Monday, September 7, 2009

Monarchs and Milkweeds

Yesterday we walked just up the road a piece to the Wiggin Farm Conservation Area. A few years ago, after the town purchased the land from a developer and averted 22 houses, the local boy scout troup helped lay out a trail and installed a bridge over a small stream.

The trail winds through a 30-acre field, which the town mows once a year in the fall. This gives the bobolinks, turkeys, deer, bees, butterflies, and other meadow creatures time to raise a new generation or two and feed before migrating or settling in for winter.

The meadow is full of flowers at this time of year. Beneath a deep blue sky, the goldenrod and milkweed form a nearly continuous canopy three feet high. Bees are busy gathering nectar. Common milkweed pods are plump with seed and ready to burst.

Milkweed plants (Asclepias spp.) are the required food of monarch caterpillars. Despite the profusion of milkweed, we have seen few monarchs this year. According to the Monarch Blog Watch monarch populations this year are lower throughout their range. The North American Monarch Conservation Plan says the population does fluctuate year to year, but many factors are causing long-term decline.

Monarchs are the only butterfly to make a round trip migration. Several generations are born here on the breeding grounds, but the final, migratory generation (the ones you see now) make a very long trip to central Mexico where they overwinter with millions of other monarchs. These same adults then start the trip north again in the spring.

As with other migrants, monarchs face many challenges on this long journey--loss of habitat all along the way, pesticides, parasites, climate change, loss of milkweed populations, genetically modified organisms. The latter include soybeans that tolerate herbicides, whereas milkweed plants do not.

Although monarch butterfly larvae require milkweed as a food source, the adults rely on a variety of plants for nectar. In the fall, monarchs need carbs to fuel their long flight to Mexico. Goldenrods and asters are good sources. This is an important reason to leave at least a portion of wildflower meadows unmowed until late fall so these beautiful orange and black butterflies can bulk up before flight.

If this is the year of the missing monarch, 2006 was a population explosion, especially in our yard. My next post will include a pictorial tale of a monarch transformation.

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