James Godwin, Mary Locke, Emily Locke

Lakewood Academy
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Abstract  |  Introduction  |  Methods  |  Results  |  Discussion  |  Acknowledgements  |  Literature Cited  |  Research Projects

Abstract

We tested to see which milkweed plants monarchs prefer to lay their eggs on. In our experiment we placed 4 potted milkweed plants in a screen tent on a ½ mowed vs. unmowed field. This field was also ½ shaded. We released female butterflies singly for 10 minutes to see which plants they laid eggs on. We found monarchs preferred the most accessible plant, which was the top of the milkweed in the mowed area. They didn't favor either sun or shade. This project brought new questions which we would like to study more, i.e., "Do monarchs lay eggs better at different times of the day? Should we have released the monarchs in the tent for longer periods of time? Would the monarchs do better if released all at the same time? How soon after mating do monarchs lay their eggs?"

Introduction

Oviposition dynamics in monarchs are important because they help us better understand the monarch life cycle. We looked at how three factors affected oviposition site choice: The relative height of the milkweed plant to its surroundings; if the plant was in the sun or shade; and where on the plant the eggs were laid. We wanted to find out where monarchs lay their eggs and which factors were most favorable for egg laying.

Hypotheses

H_O1 =The relative height of a milkweed plant does not affect where a monarch lays her eggs.
H_A1=Monarchs lay eggs on taller (or shorter) plants relative to surrounding vegetation.

H_O2=Monarchs do not have a preference over sun or shade for laying their eggs.
H_A2=Monarchs prefer to lay their eggs on milkweed plants in the sun.
H_A3=Monarchs prefer to lay their eggs on milkweed plants in the shade.

H_O3=Monarchs do not have a preference as to where on a milkweed plant they lay their eggs.
H_A4=Monarchs show a preference as to where on a milkweed plant they lay their eggs.

Methods

To test our hypothesis, we used forty-eight mated female monarch butterflies, a 12x12 foot screen tent, and four potted milkweed plants, (Asclepias currasavica). The screen tent was set up over a section of field grass, half of which was mowed. We further divided our test area into fourths by shading half the mowed and the unmowed grass. Unmowed grass was taller than the milkweed plants, which were taller than the mowed grass. We put a potted milkweed plant in each of the four sections. We released each butterfly singly by placing it on the ground in the center of the screen tent and then observed its behavior for a ten minute period. We used contingency tables and Chi² tests to determine the significance of relative height, sun vs. shade preference, and egg laying position on milkweed plants for the data we collected.

Results

Only ten of our 48 females laid eggs during the 10 minute observation period.

Oviposition and Relative Milkweed Height

We tabulated the number of eggs deposited on milkweed relative to tall and short surrounding vegetation.  In our experiment we had 8 butterflies lay eggs on mw taller than surrounding plants and 2 lay eggs on mw shorter than surrounding plants. This is shown in Figure 1.  There was a weak trend toward preference for taller plants ( p<.1, calculated chi-square value = 3.84, critical chi-square value (0.05) = 2.70 ).

Figure 1.  Number of eggs laid on relatively tall versus short milkweed plants.

Oviposition Preference on Milkweed in the Sun vs the Shade

Of the ten butterflies that laid eggs, none showed preference for milkweed plants in the sun vs milkweed plants in the shade.  This is shown in Figure 2.

Figure 2. Number of eggs laid on shaded versus unshaded milkweed.

 Oviposition Location Preference

We also tested if butterflies preferred to lay eggs on the top (T), middle (M), or bottom (B) of milkweed plants.  Eleven eggs total were laid with 8 laid on the top, 2 laid on the middle section, and 1 egg laid on the bottom section of the milkweed plant as shown in Figure 3.  Monarchs strongly preferred to lay eggs on the tops of plants ( p<.025, calculated chi-square value = 7.8, critical chi-square value (0.05) = 7.4).

Figure 3. Number of eggs laid on the top, middle, and bottom of milkweed plants.

Discussion

Monarchs seem to prefer the most accessible milkweed on which to lay their eggs.   Our data indicates that monarchs prefer the top of the milkweed plant and the milkweed plant in the mowed grass.  Monarchs do not seem to discriminate between milkweed located in the sun or the shade.  Monarchs may have chosen the relatively taller plants because the taller plants were seen first, these milkweed plants would provide a better food supply for the offspring, or the taller milkweed plants provided a better lookout for predators for the egg laying monarch.  All of the observed egg laying was on the underside of a milkweed leaf where the effects of direct sun versus shade had minimal effect which could account for the lack of preference for sun versus shade.  We wondered whether a monarch ever laid eggs on an upper surface of a milkweed leaf, or if eggs were laid on leaves that were oriented in some respect to the earth's magnetic poles.  We found an article that would help in repeating our research which stated that egg laying seems to occur more in the morning hours than in the afternoon (http://monarchwatch.org/read/articles/eggs.htm), with possible reasons being the need to feed or rest after egg laying.  If we repeated this experiment we would conduct it in the morning hours when more egg laying seems to occur, increase our sample size and we would have a longer time period for observation. We could also try putting all the butterflies in our test area at the same time.  Implications from our study are that early mowing could make milkweed plants more attractive to female monarchs for egg laying monarchs if it caused the surrounding vegetation to be shorter than the milkweed.

Acknowledgments

We thank our families, the Godwins and the Lockes for their help and support; Dr. K. Oberhauser and her staff, Stacey Halpern, Liz Goehring for their advice, encouragement and patience. Monarchs in the Classroom, U of M, provided us with potted milkweed plants, mated butterflies and larva for rearing.  Research was supported by the Natural Science Foundation (ESI-9731429).

Literature Cited

Byerley, C., O.R. Taylor, S. Darnell and S. Collins. 2000. Do Female Monarchs Show Egg-Laying Preferences? http://monarchwatch.org/read/articles/eggs.htm

Oberhauser, K., 1997.  Monarchs in the Classroom.  University of Minnesota.

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