Abstract  |  Introduction   |  Hypotheses  |  Methods   |  Results  |  Discussion  |   Research Projects

RESULTS

We defined preference as the plant that each individual larva consumed the most of. In most cases, the larva only ate from one plant type. We excluded larvae that didn’t eat anything from the analysis. For each two way comparison we answered three questions:

1. Did the larvae eat more of (prefer) the plant that they first contacted?
2. Which plant did they prefer?
3. Once the preferred plant had been chosen, how much of it was eaten?

We used contingency tables and Chi² tests to determine the significance of fidelity to the first plant contacted and preference, and t-tests to determine the significance of differences in the area of leaf material consumed.

Young Common versus Old Common

1. Ten larvae initially contacted old common (OC) milkweed, and 90% of these stayed on that plant. Ten larvae initially contacted young common (YC) milkweed, and of these 100% stayed on that plant. This difference was not significant. (Fig. 1 YC vs. OC, Chi-square=1.06, p > 0.05)
2. Eleven larvae preferred young common where only nine preferred old common milkweed. This difference was not significant. (Fig. 2 YC vs. OC, Chi-square=0.2, p > 0.05)
3. Larva consumed a greater area of young common than old common milkweed. The results of this test were significant. (Fig. 3 YC vs. OC, t=7.2, p < 0.005)

Figure 1. The proportion of the larvae contacting each plant type first that showed a preference for that type. Sample sizes are shown on the bars. Only larvae that ended up showing a preference are shown.

Young Swamp versus Old Swamp

1. Six larvae initially contacted young swamp (YS) milkweed and also showed a preference (two additional larvae contacted young swamp milkweed, but then did not eat any leaf material). Of these six, 83% stayed on that plant. Seven larvae initially contacted old swamp (OS) milkweed (7), and 29% of these stayed on that plant. This difference was not significant. (Fig. 1 YS vs. OS, Chi-square=4.03, p > 0.05)

2. Ten larvae preferred young swamp where only 3 preferred old swamp milkweed. This difference was significant. (Fig. 2 YS vs. OS, Chi-square=3.8, p < 0.05)
3. Larvae consumed a greater area, on average, of young swamp than old swamp milkweed. The results of this test were significant. (Fig. 3 YS vs. OS, t=5.0, p < 0.005)

Seven of the twenty larvae tested in this group did not consume any milkweed, and five of these did not make any contact with either leaf.

Figure 2. The proportion of larvae that preferred (ate more of) each type of plant after a 60 minute test period.

Young Swamp versus Young Common

1. Seven larvae initially contacted young swamp milkweed and also showed a preference (one additional larva contacted young swamp milkweed, but then did not eat any leaf material). 43% of these seven stayed on that plant. Seven larvae initially contacted young common milkweed and also showed a preference (three additional larvae contacted young common milkweed, but then did not eat any leaf material). Of these, 100% stayed on that plant. This difference was significant. (Fig. 1 YC vs. YS, Chi-square=8, p < 0.05)
2. Eleven larvae preferred young common where only three preferred young swamp milkweed. This difference was significant. (Fig. 2 YC vs. YS, Chi-square=4, p < 0.05)
3. Larva ate more young common than swamp milkweed, on average. The results of this test were significant. (Fig. 3 YC vs. YS, t=7.24, p < 0.005)

Six of the larvae in this test group did not consume any milkweed, and two of these did not make any contact with either leaf.

Figure 3. The mean number of mm² of each plant type consumed in 60 minutes.

DISCUSSION

In the common versus swamp milkweed tests, we found that the common milkweed was preferred. This was not simply because they were more likely to contact common milkweed first. Seven larvae contacted each leaf type first, but while all seven stayed on the common milkweed, four of those that initially contacted swamp milkweed ended up preferring (eating more) common milkweed. Not only did the larvae prefer common to swamp milkweed, but those that preferred common milkweed consumed a larger area than those that preferred swamp milkweed, on average. Thus we can reject the null hypothesis with respect to milkweed species.

When given the choice of an old or young leaf of the same species, we did not see significant differences in fidelity to either age. 83% of those that contacted young swamp milkweed ended up preferring this leaf, while only 29% of those that first contact old swamp milkweed preferred the old leaf, but this difference was not significant at the 95% confidence level. Values for young and old common milkweed were 100% and 90%, respectively. There was also no difference in preference for young vs. old common, but larvae preferred young to old swamp milkweed. After making their choices, larvae tended to eat a larger area of the younger leaves of both species. We cannot reject the null hypothesis with respect to age in the case of common milkweed, even though the larvae tended to eat a larger area of young leaves. We can, however, reject the null hypothesis with respect to age of swamp milkweed plants; larvae tended to prefer young swamp milkweed plant to old.

It does not appear that larval preference is determined simply by being able to sense one plant more easily than the other one. In all tests, there were no differences in the proportion of larvae that first contacted either plant.

Roxanne after being asked if she’d
prefer old or young swamp milkweed

Back to Top  |  Interactions with Milkweed  |  Other Research Topics  |  Site Overview