Does larval host plant affect female oviposition preference?
Genny Clute, Eva H., Mary Gruber Thomas
St. Mark's School
St. Paul, MN
Katie Bovee, Jes Norman, Maribel Vega Martinez
Minnesota New Country School
Henderson, MN
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. 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 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.
- HA1=Monarch butterflies prefer to oviposit on larval host plant species.
- HA2=Monarch butterflies prefer to oviposit on species other than their
larval host plant.
- Ho=Larval host plant has no affect on female monarch fecundity (number
of eggs laid).
- HA1=Monarchs raised on Asclepias incarnata have greater fecundity
than monarch raised on Asclepias syriaca.
- HA2=Monarchs raised on Asclepias syriaca have greater fecundity
than monarchs raised on Asclepias incarnata
- Ho=Adult female monarchs do not have an overall preference of milkweed
species for oviposition.
- HA1=Adult female monarchs prefer to oviposit on Asclepias incarnata.
- HA2= Adult female monarchs prefer to oviposit on Asclepias syriaca.
Methods
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
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.
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.
Results
Table 1 shows the number of eggs laid by each of the females tested.
Table 1. Data from the 16 individual oviposition trials.
Female
number |
Treatment |
# Eggs laid
on syriaca |
# Eggs laid
on incarnata |
Total #
eggs laid |
|
1 |
incarnata reared |
4 |
26 |
30 |
|
2 |
incarnata reared |
10 |
14 |
24 |
|
3 |
incarnata reared |
18 |
12 |
30 |
|
4 |
incarnata reared |
5 |
1 |
6 |
|
5 |
incarnata reared |
0 |
0 |
0 |
|
6 |
incarnata reared |
3 |
6 |
9 |
|
7 |
incarnata reared |
1 |
6 |
7 |
|
8 |
incarnata reared |
0 |
0 |
0 |
|
9 |
incarnata reared |
0 |
0 |
0 |
|
10 |
syriaca reared |
2 |
16 |
18 |
|
11 |
syriaca reared |
7 |
14 |
21 |
|
12 |
syriaca reared |
0 |
0 |
0 |
|
13 |
syriaca reared |
1 |
26 |
27 |
|
14 |
syriaca reared |
7 |
32 |
39 |
|
15 |
syriaca reared |
11 |
15 |
26 |
|
16 |
syriaca reared |
16 |
12 |
28 |
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.
- HA1=Monarch butterflies will prefer to oviposit on larval host plant
species.
- HA2=Monarch butterflies will prefer to oviposit on species other than
their larval host plant.
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).
|
Observed values: |
|
|
incarnata reared |
syriaca reared |
|
incarnata preference |
4 |
5 |
|
syriaca preference |
2 |
1 |
|
Expected values: |
|
|
incarnata reared |
syriaca reared |
|
incarnata preference |
4.5 |
4.5 |
|
syriaca preference |
1.5 |
1.5 |
Figure 1. Average number of eggs laid on A. syriaca and A. incarnata by monarchs
reared on A. syriaca and A. incarnata.
Our second set of hypotheses addressed monarch fecundity.
- Ho = Larval host plant has no affect on female monarch fecundity (number
of eggs laid).
- HA1 = Monarchs raised on Asclepias incarnata will have greater fecundity
than monarch raised on Asclepias syriaca.
- HA2 = Monarchs raised on Asclepias syriaca will have greater fecundity
than monarchs raised on Asclepias incarnata.
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).
Figure 2. Frequency histogram for number of eggs laid for 16 butterflies.
Figure 3. Average total number of eggs laid by monarchs reared on A. syriaca and
A. incarnata.
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.
- HA1=Adult female monarchs will prefer to oviposition on Asclepias
incarnata.
- HA2= Adult female monarchs will prefer to oviposition on Asclepias
syriaca.
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.
Figure 4. Average number of eggs laid on A. syriaca and A. incarnata by all monarchs
tested.
(A)
(B)
(C)
Figure 5. Percentage of eggs laid on A. syriaca and A. incarnata by (A) monarchs
reared on incarnata, (B) monarchs reared on A. syriaca and (C) all monarchs.
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)
- Monarchs in the Classroom, University of Minnesota
- Stacey Halpern and Michelle Prysby, University of Minnesota
- Karen Oberhauser, University of Minnesota
- Science Museum of Minnesota
- Selah, Johnson City, Texas
- Wilder Foundation
- St. Mark's School, St. Paul
- Minnesota New Country School, Henderson
- Loyola High School, Mankato
- Parents and families of students and teachers involved
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.aspx
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.aspx
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