aphid destroys the plants.  All of this is bad news for Monarchs and their reproduction. 

A warming climate could also be a threat.

"Maybe the milkweed would start coming up earlier and going through its lifecycle much more rapidly.  Butterflies would have to adjust to that," says Hughes.

Hughes hopes participants in the classes she teaches will participate in a Monarch monitoring program which supplies scientists with more data to find out what’s happening with the butterfly’s populations.  She says planting a butterfly weigh station of native plants is something anyone can do to help Monarchs.  A garden full of insecticide-free bee balm, coneflowers, daisies, and of course, milkweed, provides for young caterpillars and also adult butterflies.

Karen Oberhauser is director of the national Monarch Larva Monitoring Project.  She says on a larger scale, people can pay attention to legislation which affects the species.

"The Farm Bill is really important to insect conservation, including Monarchs, because so much land is controlled by agricultural policies in this country," Oberhauser says.

The bill, stalled in the House, could include conservation provisions for wildlife.  Oberhauser says it’s a critical time for Monarchs.  Last winter the area in Mexico occupied by Monarchs was only 60 percent of the area in the lowest recorded year, which was 2009.  She says that’s a significant number for not only the Monarchs’ future.  They are an indicator of habitat health all along their migratory path.

"If we are seeing good number of Monarchs, we also know that we are supporting other pollinator populations.  And those pollinator are really important to human well being," Oberhauser says.

A recent study found that wild pollinators like butterflies are more effective at pollinating certain food crops than their honeybee counterparts.