Although this was an incredible program, it had many errors.
It is the fourth generation that makes the journey to Mexico in the fall. But, smaller colonies of Monarchs exist in California, the Gulf of Mexico, and Florida. I read in my natural history of butterflies of north america that Dr. Urquhart (a Canadian scientist) who studied Monarchs for 50 years and started the initial tagging of Monarchs had tagged a Monarchs near Mexico City that was recovered in New York. This generation that travels to Mexico from the USA and Canada does not reproduce until they return in the spring (if they are the generation that is in sexual diapause).
Find out more about their biology at the Monarch Watch website:http://www.monarchwatch.org/tagmig/index.htm