Elaine Dunham, Michelle Prysby and Karen Oberhauser
University Scientist
University of Minnesota
St. Paul MN

  Research Projects

Abstract 

Since female monarchs tend to lay their eggs on newer milkweed plants, we were interested in studying the effects plant age had on monarch growth and development.  We reared eggs laid by five different females on milkweed plants that were classified as either "young" or "old" based on their appearance in the field.  There were no significant differences in the mass of individuals reared in the two treatments, but monarchs in the "old plant" treatment developed more slowly both as larvae and pupae.  This suggests that they compensated for the lower nutritional value in the older plants by eating more.  Larvae increased their mass exponentially as they progressed between instars, and males tended to be larger than females, although this difference was not significant.  Adults were less than half the mass of larvae just before pupation.

 Introduction

 Our observations, and those of several citizen science volunteers (see www.mlmp.org) suggest that female monarchs prefer to lay eggs on milkweed plants with new growth.  Since new growth in many plants, including milkweed, tends to be higher in nitrogen (Mattson 1980, Slansky 1993, Lavoie 2002a, MonarchLab 2002), we hypothesized that this preference may result in faster-growing or larger offspring.  We tested this hypothesis by rearing larvae on wild-collected plants that appeared to be old and young, and measuring their mass throughout their development.

Methods

We collected five to 12 eggs each from five different field-captured females, and divided the eggs from each female into two groups that would receive either young- or old-looking plants as they developed.  We started a total of 21 larvae on young plants and 23 on old plants.  All eggs were laid on the same day. 

We kept larvae in individual containers throughout their development, and gave them fresh leaves daily.  We picked wild-growing milkweed plants (Asclepias syriaca) daily, and classified them as old or young based on their appearance; young plants had no damage from herbivores, tended to be smaller, and looked fresher than old plants.  We recorded the time from egg-laying to both pupation and adult emergence for each butterfly, and weighed larvae daily after the end of the first instar.  First instar larvae were too small to weigh accurately.  We obtained a final larval mass before each larva had attached its rear pair of prolegs to the rearing container, and weighed each adult approximately 24 hours after emergence.  Monarchs were kept in an air-conditioned lab (range from 21 - 25 ºC) and exposed to natural daylight from a west-facing window during their development.

 We used t-tests to compare masses and development times of individuals in the two treatments, separating the sexes for the analyses of mass.

 Results

 Table 1 summarizes adult masses for individuals in the two plant treatments.  Even though the difference in male and female mass was not significant at the 0.05 level of confidence (Table 1), we analyzed males and females separately when comparing the effect of plant treatment in order to minimize within-treatment variation.  There were no significant differences between individuals reared on the two plant types. 

  Table 1. Analysis of adult mass. Table 2. Analysis of development time

 Since males and females did not differ in development time (Table 2), we combined the sexes to compare development time (from egg to pupation and from pupation to adult emergence) for individuals reared on the two plant types.  Individuals reared on old plants took longer to develop; this difference was significant during both the larva stage and the pupa stage. 

 Figure 1 shows the average masses of individuals of each sex on their first measurement in each instar, the final larval mass and adult mass 24 hours after emergence.  Growth throughout the larval stage was exponential; larvae increased their mass by approximately 4.3 to 5.3 times during each instar (table 3).  The rate of growth did not vary among stages (ANOVA F = 2.35, p = 0.07), although they tended to grow at slower rates later in development.  While differences between the sexes were only significant in the second instar stage (see table 3), males were larger than females throughout their development.

Figure 1. Monarch mass.  Larval masses are shown for the first day in each instar, with the exception of the last mass, which was recorded on the day before the larva hung in the pre-pupal "J" stage.  Adult masses were recorded 24 hours after emergence.  Means of 19 males and 20 females are shown.

Table 3.  Male and female larval masses (measured on the first day of each instar stage, except where noted)

 Discussion

Leaves from older plants tend to be lower in nitrogen (Mattson 1980, Slansky 1993, Lavoie 2002a, MonarchLab 2002).  We hypothesized that larvae fed older milkweed plants would become smaller adults, since nitrogen is a limiting resource for most insects (McNeil and Southwood 1978, Mattson 1980, White 1993).  However, this did not happen; there were no significant differences between individuals reared on different plant types.  While monarchs that ate plants that appeared to be older do not suffer reduced mass, they did take longer to develop.  This suggests that they may compensate for lower quality food by eating more, and thus requiring longer to develop.  Lavoie (2002) found that this was the case when larvae were fed plants that received varying amounts of nitrogen fertilizer.

 Male monarchs tend to be larger than females (Oberhauser and Frey 1999, MonarchLab 2002).  While there was a trend toward this difference in our study, this difference was not significant at the 0.05 level of confidence.  However, our sample size was small, and the large natural variation of size may mean that we could not detect this effect.  Interestingly, the mean mass of male larvae was greater than the mean mass of female larvae during each stage.  Since males did not take longer to develop than females, it appears that they may be more efficient at converting food nutrients to growth, or may be able to consume more in a given amount of time.

 Note: For a detailed study of growth of larvae fed plants that vary in nutritional quality, see (insert the two studies by Beth here).

 References

Lavoie, B. 2002. The Effects of Varying Nitrogen Supply on Common Milkweed (Asclepias syriaca) Leaf Nitrogen Content and Condition and on Monarch (Danaus plexippus) Consumption Rates and Performance. Master's thesis.  University of Minnesota.

Mattson, W.  J., Jr.  1980.  Herbivory in relation to plant nitrogen content.  Annual Review of Ecology and Systematics, 11:119-161.

McNeil, S., and Southwood, T. R. E. 1978. The role of nitrogen in the development of insect/plant relationships.  Pages 77-98 in J. B. Harborne, editor. Biochemical aspects of plant and animal coevolution.  Academic, London, UK.

MonarchLab. 2002. Sample Monarch Vital Statistics. http://www.monarchlab.umn.edu/research/VS/sample.html

Scriber, J. M. 1984. Host-plant suitability.  Pages 159-202 in W. J. Bell and R. T. Carde, editors.  Chemical ecology of insects. Sinauer, Sunderland, Massachusetts, USA.

Slansky, F., Jr., and Scriber, J. M.  1985.  Food consumption and utilization.  Pages 87-163 in G. A. Kerkut and L. I. Gilbert, editors.  Comprehensive insect physiology, biochemistry, and pharmacology, Vol. 4. Pergamon, Oxford, UK.

White, T. C. R.  1993.  The inadequate environment: Nitrogen and the abundance of animals. Springer-Verlag, New York, New York, USA.  http://www.monarchlab.umn.edu/research/VS/sample.html

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