Growth and Survival of Giant Swallowtail Butterflies
on Three Different Host Plants
Colin Hughes, Faculty Advisor
Annemarie Jameson, Graduate Assistant
Mercy Rivera, Undergraduate Assistant
Henry Pierce and Barbara Prado, Teachers
Julio Aponte, Brittny Capron,
Chaquana Lee and Jeevan Rampersad
Butterflies are curiously beautiful insects that continue to fascinate people worldwide. The thing that intrigues people most is the process of transformation called metamorphosis. This occurs in the shift of phases from caterpillar to pupa to butterfly (1). The egg is the first stage in the butterfly life cycle. The butterfly usually recognizes the right plant by a combination of sight and smell (1). They have chemoreceptors, cells that respond to" taste" and "smell", both on their antennas and the bottom end of their legs. Most species lay their eggs on a plant that the newly hatched caterpillar will eat (host plants) (1). For example, the Gulf Fritillarys host plant is the passion vine (Glassberg et al. 2000). When the caterpillar hatches it first consumes its outer shell (1). Then it goes about eating as many leaves as it can in order to pupate (1). The pupa stage, which seems lifeless, is when the caterpillar is encased in its hard outer shell or chrysalis; however, inside a surprising metamorphosis is taking place. The pupa splits open and an adult butterfly emerges.
The Giant swallowtail butterfly is also known as the "orange dogs" because they eat leaves from the orange tree (2). The caterpillars eat from the Rutaceae plant family including Citrus, Prickly Ash, and Hop Tree (3,4). Its caterpillar has a high survival rate because of their bird dropping mimicry (5). The White Sapote , Wild Lime, and Citrus are some of the plants they use to lay eggs on in south Florida (5).
To determine if butterflies develop differently on different host plants, we studied Giant swallowtails and three of their host plants, White Sapote, Wild Lime and Citrus.
Questions and Hypotheses
Question 1: Do butterflies survive better on different host plants?
Hypothesis 1 null:
Giant Swallowtail butterflies will not have different survival rates on different host plants.
Hypothesis 1 alternate:
Giant Swallowtail butterflies will have different survival rates on different host plants.
Question 2: Do butterflies grow better on different host plants?
Hypothesis 2 null:
Giant Swallowtail butterflies will not have different growth rates on different host plants.
Hypothesis 2 alternate:
Giant Swallowtail butterflies will have different survival rates on different host plants.
For the purpose of our experiment we collected Giant swallowtail eggs from the arboretum in University of Miami. We searched their host plants Wild Lime (WL), White Sapote (WS), and Citrus (C), specifically the young leaves on the trees. Spotting the eggs was simple due to their bright color (reddish-orange). Eggs were found on WS and WL, but no eggs were found on C. Therefore, we switched some of the eggs on WS and WL to C. The remaining eggs found on WS or WL were kept on that host plant. Then we placed the eggs with its leaf in the petri dish and labeled them accordingly. For example, the dishes with White Sapote leaves were labeled with WS and the date the caterpillar hatched. We did this to keep track and compare their survival and measurements (weight and head capsule width) after seven days.
We did many things in order to keep our larvae alive. Before handling the caterpillars, we washed our hands. Since caterpillars are very delicate, strong smells, chemicals, hand lotion or even bacteria on your hands can kill them. In addition to this, we cleaned the petri dishes every other day with Kim Wipes so that the caterpillars would not die from chemicals in its body waste. They need a lot of food to survive and grow. Every other day, we checked the food supply for each treatment group (WS, WL, and C) and added fresh leaves if necessary. We also had to keep them properly moistened. To keep them from drying out, we added wet (4-5 drops) paper towels (1" x 2") to the bottom of each petri dish. We placed the petri dishes in three different copy paper boxes (labeled WS, WL, C), cut a square piece from each side and inserted a piece of foil to accommodate the light source. Caterpillars need light to help them grow. To provide them with a source of light, we used General Electric 33watt florescent lights and placed them gently on the top of the foil. These lights were connected to an Intermatic - Heavy-Duty timer, which provided them with 20 hrs of light a day. We then covered all of the boxes with lids. While we were taking proper care of our caterpillars, we began to collect our data.
On the seventh day after the Giant swallowtail eggs hatched, we measured the width of the head capsule and the weight of the caterpillar. We did this to see if there was any difference in growth of the caterpillars raised on different host plants. Because caterpillars are easily injured, we gently loosened them from the petri dishes where we were raising them and one at a time we weighed them. We used a precalibrated electronic scale which measured the weight within 1/10,000 of a gram. We also measured the width of the head capsule in millimeters by holding a ruler above the head. We were also careful of our caterpillars because they are very delicate. To see if the caterpillars survived differently on the different host plants, we used a Chi-squared tests. Three t-tests were used to test for differences in growth (weight and head capsule width) on the different host plants.
