Monarch Butterflies' Population Steadily Declining, But Why?

First Posted: Mar 14, 2013 11:52 AM EDT

The Monarch butterfly population is fast dwindling, as scientists record the lowest number of butterflies during their annual winter migration to Mexico. Their numbers have dropped 59 percent this year, dropping to the lowest level since comparable record-keeping began 20 years ago, the Mexican government and a conservation alliance reported on Wednesday.

Scientists say the drop is from drought, wildfires and a loss of food plants. The World Wildlife Fund, one of the groups that sponsored the butterfly census, attributed the decline to climate conditions and agricultural practices, especially the use of pesticides that kill off the monarchs' main food source, milkweed.

"The decrease of monarch butterflies ... probably is due to the negative effects of reduction in milkweed and extreme variation in the United States and Canada," the fund and its partner organizations said in a statement.

The decline of milkweed in the monarchs' summering areas in the north can make it hard for the butterflies to lay eggs, and for the offspring that do hatch to find enough food to grow to maturity.

Many bird species migrate each winter to warmer climates, but Monarch butterflies are the only known butterflies that make the 3,000 mile journey from their breeding grounds in the United States and Canada to northern Mexico and southern California.

The hot or dry weather can kill eggs, meaning fewer adult butterflies. For butterflies that reach adulthood, unusual cold, lack of water or tree cover in Mexico can mean they're less likely to survive the winter.

"The conservation of the monarch butterfly is a shared responsibility between Mexico, the United States and Canada. By protecting the reserves and having practically eliminated large-scale illegal logging, Mexico has done its part," said Omar Vidal, the World Wildlife Fund director in Mexico, according to the Associated Press.

"It is now necessary for the United States and Canada to do their part and protect the butterflies' habitat in their territories," Vidal added.

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