According Monarch Watch there are many reasons there has been a decline in the monarch butterfly population.Widespread use of herbicides, developement of subdivisions and shopping centers and the mowing along highways has contributed to the decline of nectar sources.
But there are some things we can do. Why not create a monarch waystation at your home, business, or school? It can be as large or small as you like. You need to plant milkweed that are native to your area, so below I have listed a few for your region.
Northeast: common milkweed, swamp milkweed, butterfly milkweed, poke milkweed
South Central: antelope horn milkweed, green antelope horn milkweed, zizotes milkweed
Southeast: aquatic milkweed, white milkweed, butterfly milkweed
West: showy milkweed, antelope horn milkweed (only in NV AZ NM CO ID KS, and OK)
Without milkweeds in the spring and summer,which provide breeding areas, the popuation cannot be maintained. Without nectar from flowers, they cannot make the long migration to Mexico each fall.
You can register your waystation and even purchase a plaque identify your garden as an official Monarch Waystation.