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Life Science: Field Population System

Selective Herbivory

How selective are the larvae of the cabbage white butterfly on the plants that they eat? These larvae prefer members of the Brassica family, which includes cabbage, broccoli, Brussels sprouts, kale, mustard, radish, and numerous others. One species — Brassica rapa — includes turnips, Chinese cabbage, pak choi, broccoli raab, and the Fast Plants that were selectively bred by Bottle Biology creator, Paul Williams. All of these are varieties of the same species, so they can be considered to be members of the same population when grown together.

Selective Herbivory tests whether or not cabbage white butterfly larvae feed selectively within a population of Brassica rapa. A population of lettuce has been sown with the other seeds to make additional comparisons. If larvae do feed preferentially, they could be considered agents of natural selection. Such agents would be expected to cause the population to evolve over many generations.

Materials Needed


  1. If you are using the same system for “Assessing Variation,” you’ll begin this study about 10 days after sprouting.
  2. To begin your study, gently gather numerous larvae from the Brassica nursery. You’ll need at least three for each system.
  3. Introduce equal numbers of larvae to each system, trying to spread them around and start them at the base of the plants’ stems.
  4. At least once each day for the duration of the study period (3 – 5 days), use your “Selective Herbivory” Data Sheet to make observations. You’ll be recording where the larvae are and what is happening to the plant populations.
  5. At the end of the study period, assess which plants were selectively fed upon, if any.

Activity Questions

Before the study period begins

  1. What traits vary among the plants in your Field Population System?
  2. Among the traits that vary for Brassica rapa, which do you think might affect feeding behavior of the butterfly larvae? Why?
  3. How will you be able to tell which variants the larvae select?
  4. What has to happen in order for selective herbivory to have an impact on the next generation of plants?

After the study period ends

  1. What occurred during the study period to the Fast Plants? The turnips? The lettuce?
  2. Was there a pattern to feeding behavior over time (i.e., an order to feeding choices)?
  3. Which of the two varieties of Brassica rapa, if either, did the larvae select preferentially?
  4. What implications does this have for the Brassica population in the next generation?
  5. Do you think that this is an example of natural selection? Why or why not?
  6. How might this result in evolution within the population?
  7. If selective herbivory was observed, which trait(s) do you think might have affected feeding choices? Why?
  8. One trait of Fast Plants that was discussed in the video during the Science Studio was hairiness of plants. Is there a way that the Fast Plants in your Field Population System might become hairier over time? Explain how this might occur.
  9. Is there a way that the larvae of the cabbage white butterfly might become more or less selective over time? Explain how this might occur.

SHARE YOUR RESULTS: Selective Herbivory

TRACK OUR PROGRESS: Selective Herbivory

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