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The Effects of Different Species of Milkweed on Monarch Development

Janelle Firl, Julia Goldberg, and others
Willow Creek Middle School and Century High School
Rochester, MN

Julia and Janelle with their insect fair project display (Photo: Janelle Firl and Julia Goldberg)

Abstract

We wanted to find out how different species of milkweed affected developing monarchs. We raised 100 monarch butterflies from eggs: 20 on swamp milkweed, 20 on common milkweed, 20 on whorled milkweed, 20 on tropical milkweed, and 20 on butterfly weed. All conditions other than food were controlled (held constant). Larvae fed on tropical milkweed had the highest survival rate and the shortest time to pupation. Larvae fed whorled milkweed and butterfly weed had the lowest survival rate and the longest time to reach pupation. Common and swamp milkweed were in the middle. We are uncertain about the causes of the high mortality in our experiment. We learned that diet is an important factor in larval development. We also learned that doing an experiment like this is a lot of fun...but also work!

Introduction

We thought that different species of milkweed might have different nutritional and physical characteristics that could affect the survival rate, rate of life cycle, mass, and fitness of monarchs. We had observed that different species of milkweed had different textures, thicknesses, sizes, and shapes, as well as different pubescences. We had also done some research and discovered that different species of milkweed have varying amounts of toxin. Just like how different kinds of lettuce have different nutritional values, we thought different milkweed species would also have different nutritional values. Therefore, consumption of a particular milkweed might lead to contrasting levels of fitness.

Hypotheses

  • Null: There will be no connection between survival to pupation and different milkweed species.
  • Actual 1: Monarchs reared on different milkweed species will have different rates of survival.
  • Actual 2: Since common milkweed is most prevalent, monarchs will survive to pupation best when reared on common milkweed.

Experimental Design

  • For each treatment we obtained a random block of 20 eggs, using 2 eggs from 10 different females. All eggs were laid within a 3-hour period.
  • The eggs were distributed into 5 identical containers.
    • Eggs were kept on damp paper towel.
    • Milkweed was picked every 2 days and stored in the refrigerator. Sprigs of 5 different types of milkweed were inserted into floral tubes:
      1. Tropical - Asclepias curassavica
      2. Swamp - Asclepias incarnata
      3. Butterfly weed - Asclepias tuberosa
      4. Common - Asclepias syriaca
      5. Whorled - Asclepias verticillata
    • Milkweed was changed daily.
    • Larvae were maintained in containers until adulthood.
  • All other variables (temperature, light, etc) were kept constant (controlled).
  • An inventory was made of instar stages and date of pupation of all larvae in each container and recorded in a data book.
  • Upon emergence, the butterflies were separated by gender and then tagged with stickers indicating their milkweed treatment.
  • On the second day of adulthood, they were weighed and tested for parasites.

Summary

Our experiment went relatively well, with few problems. The problems we experienced were that with the overwhelming speed at which the monarchs emerged from the pupae stage, we were unable to record longevity. Another problem we experienced was the problem of having 20 original eggs in a container, but being unsure of the percentage of fertilization. This meant that when we recorded our numbers and percentages, they were out of 20 original eggs, and not the number of eggs that have hatched. We also had a problem after we labeled the adult monarchs with stickers, for some of them came off. This caused us to be unable to record the date of death for many of the butterflies.

Results

Larvae fed on tropical milkweed had the highest survival rate and the shortest time to pupation. Larvae fed whorled milkweed and butterfly weed had the lowest survival rate and the longest time to reach pupation. Common and swamp milkweed were in the middle.

(Photo: Janelle Firl, Julia Goldberg, and others)

Conclusion

The conclusion was that eating different species of milkweed does affect survival rate, adult mass, and the rate of the life cycle. The results show that monarchs fed tropical milkweed had the highest survival rate, shortest time to pupation, and the heaviest adults. Larvae fed butterfly weed had the lowest survival rate, longest time to pupation, and the lightest adults. For the other species of milkweed there was not as noticeable of a difference. Although common milkweed performed well, it was second to tropical in survival to pupae and average number of days to emergence. It was third behind tropical and whorled in average weights of adults.

Next Time

We are uncertain about the causes of the high mortality in our experiment. We learned that diet is an important factor in larval development. If we were ever to do another experiment of this kind, the only things we would change would be writing identification numbers directly on the wing of the adult monarchs and we would arrange for more people to record information on the monarchs everyday.

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