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Where are all the Monarchs?

September 30, 2009

That question has kept my phone ringing, my email flashing and my social networks abuzz. No one in the Northeast is seeing many Monarchs migrating this year.

Monarchs undergo  a truly spectacular migration beginning in mid-August, peaking in mid-September and finishing by mid-October in the Northeast – a 2,500 mile journey from northern breeding grounds to remote overwintering sites in the mountains of Mexico. Enroute to Mexico they roost at night, sometimes in the thousands. In Mexico they form huge aggregations in the winter numbering in the millions clinging to the trees and each other. In March and April they leave their winter roosts and begin the spring flight north to begin the next generations.  The returning individuals are few; perhaps less than one percent survive to breed.


But this year, friends to the west of the Green Mountains have seen dismal numbers. Another in Pennsylvania last year recorded over 20 a day in September and has only seen a half dozen total this year. Most folks in Maine report extremely low numbers. I have seen a total of 6 Monarchs migrating through the upper Connecticut River valley so far and for the first time in years, I have failed to put a single Monarch Watch tag on a butterfly. Despite searching all of my favorite fields for caterpillars in August and early September, I have not found a single one. The only place I have found any Monarch larvae has been on Nantucket Island off the coast of Cape Cod in a small patch of milkweed.

Chip Taylor, director of Monarch Watch (, predicted in early September that this would be the smallest migration since 2004, which resulted in only five and a half acres of butterflies wintering in Mexico.  The best year was in 1996 when there were over 50 acres of overwintering Monarchs. This year he predicts less than 10 acres of overwintering butterflies.

Why are there such large population swings? We really don’t know yet, but long term monitoring from citizen science projects like the Monarch Larva Monitoring Project ( are starting to shed some light on this. Karen Oberhauser, director of MLMP, reported on their blog that, “unusual weather has caused some unusual patterns. In the Upper Midwest, we had extremely cool weather in June just after the monarchs returned, causing a big drop in numbers just when we normally see monarch numbers increasing toward their first peak. Since then, numbers have bounced back somewhat, but in Minnesota we’re seeing our lowest numbers since 1998 (when we had another bad drought) and 2002 (after the big die-off in Mexico).”

These same cool, wet weather patterns plagued the Northeast in June and early July. Earwigs, ants, tachinid flies and other predators and parasites that attack Monarchs seem to be favored with above normal rainfall. They can be a major influence on butterfly populations. Not to mention that butterflies need sun with warm temperatures to fly, mate and lay eggs.

It looks like this year my Monarch Watch tags are going to just sit on my desk. I hope there are no catastrophic weather events on the Mexican wintering grounds and the weather next year is just like 1996!

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