TEANECK - If you haven't seen many monarch butterflies this spring, environmentalists say there's a reason.

Wildlife experts say the monarch population has hit a 20-year low. They blame environmental changes in Mexico, where butterflies go during the winter.

Officials say there is something everyone can do to help breed more butterflies -- plant milkweed in your yard.

"Milkweed is the only plant monarchs will lay their eggs on," says Don Torino, of the Bergen Audubon Society. "No milkweed, no monarch butterflies."

Milkweed is in short supply because herbicides used on farms kill it. The Audubon Society is donating milkweed plants to schools and parks to help breed more monarch butterflies.