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
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.
5. Winter 2013/2014: New Record Low
The area of forest covered with monarchs plunged to only 1.65 acres, 90% below to the 15-acre average. This record low compares to a peak of 51.8 acres recorded in 1996. At 20 million butterflies per acre, this year's population is estimated at 33 million monarchs compared to the peak of 1 billion in 1996.
6. Summer 2014: Ideal Conditions
Following two unfavorable summers that were partially responsible for the population crash in 2012, weather conditions during last summer's breeding season were ideal. If the population had encountered poor breeding conditions in summer 2014, scientists were concerned that the migration would reach its extinction threshold.
7. Winter 2014/2015: Still Dangerously Low
Although the number of monarchs overwintering in Mexico has increased from last year's record low, the population remains 80% below the historic average. There are 57 million monarchs compared to a long-term average of 300 million and peak of 1 billion.