RESEARCH UPDATE: Painted Lady Butterflies > The Effect of Diet on Hornworm Growth

Authors: Na Nguyen, Meredith Swann, Shuaiqian Men, Deborah Haney, and Julia Smith

Our original question was "how do painted lady butterflies develop when given host and nonhost food choices?" However, our research question had to be changed because we were unable to order painted lady butterflies in time for our experiment. We also had to give the hornworms fruit instead of leaves because the plants themselves aren’t available during winter season. So, our updated question became "how do Tobacco Hornworm caterpillars develop when given fruit from host plants versus leaves from nonhost plants?"

The caterpillars were divided into groups and fed hornworm chow (the thick gel they came with) for the control, tomato fruit, green bell peppers, eggplant, Geum (a type of flowering plant), and two different types of dried tobacco. We saw an initial increase in weight for all the caterpillars with access to nightshade fruits and two of the caterpillars that still had access to the control diet. We saw an initial decrease in weight in all the caterpillars eating the Geum, both kinds of tobacco, and ⅓ of the caterpillars eating the control. Overall, the subjects eating the eggplant and the bell pepper grew the most (although ⅔ of those caterpillars actually lost weight between the second and third weighing) and the caterpillars on the tobacco and Geum plants grew the least or died. ⅚ of the caterpillars eating the nightshade fruits lived through the experiment and ⅔ of the control caterpillars also lived. On the other hand, only ⅙ of the caterpillars on the tobacco survived (⅓ gold and 0 regular) and none of the specimens on the Geum survived.

The main challenge our group experienced during this experiment was having to adapt our original idea due to time constraints and seasonal availability. The very first version of our experiment would have used painted lady eggs, to allow us the maximum amount of control over their diet and development. However, since we only had three lab sessions available, if they had not hatched immediately we would not have been able to gather enough useful data. It then turned out that even already hatched painted lady caterpillars could not be shipped to us in time. When we switched to tobacco hornworms since they were available locally, after purchasing them it turned out their typical host plants were not available at all during this time of year. We had to find alternate methods of feeding them that might still be successful.

Our main advice for other groups is to be sure that your whole group researches the organism of interest prior to the experimental setup day and take lots of notes on the characteristics of the organism. There was a lot of information we were unaware of until we did more extensive research on hornworms. A group would have greater success if they use organisms who eat food that’s in season, or cater their project towards things that work in the winter. We were forced to feed our hornworms fruit from their host plants when their primary food is the leaves. Finally, record every change you see in the organism because sometimes information that seems unimportant becomes really valuable in data analysis and interpretation.

Unfortunately, the nature and timing of our experiment did leave more room for error. We were only able to weigh the hornworms three times, which means that our data was less resistant to smaller changes (such as an individual caterpillar having a full gut at one weighing and an empty gut at another). In addition, some caterpillars clung to substrate or had food particles stuck to them, also adding error to their weight. Inability to clean waste and replace stale or molding food could have increased mortality rate. Finally, since our sample size was small it is more difficult to determine which deaths were simply due to hornworms having a high mortality rate in general as opposed to being unable to thrive on a particular food.

We have several questions for other groups, particularly anybody who chose a similar type of experiment investigating an organism's diet. What organisms did you choose, and why? What variables did you decide to measure? Lastly, do you feel you had enough time to gain all the data you wanted?