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A north-bound monarch, shot here in a field in Oklahoma. ((Billy Hefton/Associated Press))

Oh dear, oh dear, oh dear, oh dear. Where are the monarchs? Butterfly counters across the north-central U.S. and southern Canada are reporting a serious decline in the number of monarchs winging their way north this year to their summer fattening-up grounds.

Has the insect world's most intrepid traveller just been laid low for a while by the cold winds and weeks of rain that dogged the central part of the continent earlier this summer? Or is there a more dire reason, such as a rise in predators — Texas fire ants, larvae-eating insects, mosquito-spraying humans — that may have wiped out much of an entire breeding generation.

"At this point, we just don't really know what's going on," says Don Davis, the monarch guy with the Toronto Entomologists' Association. "They may be just really late this year. There is still hope.

"All I can say is that the numbers are very different than last year. They are way down."

Indeed, a month ago Toronto lepidopterists were celebrating the arrival of a rare, possibly wind-blown fellow traveller: The elegant Marine Blue butterfly, a California native, paid a surprise visit, its first Ontario sighting in at least 15 years.

But now the celebrations have turned mournful. "'Where are the monarchs?' everyone is saying," says Davis. "It's everywhere — Nova Scotia, New Brunswick, Ontario, Quebec."

Last summer, these mighty migrants seemed to be extending their summer feeding grounds, seeking out their favourite diet of milkweed. There were sightings near Jasper, Alta., in northern Saskatchewan and Manitoba and as far north as Moosonee, near James Bay.

This year, there are very few reports from the West, observes Davis. And it may not be just the monarchs that have been knocked off course.

Flight path

In the eastern half of North America, monarchs winter in Mexico, rousing themselves in late February to make their way north to Texas, Louisiana and Florida, where they lay eggs and die. (A female monarch can lay up to 100 eggs and then she dies.)

That next generation then makes its way north to the Great Lakes region and beyond, usually arriving around late May or early June, in search of milkweed, its only food.

The generation born in Canada in late July and August is the one that will eventually head south, usually around the end of August.

Monarchs head to the East Coast first, often in great number, and then south, occasionally getting blown off course all the way to England.

Scientists discovered a few years ago that monarchs navigate by an internal compass tuned to magnetic north and with special photoreceptors in their eyes that measure ultraviolet light — and therefore the angle of the sun — in the sky.

How the generation born in Canada finds its way back to that one special grove in Mexico — somewhere it's never been — is one of nature's mysteries.

Nature watchers in central Canada and the eastern U.S. are reporting fewer butterflies in general this year, as well as fewer ruby throated hummingbirds in backyard feeders and other way stations, a phenomenon that probably also has much to do with the monsoon-like period in central North America that was May and June.

Continent wide

The Monarch, of course, is one of Mother Nature's marvels, an ultralight flying machine that wings its way from its wintering grounds in forests near Mexico City to, roughly, Quebec City and back over the course of four or five generations. That's a round trip of approximately 4,000 kilometres.

Each winter, tens of millions of monarchs rest up in a near-hibernation phase in Mexican forests then cross the Gulf of Mexico to Texas, Florida and Louisiana for a mating frenzy and then the trip north.

Each brightly coloured monarch — its distinctive orange and black markings a warning to predators that it is full of poisonous milkweed — only lives for five or six weeks, its successive offspring continuing on the northern journey until something, probably the height of the sun on the horizon, says it is time to return to Mexico.

It is this homeward-bound generation, the one that is born at the end of the summer in Canada and the northern reaches of the U.S. around the Great Lakes, that is the most unusual: it alone lives for seven or even eight months and can instinctively find its way back to the same Mexican grove its grandparents came from.

It is also the generation that is not showing up here this year in anything like the numbers in the past.

Should we worry?

Some butterfly experts, like Chip Taylor, a University of Kansas biologist who founded the group Monarch Watch, says monarchs are a great breed for bouncing back from adversity. They have done this before when hurricanes or forest fires have wiped them out in great number.

What's more, the relative absence of butterfly predators this year, because of the cold rainy spring, might give late-blooming monarchs a second wind.

At the same time, Taylor writes on his blog, the general absence of butterfly larvae this summer in important breeding grounds is of much greater concern. It means that the returning generation, the Canadian one if you will, will be much smaller than normal and put the entire species behind the eight ball for the cycle that follows.

By several accounts, this year's crop of monarchs enjoyed a good winter stay in Mexico and flew out of there in a great wave and well-fattened. They also had a fine spring in Texas, according to enthusiasts' reports. But then something happened — probably cold, rainy weather that washed away larvae and drowned adults — on the way to Kansas.

That is Taylor's assessment. And the dropoff can be clearly documented on the blogs by monarch devotees on such sites as Monarch Watch and Journey North.

Where are the monarchs, the cry went out from North Dakota, central Michigan, New Jersey, Massachusetts, east central Ohio, upstate New York, the Hudson River valley and, in the past couple of weeks, most of Canada.