Does larval host plant affect female oviposition preference?
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Genny Clute,
Eva H., Mary Gruber Thomas
Abstract | Introduction | Methods | Results | Discussion | Acknowledgements | Literature Cited | Research Projects Our group had a strong interest in the adult stage of the Monarch Butterfly. This interest led to our question: Does larval host plant affect oviposition preference? To find the answer, we reared two sets of monarchs; one set on Asclepias incarnata (swamp milkweed) and the other on Asclepias syriaca (common milkweed). Once the butterflies emerged, we put one mated female monarch butterfly in a cage with a stalk of A. syriaca and a stalk of A. incarnata for a whole day. Only one butterfly was in a cage at a time so we were able to get precise counts of eggs laid on each plant. We did these trials five to ten times with each butterfly, depending on how long each butterfly lived. We did Chi-square tests to find any oviposition preference and to find any correlation between larval host plant and oviposition preference. There was no correlation between the larval host plant and oviposition preference, and both incarnata- and syriaca-reared females preferred incarnata. We then did a T-test and showed that females that were raised on syriaca laid more eggs than females raised on incarnata. Factors that may have affected the results were getting ten trials done for both sets of butterflies, having butterflies in trial tents for different lengths of time, and controlling for leaf number and leaf space. There were more leaves on the incarnata than on the syriaca, and this may be why the females preferred to lay their eggs there. The last factor leads to another related project investigating the effects of leaf space and leaf quantity on oviposition preference. Introduction We wanted to find out if female monarch butterflies have a preference for specific milkweed species when laying their eggs.. We knew that they only lay on milkweed but Minnesota has two fairly common milkweed species, Asclepias syriaca (common milkweed) and A. incarnata (swamp milkweed). This is an important and interesting question to all of us in many ways. First of all we all wanted to work with adult monarchs more than the larval stages, and this gave us opportunity to work with both. Secondly, this project could be done by both the southern MN team and the central MN team. In addition, if we were going to raise monarchs for a school class or project then this study would provide important information for raising happy and healthy monarchs. And, finally, this project just seemed fun! There have been a few studies similar to ours. Firl, Goldberg, and others did a study on the effects of milkweed species and the development of monarchs (http://www.monarchlab.umn.edu/MidSch/wcms6.html). They found no difference in monarch survival or development between monarchs reared on A. syriaca and those reared on A. incarnata. A group of people from St. Peter MN did a study on food preferences of larval monarchs (http://www.monarchlab.umn.edu/Research/Mwd/mwdpref.html) and found that monarch larvae preferred A. syriaca over A. incarnata. Both studies were performed during the larval stage of monarchs and had to do with treatments of different milkweed species. Though our study also investigates milkweed preferences in monarchs, it focuses on the adult stage. We wanted to know if larval host plant species affects adult oviposition choice. We suspected that monarchs would prefer to lay eggs on their larval host plant species. To answer our question, we developed an experiment that would work for both halves of our research group. Hypotheses:: Ho=Larval host plant has no affect on oviposition
preference in monarch butterflies. Ho=Larval host plant has no affect on female
monarch fecundity (number of eggs laid). Ho=Adult female monarchs do not have an
overall preference of milkweed species for oviposition. Our team divided into two groups for this project. One group worked in St. Paul while the other group worked in the Mankato area. Each group received four mated females from the University of Minnesota. We allowed these females to lay eggs on two types of milkweed plants: Asclepias incarnata (swamp milkweed) and Asclepias syriaca (common milkweed). After the eggs hatched, each group raised 30 larvae solely on Asclepias incarnata and 30 larvae solely on Asclepias syriaca. Once the monarchs pupated and emerged, Asclepias incarnata females were mated with Asclepias incarnata males. The same process was followed for Asclepias syriaca adults. The independent variable in our experiment was the type of milkweed the larvae were raised on. The dependent variable was the number of eggs laid on either milkweed species. To test for oviposition preference, one stem of each
milkweed species was placed in a 2-foot square cage. Each group
had two 2-foot square cages for the To analyze our data, we used a chi-square test to determine whether the larval host plant is associated with oviposition preference. After testing to be sure that data were normally distributed, we used a t-test to determine whether the larval host plant affected female fecundity (number of eggs laid by each adult). We also used a t-test to determine overall preference of milkweed species for oviposition. Table 1 shows the number of eggs laid by each of the females tested.
Table 1. Data from the 16 individual oviposition trials.
Our first set of hypotheses addressed oviposition preferences of monarchs reared on different milkweed species. Ho=Larval host plant has no affect
on oviposition preference in monarch butterflies. In looking at larval host plant oviposition preference, we failed to reject our null hypothesis. A chi-square test showed that there is no preference in oviposition for larval host plant (Figure 1, Table 2, X2 = 0.25, df = 1, p>0.1).
