The latest survey of monarch butterflies in Mexico shows numbers are way down from last year, and the lowest since measurements began 20 years ago.

Every year, the butterflies leave eastern Canada and the U.S. in late summer for a spectacular, 4,000-kilometre migration to overwinter in a small area northwest of Mexico City. The return trip, now underway, will take four generations for the monarchs to reach Canada.

In 1996-97, the monarch survey - which is now a joint project of Mexico's National Commission of Protected Areas, the World Wildlife Fund (WWF) and Telcel, a Mexican company - reported 18.19 hectares occupied by monarch butterfly colonies, the largest ever recorded. The current survey in the Oyamel forest found the area was now only 1.19 hectares.

For Lincoln Brower, who teaches biology at Sweet Briar College in Virginia and is arguably the leading expert on the monarch butterfly, the insect's plight "is symbolic of what's going on in terms of environmental degradation everywhere."

Brower has been making research trips to Mexico since shortly after a Toronto scientist named Fred Urquhart discovered and documented the monarchs' magnificent migration in 1976.

The area in Mexico with colonies fluctuates over the years, but it certainly appears to be on a downward trajectory. Before the record low this winter, the median area for the years 1993-94 and 2003-2004 was 7.81 hectares, then down to 4.32 between 2004-5 and 2011-12.

Brower says they may be facing a perfect storm of negative effects.

King of the herbicides

In Canada and the U.S., the monarchs face a huge challenge finding milkweed, which is the plant on which they lay their eggs and then the caterpillars feed. Brower says the widespread use of the herbicide glyphosate is now the critical threat to the monarchs' food supply.

Glyphosate was developed by the U.S. agricultural biotech company Monsanto, which they sold as Roundup, and its use took off once Monsanto's seeds, genetically modified to be resistant to glyphosate, hit the market. Other herbicide manufacturers have helped make glyphosate the top-selling herbicide in the U.S.

It brings to mind the lines of a Robert Frost poem from more than five decades ago:

"Where have those flowers and butterflies all gone / That science may have staked the future on?"

Butterfly enthusiast Craig Wilson of Texas A&M University has been tagging monarchs on their way to Mexico for the last five years, and his result last year portended the survey results in Mexico. Wilson's count in 2012 was down 60 per cent from 2011.

Wilson says the severe drought that hit Texas and other parts of the U.S. last year had a negative impact on milkweed and other plants that provide food for monarchs. As well, the monarch eggs on the milkweed often dried up because of the heat and drought.

And it's not just the milkweed's decline that directly impacts monarchs. At their adult stage, the insects need nectar that can come from all kinds of plants, especially wildflowers, which are also being decimated by glyphosate.

An umbrella and a blanket

The World Wildlife Fund's Mexico director, Omar Vidal, says in a news release, "The conservation of monarch butterflies is a responsibility shared by Mexico, the US and Canada. By protecting its sanctuaries and practically eliminating large-scale deforestation, Mexico is doing its part. It is necessary that the US and Canada also do their part and protect the habitat of the monarch in their countries."

While the large-scale deforestation of the past has been virtually stopped, says Brower, logging that is fatal for the butterflies still takes place at higher elevations in the Oyamel forest, where the monarchs spend winters in Mexico. They favor the coniferous sacred fir tree for their colonies, which are largely inactive.

Brower says he has witnessed what he calls "predatory logging" in almost all of the last 35 years while researching the monarchs. That's when loggers fell one or two trees on one trip that, in total, could amount to about a hundred trees over the summer.

The butterflies form their colonies at high elevations, and "those trees provide the micro-climate for monarchs to survive," Brower explains. The trees provide shelter so the butterflies don't get wet as well as a blanket effect that moderates the temperatures inside the colonies.

Because of the high elevation, temperatures drop below freezing almost every night while the butterflies are there. If the butterflies get wet, their resistance to freezing drops way down.

"If you cut holes in the forest," Brower explains, the butterflies are both more likely to get wet and more likely to freeze. In addition, removing one tree also may expose the monarchs to too much sunlight, which can seriously shorten their lifespan.

Despite the threats, so far the monarchs are sticking with their current locations in Mexico. A few years ago, Brower worked with NASA to look for other colonies and they found none.

Eco-tourism risk

According to Brower, "hugely excessive numbers of tourists are visiting" the Monarch Butterfly Biosphere Reserve.

On Brower's last Mexico trip, in late February, he helped lead a visit by former U.S. President Jimmy Carter and reports counting 24 large passenger buses in the visitor centre parking lot. Yet on a nearby trail, Brower spotted a sign stating a limit of 20 visitors at a time to the colony.

Without better management, Brower says ecotourism will overwhelm the butterflies.

Brower says that unless something is done about the various threats to the monarchs' survival, the migration will be diminished or "we may lose the migration altogether." It's not an extinction issue, at least, due to the monarch's widespread distribution, which includes the South Pacific and Australia.

To replace the milkweed, Craig Wilson is overseeing a program in his part of Texas to plant milkweed, with local farmer's cooperative stores providing potted plants. He would like to see a feeding corridor from Texas to Canada for the monarchs.

He has also helped set up four butterfly garden waystations around elementary schools.

Brower would like to see more of these programs, which help to some extent, but also because they get people engaged.

Brower also says that damaged areas of the Oyamel forest may recover.

"An area that's been seriously horse-logged is coming back under its own steam and if it's left alone for 10 or 15 years, it will recover to the point where the butterflies can survive pretty handily in that area."

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