Some monarch butterflies on the West Coast apparently are losing their instinct to migrate away from Southern California. No one knows for sure why, but gardeners in Ventura and Santa Barbara counties planting tropical species of the plant the butterflies depend on — milkweed — might have something to do with it.
http://www.vcstar.com/news/2010/jul/24/monarchs-sticking-around-butterfly-gardens-give/?partner=RSS