Table 2. Observed and expected values for oviposition preferences. Shown are the number of butterflies from each treatment that exhibited a preference for incarnata or syriaca (i.e. more eggs laid on one species than the other).
Our second set of hypotheses addressed monarch fecundity. Ho=Larval host plant has no affect
on female monarch fecundity (number of eggs laid). To look at the distribution of the number of eggs laid, we created a histogram (Figure 2). The data were approximately normally distributed, justifying our use of the t-test. The t-test determined whether the larval host plant affected female fecundity. We were able to reject our null hypothesis with 90-95% confidence. Our results showed that there is a trend toward monarchs reared on Asclepias syriaca laying more eggs than those raised on Asclepias incarnata by a margin of almost two to one (Figure 3).
Our third test questioned the overall preference of female monarch butterflies for oviposition. Ho=Adult female monarchs will not
have an overall preference of milkweed species for oviposition. A t-test showed that we were able to reject our null hypothesis with a 90-95% confidence. (T = 1.69, df = 6, 0.1 < p< 0.05) The female monarch butterfly prefers to oviposit on Asclepias incarnata no matter which species it was raised on. Figure 4 illustrates the average preference of Asclepias incarnata to Asclepias syriaca for oviposition. (Figure 4). Figure 5 illustrates the percent of eggs laid on Asclepias incarnata and Asclepias syriaca by female monarchs in each treatment.
Discussion We were surprised by the results of our experiment on oviposition preference of female monarchs. We had expected the female adult monarch to preferentially oviposit on the larval host plant species. Our conclusion, which showed that monarchs preferred to oviposit on Asclepias incarnata by almost a factor of two begs the question as to why. Although we were careful to make sure the leaf area of both the Asclepias syriaca and the Asclepias incarnata was similar, we wondered if there were other factors which drew the female monarch to the Asclepias incarnata for oviposition. Possibly monarchs may prefer to oviposit on Asclepias incarnata because it is inherently more appealing. In addition, the relative size of the Asclepias incarnata leaf may be important. Could the smaller leaf size of the Asclepias incarnata be more appealing to the female adult? Another question that arose during our experiment relates to the number of eggs laid on different days. Although the temperature remained fairly constant, on some days a monarch would lay as many as 30 eggs while on other days, she would lay very few eggs. In reviewing our records, we questioned the age and environment of the monarchs. Due to problems with mortality, the age of the adult monarchs varied. The average age of the monarchs raised on Asclepias incarnata was 5 days old; whereas the monarchs that were raised on Asclepias syriaca were generally 7-10 days old, being kept in a cooler while waiting to oviposition. However, age of the monarch did not appear to affect the amount of eggs laid. Monarchs of the same age group would lay differing amounts of eggs. A final note of interest was the fact that, upon emergence, most larvae preferred to eat from the Asclepias syriaca, showing no particular preference to where they were laid. This would be another topic that would be fascinating to pursue. Acknowledgments National Science Foundation (ESI-9731429) Literature Cited Firl, J., J. Goldberg et al. 1999. The Effects of Different Species of Milkweed on Monarch Development. Monarch Lab Web site. URL: http://www.monarchlab.umn.edu/MidSch/wcms6.html Kuerschner, R., J. Peterson, N. Pherson, N. Schliemann, R. Shoemaker and D. Wunderlich. 1999. Food preferences of Danaus plexippus larvae. Monarch Lab Web site. URL: http://www.monarchlab.umn.edu/Research/Mwd/mwdpref.html Back to top | Interactions with Milkweed | Student/Teacher Research | Research Topics | Site Overview |
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experiment. The Asclepias incarnata stem was cut from a
garden area in St. Paul and a wild habitat area in the Mankato area.
The Asclepias syriaca was cut from plants grown in pots.
We ensured that both stems were of the same approximate height,
had the same approximate leaf area and included no yellowing leaves.
In addition to the milkweed, we included fresh cut flowers for necturing.
Beginning at 8:00 a.m. each day for a period of 12 days, one female
mated monarch was placed in a cage containing the stems from both
milkweed species. We placed each cage outdoors in a sunny area.
The temperature average during the time of the experiment was 80
degrees Fahrenheit. We allowed adult females to lay eggs freely
on either plant until 4:00 p.m. At this time, we removed the
adult from the cage and counted the number of eggs on each stem.
We repeated this process for each of the adult female monarchs.
Final sample size was seven female Asclepias syriaca-reared
adults and nine female Asclepias incarnata-reared adults.
In stating our conclusion, we omitted days when no eggs were laid
on either plant choice.
A
B