Phase I of the anti-rat campaign began in January 2011 on Rabida island and about a dozen islets, which like Pinzon and Plaza Sur are also uninhabited by humans.

The goal is to kill off all non-native rodents, beginning with the Galapagos' smaller islands, without endangering other wildlife. The islands where humans reside, Isabela and Santa Cruz, will come last.

Previous efforts to eradicate invasive species have removed goats, cats, burros and pigs from various islands.

Pinzon is about seven square miles (1,812 hectares) in area, while Plaza Sur encompasses just 24 acres (9.6 hectares).

"This is a very expensive but totally necessary war," said Gonzalez.

The rat infestation has now reached one per square foot (about 10 per square meter) on Pinzon, where an estimated 180 million rodents reside.

The director of conservation for the Galapagos National Park Service, Danny Rueda, called the raticide the largest ever in South America.

The poisoned bait, developed by Bell Laboratories in the United States, is contained in light blue cubes that attract rats but are repulsive to other inhabitants of the islands. The one-centimeter-square cubes disintegrate in a week or so.

Park official Cristian Sevilla said the poison will be dropped on Pinzon and Plaza Sur through the end of November.

A total of 34 hawks from Pinzon were trapped in order to protect them from eating rodents that consume the poison, Sevilla said. They are to be released in early January.

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Galapagos Islands steps up extermination of 180 million rats