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Do Monarch Larvae Prefer Young or Mature Milkweed Leaves?

Beth Zaiken
Willow Creek Middle School
Rochester, MN

Beth with her insect fair project display (Photo: Beth Zaiken)

Abstract

I gave 7 monarch larvae a choice of ingesting newly sprouted common milkweed leaves or mature leaves in a controlled environment and recorded their daily consumption. The purpose of this experiment was to determine whether or not monarch larvae would display a diet preference.

In the end, 48% of the larvae tested consumed new leaves, 14% consumed mature leaves, 19% did not show a preference, and 19% were not active due to molting. Only 7 larvae (A-G) were tested (although their milkweed consumption was observed daily for a total of 36 experimental repetitions). Individual larvae results were as follows: 4 preferred new leaves, 1 preferred mature leaves, and 2 did not show any observable preference.

I believe testing a greater number of larvae and observing their preference for only one day per larvae would increase the accuracy of my results. My data does seem to indicate that monarch larvae do prefer the newly sprouted milkweed leaves over the more mature ones. It is interesting to note that individual larva were sometimes observed on both sides of the experimental container but only ate the new leaves.

Observation

I have been feeding my monarch larvae a mixture of very newly sprouted, small common milkweed leaves and large mature leaves for many summers now. It seems that they will eat both types, but I've wondered if they might actually have a preference. In mid-August and September, I had consistently observed an unusually high number of monarch butterflies in a field near my home (just north of Apache Mall). The weeds and grass in this field are typically cut in mid-summer so the milkweed at the time of my observations was relatively young.

Question

In my experiment, I attempted to determine whether or not larvae prefer new or mature milkweed leaves. Of course this refers to raising them in captivity. So the question I tried to answer was..."If given a choice of new vs. mature milkweed leaves, which type do monarch larvae raised in captivity prefer?".

Hypothesis

If captive monarch larvae are given a diet choice between new (6 to 10 cm) milkweed leaves, or mature (15 cm or greater) milkweed leaves, I thought that they would eat the new leaves.

Experiment

In my experiment, I used monarch larvae in various stages of molting. Small larvae (1st - 2nd instar) were placed in the center of petri dishes and large larvae (3rd - 5th instar) were placed in the center of a 7 x 11 x 2 in white plastic container covered with a screen. To the sides of each container, I placed equal amounts of fresh (picked daily) milkweed leaves cut to the same size. One side of each container held only pieces of new milkweed leaves, while the other side had only pieces of mature milkweed leaves.

The maturity of milkweed leaves, new (6-10 cm) vs. mature (>15 cm) was my experimental variable. Each larvae was placed directly in the center of each container, in a buffer zone where no milkweed was present. Each individual larva was required to locate a pile of food (new or old). After 24 hours, I recorded how much of each type of milkweed was eaten. No variation in lighting or background color (white) was present during my experiment.

This experiment was repeated with 7 larvae in various instars, 36 times. My experiment was replicated as many times as possible before the first frost killed my source of fresh milkweed leaves. Freshly picked (daily) milkweed was one of the control requirements for my testing. The size of the milkweed pieces, lighting, background color, and equal amounts of milkweed in each corner of my testing containers were other factors that I kept the same in each experiment.

Analysis of Results

(Photo: Beth Zaiken)

Daily experiment results on each larva (A-G) were recorded on my Larvae Preference Data Spreadsheet. My results were then compiled into percentages and visually represented in a pie chart according to the following categories:

  1. more new leaves consumed
  2. more mature leaves consumed
  3. equal amounts consumed
  4. molting

Conclusion

My captive raised larvae clearly seem to prefer young milkweed leaves over mature ones. This conclusion is drawn from the fact that 48% of the larvae tested consumed the new milkweed leaves, 14% ate mature leaves, 19% showed no preference, and 19% of the larvae on a given day were not consuming any milkweed because they were molting.

Next Time

I would like to see my data reproduced by another researcher and compare our conclusions. If I were to repeat my experiment, I would do so in August, perform 100 repetitions, and compile data on 1st-2nd instar larvae and 3rd-5th instar larvae separately.

I am specifically interested in this research because of my observation of a field near my home. This field is mowed mid-summer and by late summer, the milkweed regrowth is predominantly new plants. I wonder if timed mowing of milkweed fields could possibly affect the number of surviving larvae and ultimately, monarch numbers overall.

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