We compared whether Giant Swallowtail caterpillars survived differently on different host plants. While observing the caterpillars we noticed that some of them did die. The average survival rate for caterpillars feeding on Citrus is 0.8, Wild Lime is 0.85, and White Sapote is 0.94 (Figure 1). Analyzing these data we found out that it was all practically the same, based on our p-value which is 0.91. Therefore, there was no significant difference in the survival rate of the caterpillars that were fed on different host plants.
In this experiment we were measuring the growth of the caterpillars by measuring the head capsule and weights to see if the growth is different as the caterpillars feed on different host plants. There were differences in the results for both head capsule width (Figure 2) and for weight (Figure 3). We will introduce our results for growth by discussing pairs of observations.
We observed that the caterpillars on the White Sapote plant had larger head capsules and weighed more than the caterpillars on the Wild Lime. The average head capsule size of the caterpillars grown on the Wild Lime was 1.27 mm while the average size of the head capsule of caterpillars grown on White Sapote was 2.13mm (t-test p value <.0001). Similarly weight was greater on White Sapote (.1463 grams) than Wild Lime (.0284) (t-test p value < .0001).
The growth of Giant Swallowtail caterpillars feeding on Citrus and White Sapote was also compared using t-tests. The head capsules of the caterpillars on White Sapote were bigger than the ones on Citrus. For Citrus the average width was 1.68 mm and 2.1 mm for White Sapote. We found that the p-value was 0.02. White Sapote also produced heavier caterpillars than Citrus. The average on Citrus was 0.093 grams and 0.146 grams on White Sapote. The p-value was 0.021. There was a significant difference between the two averages. Giant swallowtail caterpillars grow larger on White Sapote than those on both Citrus and Wild Lime.
Finally, we compared growth of Giant Swallowtail caterpillars on Wild Lime than on Citrus. For the head width, our results showed that Citrus rears bigger caterpillars than Wild Lime (wl=1.27mm, c=1.68mm). The head width p value is p < .001 . Our results for the weights shows that caterpillars reared on Citrus are heavier (wl=.028g , c=.094g). The p value is p < .001. Caterpillars grow better on Citrus than on Wild Lime.
Every comparison we made of Giant Swallowtail growth on different host plants had a p value < .001, which means we could reject the null hypothesis that states that there is no difference between the average weights and head capsules of the caterpillars reared on the two plants. The best plant was White Sapote, with the worst being Wild Lime (Figure 3).
Survival of larvae is important to butterfly mothers. When butterflies lay their eggs, they lay them on plants that the larvae will hopefully eat and survive on. Based on the Chi-Squared test that we performed, we could not reject the null hypothesis that survival would be the same on different host plants. We conclude that Giant Swallowtail caterpillars have similar survival rates on different host plants. Some variables that could have affected the survival rates were disease and environmental stress (e.g. moisture, temperature, and handling). While there was no difference in survival rates of caterpillars on three host plant species, we were also able to compare growth on these plants.
Our hypothesis that the caterpillars would grow differently on the different host plants was correct. The caterpillars on the White Sapote grew quicker than the caterpillars on the Citrus and Wild Lime plants. We know that the caterpillars grew faster based on their weight and head capsule width which were measured 7 days after they hatched. The faster and more caterpillars eat, the earlier they pupate. It is better for them to pupate faster to prevent diseases or to avoid getting eaten by predators. Also, the bigger the caterpillars are the greater the size of the chrysalis and butterfly. It is more advantageous for the butterfly to be larger because this may allows them to produce more and healthier eggs.
Giant swallowtails lay their eggs on a variety of host plants and we measured growth of caterpillars on three of them, the Wild Lime, Citrus, and White Sapote. Of these plants by far the most abundant plant in our area is the Wild Lime. At this time the Citrus plants, which are not native, are being eradicated in South Florida because of a disease outbreak called Citrus Canker. White Sapote is a newly introduced exotic fruit tree to our area and very few exist. Although our research indicates that Giant Swallowtail caterpillars grow better on White Sapote and Citrus than on Wild Lime, Citrus and White Sapote are rare in South Florida at this time. It is a good thing that the Wild Lime is the most abundant host plant in our area so the Giant Swallowtail will have a host plant on which to lay their eggs.
Why do butterflies lay their eggs on different host plant species? Some butterflies lay their eggs on many host plants even though their caterpillars grow best on one particular plant. Maybe female butterflies cant tell the difference between the different host plants for the caterpillars, but their chemoreceptors do. Butterflies touch and smell plants and if the plant they are on is a suitable host they may lay their eggs on it. Another reason why butterflies might not lay their eggs on the plant they land on first is because they might hold out and try to see if they can find another host plant better than the one they are on. Holding out until the butterfly finds the best host plant, however, may not be best strategy for the butterfly. If the best host plant is rare, than the butterfly might have a hard time finding that best plant. A butterfly that cant find the right plant cant lay its eggs at all. How and why butterflies pick their host plants is really interesting.