1. Summer 2012: Drought
Drought and excessive heat in the mid-continent during summer 2012 resulted in low reproduction. Historically, the U.S. corn belt has produced half of the monarchs that migrate to Mexico. Drought in that region is cited as major factor that contributed to the decline.
2. Winter 2012/2013: Record Low Population
Once each year, the population is measured at the overwintering sites in Mexico. This graph shows population estimates since record-keeping began in 1994. The 2012/2013 population was 80% below average. There were only 60 million monarchs compared to the 350 million average. The area of forest covered with monarchs was only 3 acres, compared to the 17-acre average.
3. Spring 2013: Cold Temperatures
Monarchs migrate from Mexico into the southern U.S. in March. There they produce the first spring generation, then die. The new generation was slow to develop and late to migrate northward. Unusually cold temperatures across the mid-continent was the cause.
4. Summer 2013: Unproductive Breeding
Low winter numbers, cold spring temperatures, and a delayed spring migration meant few monarchs entered the northern breeding grounds in June. Monarchs were scarce all summer. Many observers didn't see a monarch until August, other people saw none at all. Observers also saw little evidence of reproduction. Population recovery depended on a productive breeding season.