Fears for Mexico's butterfly population

President Obama is due to meet the leaders of Mexico and Canada for a summit marking the 20th anniversary of the North American Free Trade Agreement.

While border security and trade will be high on the agenda, environmentalists are lobbying for an unlikely topic - butterflies.

The monarch butterfly was named the symbol and logo of the NAFTA group because of its incredible annual migration from Canada to Mexico. But in recent years, the number of monarchs migrating has fallen sharply.

Will Grant reports from Mexico.