Is there an association between female Monarch oviposition and host plant age?
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Andrew Hoffarth
Cloquet High
School Abstract | Introduction | Methods | Results | Discussion | Acknowledgments | Literature Cited | Research Projects I studied the relationship of milkweed age and female monarch oviposition preference to see if the age of milkweed has any impact on where the female monarchs choose to oviposit. Mated females were placed in cages containing three different ages of Asclepias syriaca (Common Milkweed): young, mature, and senescing. The number of eggs deposited on each was counted after two to four days. The conclusion was made that female monarchs prefer to lay eggs on young and mature milkweed over senescing milkweed. I studied female monarch oviposition on the milkweed species Asclepias syriaca. Female monarchs have been found to prefer to lay eggs on plants with cardenolide concentrations of 300-400 (Zalucki, Brower, and Malcolm 1990). One possible cause of this preference may be that they recognize the plants' age and determine whether or not their caterpillars can eat it and survive. This may be due to natural selection over the years, leaving only the strongest to survive. I predicted several things: 1) Female monarchs will oviposit on the young plants exclusively, 2) Female monarchs will oviposit on the young and mature plants exclusively, 3) Female monarchs will oviposit exclusively on the mature plants, 4) Female monarchs will have no preference and will lay eggs equally on all three plant types. The project started off with the building of cages. Supplies were purchased from the local hardware stores and two cages were built. The cages were 3'x2'x3' and built of PVC Pipe and mesh netting. The next step was to acquire the females. Mated females from the U of MN were released in the cages. I was able to keep them alive for a long time using a feeding cage and a concoction of honey-water containing 10% honey and 90% water. In the meantime, I went out to some nearby fields and cut milkweed of each of the categories. To set up the experiment, I clipped the milkweed at the stem with 5" of leaf space and recorded data on all the plants used. Thus, standardizing the plants and reducing any bias. The plants were placed in bottles of water and lined up in a straight row exactly one foot away from each other. The plants were also rotated from front to back in each experiment, to further reduce any preference the monarchs might have. The cages were then placed over the plants, and at a certain time, usually in the early morning, the monarchs were placed in the cages. The monarchs were left in the cages until a significant amount of eggs were laid on the plants. The standard was over 100, but frequently I found that they had laid several hundred. The results collected showed that female monarchs have an oviposition preference. The female monarchs tested showed a preference towards the young and mature milkweed in comparison to the senescing. The conclusion was made using data and averages. There is a direct contrast between the average number of eggs per leaf. Both the young and mature milkweed have an average of 6 per leaf, while the senescing (dying) have an average of about 0.5 per leaf.
Figure 1, above left, shows that the average number of eggs found on one leaf of a plant is much higher for both the young and mature milkweed. There is one other major difference I found when comparing my data, the total number of eggs found. I collected 943 eggs total (see distribution in Figure 2, above right). This figure represents the total data found in this project. As can be seen, there is a great difference between the young/mature totals and the senescing totals. Based on this data, I believe a preference was established. After finding this, I would like to continue the experiment and find out what cues the female monarchs to make this preference. The data was analyzed using the chi-square test. With two degrees of freedom, and three variables, a 99.5% confidence was found.. When the same test was done using just the young and mature data, only a 97.5% confidence level was found. This test rejects the null hypothesis and proves that data to be statistically significant. They hypothesis that there would be a preference against the senescing plants was shown to be true. Acknowledgments I would like to thank Tim Anderson, Cynthia Edwardson, Sonia Altizer, Michelle Solensky, Liz Goehring, Michelle Prysby, and Karen Oberhauser for their support and knowledge. I would also like to thank the University of Minnesota and the Science Museum of Minnesota for giving me the opportunity to explore the monarch, an experience I shall cherish for life. Literature Cited Oyeyele, S.O. 1990. Cardiac Glycosides and Oviposition by Danaus plexippus on Asclepias FR. Minitex Library Information Network. Zalucki, M.P. 1990. Oviposition by Danaus plexippus in relation to Cardenolide Contents. Minitex Library Information Network. Back to top | Interactions With Milkweed | Student/Teacher Research | Research Topics | Site Overview |
