Percentage of milkweed plants carrying Monarch butterfly larvae:

2001 - nearly 25%
2002 - about 7%
2003 - about 8%
2004 - under 5%

In the same period a massive increase in the use of herbicide-tolerant GM soya beans occurred.

"...one factor in the decline in the number of egg-carrying plants could be the growing use of herbicide-tolerant soybeans, which are genetically engineered to permit larger amounts of weed-killing chemicals to be applied without hurting the crop. This method may have increased the spraying of herbicides and thereby the destruction of milkweed, which can be common in farm fields."
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A butterfly mystery
Chao Xiong, Star Tribune
August 21, 2004 MONARCHS0821
Last update: August 21, 2004 at 12:04 AM
http://www.startribune.com/stories/462/4940059.html

The number of milkweed plants in the Upper Midwest carrying the monarch butterfly's larvae is in its third consecutive slump, due to factors that elude researchers, a local monitoring project shows.

Research by the Monarch Larva Monitoring Project at the University of Minnesota shows that the numbers are below average and at their lowest level since 1998. Milkweed is the only plant on which monarchs will lay their eggs, and also serves as a the sole food source for larvae.

In 2002, the project found that about 7 percent of milkweed plants examined in the Upper Midwest carried larvae. In 2003, that number was about 8 percent. This year, volunteers are finding that slightly fewer than 5 percent of milkweed plants carry larvae.

That's extremely low, said Karen Oberhauser, the project's founder and an assistant professor in the university's Department of Fisheries, Wildlife and Conservation Biology.

In 2001, nearly 25 percent of all milkweed plants carried larvae. Since the project began keeping tabs in 1996, the average has been 13 percent.

The data are collected by hundreds of volunteers who take to the fields to hand count the butterfly's tiny white eggs and striped caterpillars.

"There's a disturbing indication that [the population] is going down, because we've had three low years in a row," Oberhauser said.

The low population reports are especially troubling because Oberhauser can only speculate as to the causes.

Storms in the monarchs' wintering grounds in Mexico killed millions of butterflies in 2002 and again this year, she said.

Monarchs are famous for their unusual mass migrations from summer habitats in the United States and Canada to specific locales in Mexico.

Although a link has not been proved, Oberhauser said, one factor in the decline in the number of egg-carrying plants could be the growing use of herbicide-tolerant soybeans, which are genetically engineered to permit larger amounts of weed-killing chemicals to be applied without hurting the crop. This method may have increased the spraying of herbicides and thereby the destruction of milkweed, which can be common in farm fields. The project's findings show that the use of herbicide-tolerant soybeans grew from 50 percent in 2000 to 85 percent in 2003.

"It's kind of a complicated problem because there's not one smoking gun," Oberhauser said.

Teaching caring

She oversees the monarch project with a gentle hand and glacial patience in a pair of small second-story offices on the university's St. Paul campus. At ground level, her life's work comes full circle in a sunny greenhouse and lab overrun with eager young staff members.

Under careful observation, countless caterpillars munch on milkweed .

They increase their mass by 2,000 times in up to two weeks. Adults emerge from emerald-green chrysalises daily, and others embrace in 16-hour mating sessions.

It's the largest and most intensive study of any butterfly species or insect, Oberhauser said.

Monarchs aren't considered a keystone species, which when removed from their environment lead to greater damage. But as a nonthreatening ambassador to the animal world, Oberhauser said, they are priceless when it comes to introducing the public to conservation.

"People need a hook to be concerned about the environment," she said. "To have something like monarchs that people can understand and care about, it helps them care about the environment."

The project also works aggressively in outreach and education. It breeds about 50,000 monarchs in captivity each year. Most are sold at a minimal cost to local schools for educational purposes. A few thousand are used in experiments carried out by Oberhauser and her staff.

The project researches factors that affect monarch populations, including pesticide use, predation and weather patterns.

Oberhauser said it will take more data to find possible correlations.

Chao Xiong is at This email address is being protected from spambots. You need JavaScript enabled to view it..