University of Minnesota

How Does the Moisture Level of Milkweed Affect the Growth of Caterpillars?

McGuire Jr. High School
Lakeville, MN


The purpose was to see how the caterpillars grow in response to different moisture levels of milkweed. I put the leaves in a box with two sides: dry leaves in one side, leaves with 2 spritz of water on the other side. We placed caterpillars in each side. Then we recorded length of caterpillars in cm and recorded observations everyday. We placed the box in back of the science room by the window. We checked the box daily for changes in the caterpillars.

The caterpillars with 2 spritz of water and the caterpillars with no spritz of water grew the same length, in general. Problem: the caterpillars went into pupae unexpectedly. Also, being close to the window, the sun may have dried out the leaves. The test ran smoothly, and we found out that the moisture level of milkweed doesn't affect the caterpillars' growth.


We believe that the group of caterpillars with two spritz of water will grow the largest because caterpillars eat moist leaves in the wild. We don't want the leaves to get too wet for they would grow mold and if they were too dry the leaves would die. In conclusion the caterpillars will grow larger with two spritz of water, because they eat moist leaves in the wild and and two spritz isn't too wet or too dry compared to what they eat in their natural habitat.

Background Information

There are five types of milkweed in the Northern United States: Showy, Swamp, Common, Whorled, and Butterfly milkweed. It grows from 60-200 centimeters in places like ditches, fields and swamps. Milkweed makes the caterpillars grow 2,700 times larger than when they first emerge from the egg. After the caterpillars have eaten the milkweed, it will grow back in a few weeks so the caterpillars can always go back to a certain spot to get food. If milkweed is too dry it will die. If it is too wet it will grow mold. Two spritz of water is the perfect moisture level for the caterpillars since that is about how much moisture there would be in the wild. Milkweed is moist in the wild because of rain and morning dew. Since humans need about eight cups of water each day, we think that caterpillars might need water also.


  • one box split into two sections
  • six milkweed leaves each day
  • two caterpillars
  • spray bottle
  • paper towels
  • ruler
  • data chart


  • number of leaves
  • number of spritzes to each leaf
  • all larvae start around same size or instar
  • moist towels in each side (A & B)

Dependent Variable

The growth of each caterpillar

Independent Variable

The moisture level of the leaves


  1. Get box with two equal sides
  2. Put wet paper towel in both sides
  3. Put dry leaves on one side
  4. Put leaves with 2 spritz of water on the other side
  5. Put one 1.3 cm caterpillar in each side
  6. Record length of caterpillars with ruler in cm
  7. Record observations of caterpillars by size and body features
  8. Put box back in the back corner by the window
  9. Feed caterpillars everyday
  10. Clean and check box daily


Our hypothesis ended up being wrong. The caterpillars with zero spritz, and the caterpillars with two spritz of water on their leaves actually grew about the same size. This information can be seen on the graphs.

(Photo: McGuire Jr High School)
(Photo: McGuire Jr High School)

There were some unusual occurrences. Our first set of caterpillars pupated earlier than expected, forcing the experiment to be tried again. In the second experiment, we got results though the caterpillars pupated around the sixth day of observations.

If we did this experiment again, we would add one more independent variable, the amount of light the caterpillars receive. We believe this would make the conditions even more like their natural habitat for the caterpillars.

The experiment went very well. We learned that although caterpillars are very different than us, they still need some of the same things as humans. The experiment was done to see if the moisture level of the leaves would effect the caterpillars' growth. Things ran smoothly and we found that the moisture level of milkweed does not affect the caterpillars' growth.

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