photo from left to right:  Jody Hieb, team scientist Michelle Solensky, Shari James, team scientist Bill Calvert, Kevin Hieb

Shari James, Jody Hieb and Kevin Hieb

Scottsburg High School
Scottsburg, Indiana

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

Abstract

We investigated the effect of weather conditions relative to a storm front on monarch female oviposition. We measured temperature, humidity, and barometric pressure during a period of ten days, and recorded how many eggs were laid by several mated females on each day. High temperature appears to increase female oviposition. Females only laid eggs on days during which the barometric pressure was rising, while humidity had no clear impact on female oviposition. Our observations suggest that the weather does affect female oviposition, although additional observation would increase our confidence in these conclusions.

Introduction

In our experiment, the objective was to determine if weather affects female monarch butterfly (Danaus plexippus) oviposition. Weather factors measured in our project included temperature, humidity, and barometric pressure.

Butterflies are active during the warm parts of the day and will even bask on rocks on cool sunny mornings to warm muscles to power flight (Oberhauser and Goehring 1997). Since butterflies are ectothermic, temperature strongly affects their metabolism and growth so a warm habitat can actually enable a butterfly to develop more rapidly, up to fifty per cent faster (Stokes and Williams 1991). Butterflies need body temperatures of 85-100 degrees Fahrenheit so they can fly well. To help maintain this temperature they use the heat of the sun. On cloudy days a person may not see butterflies at all, since the butterflies can become inactive if their body temperature becomes too cool (Stokes and Williams 1991).

It is known that rainy weather will lead to fewer eggs but we were interested in whether other weather factors that are indicative of approaching storm systems also affect oviposition. The three factors measured in this study (temperature, humidity, and barometric pressure) were chosen because these are the three main variables associated with an approaching storm system (Asher, pers. comm.). These factors could also be measured easily and with a high degree of accuracy.

We hypothesized that female oviposition would increase when the weather was sunny and warmer and decrease when the weather was cooler and rainy.

Hypotheses

H₀ = Weather will have no effect on female oviposition behavior.

H_A1 = Storm systems (high temperatures, increasing barometric pressure, and decreased humidity) will increase female oviposition.

H_A2 = Storm systems (low temperatures, decreasing barometric pressure, and increased humidity) will decrease female oviposition.

Methods

For the research project, our team reared forty monarchs (from the University of Minnesota monarch lab) to the adult stage. When the monarchs emerged from the chrysalis we moved them to an outdoor screen tent (10 ft x 10 ft) that contained fresh flowers, sponges with honey water, and fresh common milkweed (Asclepias syriaca). We numbered the female monarchs to allow us to keep track of which females had mated. We checked every night to record which females had mated. We began with seventeen females and after one night, two had died leaving us with fifteen female monarchs. After all of the female monarchs mated we started to take temperature, humidity, and barometric pressure readings. We obtained these readings by checking the weather on the Internet each morning. Every night we took the milkweed out of the tent and counted the number of eggs on the milkweed. Every morning we put fresh milkweed in the tent. After we counted the number of eggs on the milkweed, we divided that number by the number of female monarchs in the cage to yield the average number of eggs laid per mated female. We continued to collect data until all of the females died. Since we had inclement weather and our female monarchs died early in the study, we repeated our study using the same methods.

Results

As previously stated, due to inclement weather, all our female monarchs died in our first trial, resulting in no data. During the second trial, females laid eggs on only two of the ten days. Of the two days on which eggs were laid, one day was warmer than average and the other was slightly cooler than average. These eggs were laid on the 2^nd warmest and the 6^th warmest days (Figure 1). These data suggest that temperature might be important, but the data is insufficient to conclude that temperature had an effect on female oviposition. There did not appear to be a correlation between humidity and female oviposition (Figure 2). On both days during which eggs were laid we noted that the barometric pressure was falling (Figure 3).

Figure 1.  Number of eggs laid per female monarch with temperature as a weather factor.	Figure 1.  Number of eggs laid per female monarch with humidity as a weather factor.
Figure 3. Number of eggs laid per female monarch with barometric pressure as a weather factor. Green arrows indicate days on which the barometric pressure was falling.

Discussion

Although our sample size is small due to bad weather and high mortality, this project looks promising for future researchers.

Our investigation suggests that weather might influence female oviposition. Storm systems with low temperatures might have a negative effect on female oviposition, while falling barometric pressure appears to have a positively affect female oviposition. It is possible that falling barometric pressure, brought about by an approaching storm, stimulates females to lay more eggs before the storm hits and temporarily reduces the opportunity for oviposition. Unfortunately, our small sample sizes prevented us from having a high level of confidence in our conclusions.

In the future we would start no later than the third week of August, helping to eliminate the possibility of the females being in diapause. We would hand mate the butterflies, ensuring all the female butterflies were mated, which would allow us to start counting the number of eggs earlier. This would allow us to collect data earlier in the life of the butterflies and thereby increase our sample sizes.

For future research we would also include cloud coverage as another weather variable (for reasons previously cited) and we would hand feed the butterflies to help ensure that the butterflies are nutritionally healthy. To do this method of feeding we would place plastic lids, upside down (the lids have ridges and can serve as a small feeding trough), to hold honey water. The butterflies are then placed with their front feet dipped in the trough-like area. The butterflies can be lined up side by side to begin feeding. (James, pers. comm.) This is a simple method to keep track of which butterflies are eating. If the proboscis does not unwind then place a probe or a pin in the loop of the proboscis and pull the pin away from the head so the proboscis is extended and touches the honey solution (Monarch Watch 2001). Allow the butterflies to feed for fifteen minutes.

Acknowledgments

We would like to thank the following people who helped assist us on our project:

Shari James’ family who raised the larvae to the adult stage
National Science Foundation
University of Minnesota (Research Institute Staff especially Michelle Solensky and Bill Calvert)

Literature Cited

Asher, Phil. Personal communication. January 4, 2001.

James, Bill. Personal communication. October12, 2000.

Monarch Watch. 2001. http://www.monarchwatch.org/rear/rear4.htm

Oberhauser, K. and L Goehring. 1997. Monarchs in the Classroom. University of Minnesota, St. Paul, MN. p.31.

Stokes, D. and L., and E. Williams. 1991. The Butterfly Book. Little, Brown and Company, Boston, MA. p.9.

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