Adopt-a-Bug!


Paul C. Johnson
Department of Natural Resources
University of New Hampshire
Durham, NH 03824


Congratulations! You have just adopted a cute and cuddly milkweed bug. Although we feel sure that your bug will provide you with weeks of enjoyment and fascinating entertainment, along with it comes a heavy responsibility. Its quality of life, and in fact its very survival, are in your hands. Follow the directions below carefully and they will help you become a good adopted parent. We feel sure you are up to the challenge.

Milkweed Bug Life History: The large milkweed bug, Oncopletus fasciatus (Dallas) (Hemiptera:Lygaeidae), exhibits gradual metamorphosis. Eggs are yellow, about one mm in diameter and change gradually to an orange color as incubation proceeds. First instar nymphs are about 1.5 mm long (tip of head to tip of abdomen). They grow through five nymphal instars, molting their old exoskeleton during transition between instars. As their name implies, they normally feed on milkweed seed.

Materials

One medium sized plastic vial with cap.

One strip of metric graph paper about 5 cm x 2 cm.

Several milkweed seeds or uncooked, unsalted, shelled sunflower seeds.

One microcentrifuge tube with cap.

One cotton ball.

One milkweed bug egg.

Procedure

Prepare an appropriate habitat for your egg -- a plastic vial with some milkweed seeds, graph paper strip and a water supply. Add a few drops of water to the dispenser as needed. The cotton should be kept moist, but not saturated. If the cotton becomes dirty or moldy, get a replacement from your teacher. Keep the vial capped except when adding water. Be careful that you bug does not escape while you are adding water. They are quite active, and very delicate. If you must handle them, a soft camel hair brush can be used to manipulate them.

Mark the cap of the vial with your initials using a wax marker or fine tip pen.

Insert the paper strip into the bottom of the vial.

Place several milkweed seeds or uncooked, unsalted, shelled sunflower seeds into the vial.

Carefully cut off about 4 mm of the tip of the microcentrifuge tube using a razor blade.

Stuff the cotton ball into the microcentrifuge tube, making sure the cotton is tightly packed into the tip of the tube and fills most of the space inside the tube.

Saturate the cotton with water and close the microcentrifuge tube cap.

Have the teacher place a milkweed bug egg into the vial.

Rest the microcentrifuge tube in the opening to the vial (tip down), and screw the vial cap on to hold it in place.

Check your adopted bug at least once daily, and measure it at least once a week and whenever it molts. Examine the habitat for discarded exoskeletons (i.e. has it molted?). Remove the exoskeleton and measure the length of your newly molted bug (tip of head to tip of abdomen in mm) using the metric graph paper strip. Record the date and length on the datasheet provided and the life stage (place a check in the appropriate column).

You should make a sketch of your adopted bug for each life stage (use the appropriate box below). This is best done under a microscope and time will be made available for sketching. Begin with a sketch of the stage you have adopted. If possible sketches should be in color. You may want to retain a copy of this keepsake for your family album. You will probably not be able to sketch every stage, but should have several sketches during the course of your bug's development. They are intended to develop your powers of observation and make you really look at your bug.

If your bug should die, give it a proper burial with appropriate ceremony and apply to your teacher for another adoption. Record the date of death, length and the stage of development at the bottom of your datasheet. Begin a new datasheet for each bug adopted.



Adopt-a-Bug! Life Stage Sketch Sheet
Nymph 1









Nymph 2









Nymph 3









Nymph 4









Nymph 5









Adult












Adopt-a-Bug! Data Sheet
DATE LENGTH (mm) N1 N2 N3 N4 N5

ADULT

DEATH              

The formatting of this table may not work on all web browsers.


Population Growth for the Milkweed Bug

Each of your milkweed bug habitats constitutes one part of a group of milkweed bugs that has been followed from birth to adulthood, recording the number surviving at regular intervals. The collective data for the class may be used to calculate the growth rate of the population.

Since milkweed bugs reproduce only as adults, we can ignore the age classes between the egg and the adult, and deal only with the time period from egg to egg. We will use the average time in days from egg to adult for each milkweed bug that survived to the adult stage and then add one week to allow for reproductive maturity to get the time period from egg to egg.

The average female milkweed bug lays 50 eggs. Since roughly half of the milkweed bugs in our population are males, and only the female offspring will lay eggs in subsequent generations, we should reduce this figure to 25. Since the sex ratio is 1:1, we can calculate the total population in any subsequent generation by multiplying by two. To compute each successive generation, multiply the number of females in the preceding generation by the number of female offspring per adult female that survives to lay eggs. Remember, not all eggs will survive to the adult stage. Use the proportion of milkweed bugs that survive to the adult stage to adjust the number of milkweed bugs contributing offspring to the next generation.

N(t+1) = N(t) * A/2 * 25



Where: N is the number of eggs in a generation, t is the generation number, A is the proportion of eggs surviving to the adult stage (divided by 2 to give the number of females), and 25 is the number of female eggs produced by each female that survives.

 

Question?

How many eggs would there be in a population of your milkweed bugs after two generations if you started with 50 eggs and only 6 survived to the adult stage?

Answer:

Compute A, the proportion of eggs surviving to the adult stage.

A = 6/50, so A = .12

Since N(0) = 50 (the initial number of eggs), N(1) =50 * 0.12/2 * 25. Therefore N(1) = 75 eggs after one generation.

Since N(1) = 75, N(2) = 75 * 0.12/2 * 25. Therefore N(2) = 112.5, or about 113 eggs.

After five generations? After 10 generations?

Plot a graph of your population growth starting with 50 eggs and following it for 10 generation. Shade in each column above a generation (along the x-axis at the bottom of the graph) to the level of the number of eggs in that generation. You will have to fill the y-axis (along the left margin of the graph) with appropriate labels. Remember to use your class average for the proportion of eggs surviving to the adult stage.


Adopt-a-Bug! Population Growth

^Y / X>
0 1 2 3 4 5 6 7 8 9 10

 



Food for Thought

Can you see an easier way to calculate the number of eggs in the population after several generations?

What would this growth rate mean in the natural habitat of the milkweed bug?

What forces might act to prevent such population growth under natural conditions?

What would happen if some eggs were reared under cold conditions while others were reared under warm conditions?


For information
Paul C. Johnson
Department of Natural Resources
258 Spaulding Life Sciences Building
University of New Hampshire
Durham, NH 03824
Phone: (603) 862-1717
FAX: (603) 862-1713
e-Mail: pcj@christa.unh.edu


Last modified on August 18, 1997