Food preferences of Danaus plexippus larvae
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Roxanne Kuerschner
from left to right: Roxanne (teacher), Dan, Jessie, Nash, Shoe (teacher), Nicole St. Peter High School Abstract | Introduction | Hypotheses | Methods | Results | Discussion | Research Projects We tested the food preference of the Monarch butterfly (Danaus plexippus) on the two primary species of milkweed in Minnesota: swamp milkweed (Asclepias incarnata) and common milkweed (Asclepias syriaca). Individual leaves were cut into 100 mm2 pieces. One square of each variety of milkweed was placed in a container at a distance of 5 centimeters (cm) from each other. An individual third instar larva was entered into each chamber facing in a neutral position. The caterpillars in each treatment were allowed to feed undisturbed for a period of one hour and then removed. We then measured the percent of eaten leaf material by placing the partially eaten leaf on millimeter-sized graph paper. The treatments tested the larval preference for old leaves versus new leaves in each of the two varieties of milkweed, as well as their preference for common versus swamp milkweed. The data suggest that young leaf material is preferred over old in each of the two varieties, and common milkweed is preferred over swamp milkweed. In our experiment, the objective was to determine if Danaus plexippus larvae have a preference in food given the choices of common and swamp milkweed. Our experimental set-up consisted of three groups each with twenty third instar larvae. Each larva had an individual container in which there were two pieces of milkweed. The condition and species of the milkweed varied in the three groups. The larvae were given the choices of common milkweed (Asclepias syriaca) versus swamp milkweed (Asclepias incarnata), young common milkweed versus old common milkweed, and young swamp milkweed versus old swamp milkweed. Ho=There will be no difference in preference for milkweed age or species. HA1=Larvae will prefer young plants to old plants. HA2=Larvae will prefer common milkweed to swamp milkweed. For the research project, our team designed a food preference test. In the experiment, we tested which type of milkweed the larvae preferred, either common or swamp milkweed. In addition, we tested how plant age affected preference. We collected all leaves on the day of the experiment from wild plants. "Old plants" were partially yellowed, and were beginning to senesce. "Young plants" had green leaves and appeared to be in better condition than old plants. For the experiment, we placed sixty third instar larvae into individual plastic cups with two food choices. There were a total of three groups with twenty larvae in each group. One group was given a choice of young and old common milkweed. Another was given young and old swamp milkweed, and the last group was given common and swamp milkweed. To make the milkweed samples, we used a 100 mm2 paper punch to obtain samples of equal area. We handled the samples with latex gloves to avoid biases due to handling. The food choices were pinned with insect pins on both sides of the larva, five centimeters from each other. The larva was initially placed in the plastic cup facing away from both leaf samples to remove any bias.
To determine their preference, we gave the larva one hour to eat. We then collected two types of data. We observed which of the food choices was first eaten by the larva. To do this we would simply watch the cup and mark down which type of plant was the first to be eaten. We then covered up the container with a petri dish lid for the remainder of the sixty minutes and let the larva eat undisturbed. At the end of this period, we measured the area of each leaf that had been consumed.
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