Monarch Watch is an outreach educational and research program designed to use citizen scientists to learn about and aid monarch butterflies.
It turns out that almost all of the monarchs in the central and eastern portions of North America migrate each fall to one small area in central Mexico's mountains to overwinter. (The actual site was only discovered by scientists in the mid 1970's.) There the monarchs cluster on trees at higher elevations, where the temperatures are traditionally cool enough to let them almost hibernate, reducing their need to feed. In essence, they doze away the winter, then return north in the spring to lay eggs and repopulate our summer meadows and gardens.
At that important overwintering site, scientists are able to estimate the monarch population simply by measuring the acreage that their dense clusters cover. Since Monarch Watch began, the highest wintering population was in 1996/1997. That winter, the butterflies covered almost 21 hectares (over 51.5 acres). This winter (2012/3) the area covered by wintering monarchs was the lowest yet seen: 1.19 hectares (a little less than 3 acres).
There are lots of reasons for this decline, but a major cause has been the decrease in milkweeds available as larval food for monarchs. This decrease in milkweeds has occurred primarily because of increased use of herbicides - in crop fields, pastures and along roadsides.
As gardeners, this is where we can make a difference. Monarch Watch is encouraging all interested parties to develop Monarch Waystations - gardens specifically geared to provide habitat to help monarchs produce more caterpillars as well as to provide them with good nectar sources to fuel their southward migration in the fall.