Monarch Size & Condition: Comparisons at Different Times and Locations

Note: In order to compare monarchs measured at different times, you need to use statistical tests. It’s not enough to just compare average values. For an introduction to basic statistics, check out our Basic Stats section.

Migratory Generation  |  Overwintering Monarchs  |   Breeding Monarchs

Breeding Monarchs

Sonia Altizer
Karen Oberhauser
Michelle Prysby

University of Minnesota
St. Paul, MN

In 1997, we measured many monarchs throughout the course of the summer in the eastern (collected in WI and MN) migratory population. These samples were different than those from the migration and the overwintering period because they included monarchs from several different generations.

Question:

• Does the size of different summer breeding generations of monarchs vary?

Methods:

We captured monarchs weekly. Most of our sampling efforts took place at our regular field site in west-central Wisconsin, although a small proportion of the data reported here were collected at a farm about 20 miles east of the site in east-central Minnesota. All butterflies were released after we measured them.

Results:

Figure 4 summarizes winglength data from this study. Monarchs increased in size throughout the course of the summer. Those we measured in June were, on average, smallest, and those we measured in September were largest. In addition, males were larger than females. We did a statistical analysis called a multiple linear regression on these data. Both the collection month (p < 0.04) and sex (p < 0.01) had significant effects on winglength.

Discussion:

We have not studied factors that might be responsible for changes in winglength over time in the eastern population. Since winglength is determined before the adult emerges from the pupa, these factors must be acting during the larval stage. It may be that temperature changes (see study by Liz Larkin), or changes in the hostplant are responsible for the differences we observed. It is also possible that a combination of several factors is responsible for these differences. We do know that larvae fed leaves from old Asclepias syriaca plants are larger, on average, than those fed young A. syriaca (see study by Liz Goehring). The milkweed species that the larvae eat could also be important. The monarchs we measured in WI and MN in June probably migrated north from southern or central states, where they probably fed on other milkweed species (generation 1 and 2 in Yearly Life Cycle).

Discussion of winglength studies

continue to Sex Ratios