Monarch Degree Days

For the last four years we have been using the accumulated monarch degree days during the breeding season to show how the temperatures relate to the numbers of monarchs each fall. These records were particularly useful in explaining the low numbers of monarchs at the overwintering sites in the winter of 2004-2005. Although cooler in Texas and about average in the most northern portion of the breeding area, the summer of 2007 was the hottest among the last 5 years in the Midwest.

The total monarch degree days and potential number of generations for each of the last 5 years are summarized in the following table.

Table. Monarch degree day totals and potential number of generations through 23 September for 2003 – 2007.

The methods used to calculate monarch degree days can be found in the “Monarchs, Cold Summers, Jet Streams, Volcanoes, and More” article from the Monarch Watch January 2005 Update. The breeding seasons of 2006 and 2007 have been the hottest, with the greatest number of potential monarch generations, of this period. Higher temperatures can favor monarch population growth but they can also be detrimental if they co-occur with drought conditions. The summer of 2004 was the coldest during this period and the coldest since the Mount Pinatubo summer of 1992. The low temperatures during the 2004 breeding season, one that followed massive mortality at the overwintering sites in January and February 2004 and high temperatures in March in Texas, contributed to the lowest overwintering population (2.2 hectares) observed or measured at the overwintering sites. The January 2005 Update article (referenced above) contains an extensive discussion of the factors that contribute to summer temperatures.