Donna Elwell, Brandi McTee, Kim Williams

Blue Ridge High School
Blue Ridge, TX

Abstract  |  Introduction  |  Methods  |  Results  |  Discussion  |  Acknowledgements  |   Research Projects

Abstract

Natural and artificial events affect milkweed growth and apparency in Texas. Caged monarch butterflies were tested to determine preference for apparent or non-apparent milkweed with respect to oviposition. Mated females were placed in cages with native milkweed and plants under simulated summer day length conditions. None of the females in our experiment laid any eggs. Possible causes for this failure to produce eggs include suboptimal lab temperatures and human activity in the classroom. In the future, controlled environmental factors and secluded surroundings might yield different results.

Introduction

During March, in the area of Texas known as the Blackland Prairie (Figure 1), milkweed pushes up through the old vegetation mass and becomes relatively conspicuous. During early June, the Texas Highway Department begins its periodic program of mowing highway right of ways after most spring plants have set their seed (Godwin, int., Teague, int.). Once again the milkweed pushes up through the cut vegetation and becomes conspicuous (pers. obs.). We wondered whether these natural and artificial regimes that promote milkweed growth provide increased opportunities for oviposition.

    Figure 1.  Geographical regions of Texas, with the Blackland Prairie region shown in blue.  The yellow star indicates the location of Blue Ridge, TX, where the experiment was conducted.

We designed a milkweed apparency test to determine whether monarch butterflies (Danaus plexippus L). prefer apparent or non-apparent milkweed when given a choice for oviposition. We were unable to find references that specifically addressed our experiment. We did find studies, suggesting that females prefer single isolated milkweed plants to patches of milkweed plants (Zalucki and Kitching, 1981).

Hypotheses

H₀= There is no difference in female oviposition preference for apparent or non-apparent milkweed.

H_a1= Females prefer apparent milkweed.

H_a2= Females prefer non-apparent milkweed.

Methods

We obtained 10 mated females to use in our experiment. Six females were reared in the lab at the University of Minnesota and four females were reared in the classroom at Blue Ridge Middle School. All females were mated by hand pairing. A pair of females was randomly selected for placement in identical cages (4’ x 2’ x 2’). At one end of each cage we placed a container of cut milkweed (Asclepias viridis) with no other plants around it, and at the other end, milkweed surrounded by tall grasses and other plants native to the Blackland prairie. To ensure consistency, the milkweed was a uniform height of twelve inches in each container for all trials. The condition of the A. viridis was graded as equal in each treatment: plants were relatively young with <5% yellowed or dying and <5% showing evidence of herbivory or disease. Laboratory ambient temperature was 70^o F. Our goal was to raise the temperature to 75^o F, but we were unable to do so. A total of six 200 watt lamps (Fig. 2) were suspended above the cages to simulate summer sun and were connected to timers for 14 hours of daylight.

Figure 2.  We positioned lights above the cages used in the experiment to simulate sunlight.

To determine preference, we left the plant samples and females in the cages for 36 hours. We then removed the females and collected the plants to count and record the number of eggs laid on the apparent and non-apparent milkweed. Fresh plants were placed in the cages in reversed positions and a new female was released to each cage. We repeated the counting and rotation of position every 36 hours for a total of five trials (n = 10 females).

Results

The females did not appear to be very active during the day. Most rested on the netting near the lights. None of the females laid eggs.

Discussion

We found that we were unable to raise our lab temperature any higher than 72 ^o F. Although we palpated the female abdomens to verify that they had mated successfully, we found no evidence of egg laying after any of the trials. We saw less activity than anticipated. It is possible that it was too cool for our butterflies to lay eggs. The cages were in a classroom setting. It is also possible that the constant movement of students in and out of the area disturbed the butterflies. Alternatively, it is possible that our females were in diapause and therefore were unable to lay any eggs.

Future research should be carried out away from high traffic areas. Ideally, a lab with few people around would be best. The ability to effect a significant change in the surrounding environment (i.e. temperature) was unavailable in our situation. A more regulated work area would be better suited for this experiment.

Acknowledgments

We wish to thank the National Science Foundation, the University of Minnesota, and the Science Museum of Minnesota for our grant to participate in the Monarch Monitoring: a Teacher/Student/Scientist Research Project. Thanks also to Dr. Karen Oberhauser, Dr. William Calvert, and Michelle Solensky for their guidance on defining our research and providing constructive comments on drafts of this manuscript.

Literature Cited

Godwin, Royce, Telephone interview. 14 Nov. 2000.

Teague, Ernest, Telephone interview. 14 Nov. 2000.

Zalucki, M. P. and Kitching, R. L. 1981.  Dynamics of oviposition in Danaus plexippus (Insecta: Lepidoptera) on milkweed, Asclepias spp. J. Zool., Lond. 198: 103-116.

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