Monarchs in the Classroom

Monarch larvae are raised in labs in controlled temperatures to regulate the speed of their growth. The different temperatures also affect their development and coloration. Larvae were raised at three different temperatures: warm 22°C to 32°C, control 17°C to 27°C, and cold 6°C to 16°C. The larvae were closely monitored and measured for the duration of their growth and using a presence/absence method, their percent color was measured. The metabolic rates of the larvae increased in relation to the temperature surrounding them, thus the larvae increased in length, mass, and instar at a faster speed at a heightened temperature and at a decreased speed at lessened temperatures. The temperature has a direct influence on the percent of black color and light color present on larvae. The average black color was 65% in the cold treatment, 29% in the warm treatment, and 49.5% in the control treatment. The data has been graphed with a line of best fit. The line of best fit has a mean square error of 6.82x10-35, or too small to be significant.

The purpose of this project was to study the effects of temperature on the coloration of the Danaus plexippus (monarch butterfly) larvae and the development (ie: mass, length, speed of growth, and mortality at different stages) from egg stage through emergence as adults.

Although very little work has been done which studies the coloration of monarchs in relation to the temperature, previous work regarding development in different climates has shown that cold climates slow the metabolic rate of larvae and adult monarch. The first hypothesis for this project was that the coloration of larvae in a cold climate would be darker than that of larvae in a warm climate since, dark colors absorb light and warmth to maintain warmer body temperature and in a warm climate, excess warmth from light is unnecessary, so the coloration would be lighter than that of larvae in a cold climate. The second hypothesis for this study was that the development of the larvae would be slowed in a cold climate and would be sped up in a warm climate. The mortality rates would be higher in both the cold and warm climate due to extream temperatures. To keep a stable perspective, there was a control climate.

Monarchs, as well as all other moths and butterflies, go through complete metamorphosis. The life cycle of the monarch is divided into four distinct stages: egg, larva, pupa, and adult. The first stage of development is the egg stage, which lasts for four days under ideal conditions. Once the monarch hatches from its egg, it enters into the larva stage. The larva period is divided into five different stages called instars, and a second level instar can be seen in Figure 1. An instar level shows how many times a larva has molted and can be distinguished by comparing the size of the head capsule to tentacle length. Since a larva cannot grow excessively with the protection of its cuticle, it must molt frequently as it develops. This can be seen in Figure 2. After molting for the last time and reaching the fifth instar level, a larva creates a silk pad then pupates to enter into the pupal/chrysalis stage of development. A monarch remains in chrysalis for up to two weeks under ideal conditions. During its tenure in a chrysalis, a larva’s inner organs and outer appearance rapidly change into those of an adult butterfly (Kuda and Oberhauser).

Figure 1	Figure 2

A second instar	A fourth instar molting to a fifth instar

The coloration of monarch larvae varies in unique patterns of black, white, and yellow. As a larva develops, its color becomes more vivid and defined (Kuda and Oberhauser). Most larval coloration is one-half black, one quarter yellow, and one quarter white. However, different variations have been known to exist. For example, a coloration pattern consisting of black and white has been observed (Oberhauser). Those particular larvae with that color pattern are called "zebra" larvae (Solensky).

No research has yet been done that quantifies the color differences between larvae in warm temperatures and larvae in cold temperatures. However, color differences between climate larvae have been noted. Research performed by Dr. David James in New South Whales, Australia, proved that the larvae adapt to their environment by changing their colors and thus manipulating their body temperature. Although he did not quantify the color differences, Dr. James gave reason for their existence (James).

It was observed at the beginning of this project that, when moving larvae from a cold climate (used to slow metabolism and development in the monarch lab), that coloration was significantly darker than that of the larvae that remained in the room temperature (Solensky and Prysby). From this, the question was raised as to why those larvae were darker and whether larvae in warm temperatures would have lighter coloration. This developed into the question proposed by this study.