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Monarch Degree Days for the Last Three Seasons

By Janis Lentz and Chip Taylor

To get a better idea of how this season compares with the two previous seasons, we’ve calculated the degree days for each year for five cities: Winnipeg (Manitoba, Canada), St. Paul, MN; Des Moines, IA; Lawrence, KS; and Dallas, TX. The calculations start with the approximate date of first arrival of monarchs for each location. The accumulated degree days as of 23 September are shown in Table 1.

Table 1. Accumulated monarch degree days and potential monarch generations for selected cities as of 23 September for the last three years. The number of degree days for each year is listed in the year column. Gens = the potential number of generations

A comparison of the accumulated degree days for each of the last three years for the cities listed shows that 2005 has been warmer than either 2003 or 2004, except for 2003 in Winnipeg. The result is a higher number of potential monarchs generations this year. The greater potential for reproduction due to higher temperatures could explain the many reports of favorable monarch numbers this summer.

Although the total degree days are shown through 23 September for each city, we have to keep in mind that the migration begins and ends at different dates for each city. Most important for our considerations is the “getting out of town date”, i.e. the last date for departure for which a monarch has a high probability of reaching Mexico. The tagging data shows that the these dates are 6 September for Winnipeg, the 20th for St Paul, 27th for Des Moines, 4 October Lawrence and the 21st for Dallas. The accumulated degree days for Winnipeg through the 6th of September was 1202 or 1.6 generations. This figure gives us a more realistic estimate of the possible success of monarchs in the Winnipeg area than the total degree days through the 23rd shown in Table 1. Similar adjustments could be made for the other cities.

In the tables below, the monarch degree days are shown for Winnipeg, St. Paul, Des Moines, Lawrence, and Dallas for the past three seasons from 24 August through 23 September. For the complete degree day data, from the average date of first arrival of monarchs for each of these cities, please see the July Update.

The data presented in this analysis has been obtained from weather records from the official recording weather station for each city. These stations are usually located at the largest major airport. As such, these records are only an approximation of what monarchs might experience in your Monarch Waystation or the nearest milkweed patch. Clearly, we need a more refined measure of the environments monarchs experience in our gardens. We are working on a new project and expect to have a more precise way of recording the temperatures in gardens available for next season.

Winnipeg, Manitoba (Canada) 49:54:00N 97:07:48W

St. Paul, MN 44:56:52N 93:06:13W

Des Moines, IA 41:34:36N 93:37:03W

Lawence, KS 38:57:46N 95:15:19W

Dallas, TX 32:47:39N 96:45:55W