PURPOSE:  To analyze the parasite-host and predator-prey relationships in a simple plant and insect food chain.

MATERIALS: Field -- large plastic bag, wooden stake, 1.8 to 2 m twine, index card, pencil, suitable winter clothing.  Lab -- hand pruner or paring knife with glove, paper towels, dissection needle, forceps, hand lens.

INTRODUCTION:  If you look closely at goldenrod plants in the fall or winter, you will see enlarged areas called galls on the stems of some plants.  Round or elliptical galls are formed around larvae of insect parasites which feed on the goldenrod plant host.  Sometimes insect predators lay eggs in galls which contain parasite larvae; the predator larva then eats the parasite larvae as prey.  Finally, vertebrate predators sometimes open the galls in winter, and eat the parasite or predator insect inside.  You can see that the smooth gall walls hide a world of insect interactions as complex as any soap opera!

In this activity you will go outdoors to collect goldenrod plants.  You will calculate the rate at which goldenrod plants are parasitized by insects.  You will use a key to identify and count all the insects in the galls.  Then you will use the fact that predator populations are usually smaller than prey populations to identify the predator and prey insect species in the goldenrod community.

PREDICTIONS:

1.  Before you go outdoors, use what you know about energy flow in food webs to predict how the populations below will compare in size:

Goldenrod Plant Gall Insect

Population > =  < Population

Predator Insect Gall Parasite Insect

Population > = < Population

PROCEDURE:

A.Gather the field materials.  Write your name on the plastic bag and on the index card.  Tie the end of the twine to the top of the stake.

B.  Dress warmly.  Find a field that contains goldenrod plants.  Being careful not to trample the plants, push the stake securely into ground or snow.  Hold the end of the twine and walk in a circle around the stake to make a 10 m² circle with your feet around the plants.

C. Determine the number of goldenrod plants inside the circle with and without round galls on the stem.  Record these data on the index card.

D.  Break the stems off all the round galls.  Put the galls and your data card inside the plastic bag, and return to the lab.

E.  In the lab, transfer the number of goldenrod plants with and without round galls from your data card to your goldenrod data table.

F.  Record the number of galls that were opened by a vertebrate predator on your goldenrod data table.

G.Open all the galls as follows:  Unfold and stack several paper towels to make a firm cutting surface.  With pruner partially cut through gall's equator, rotate gall 90^o and cut again. The gall can usually be opened at this point without cutting the animal inside. If using a knife and glove, carefully insert the knife blade into the gall along its equator.  Twist the knife until the gall pops open.

H.  Use the key to identify each insect.  Record your data on the data sheet.

I.  Share your data with others in your class.  Record the class data on your data sheet.

J.  On your data sheet, calculate the percent of goldenrod plants that were parasitized by insects.

K.  On your data sheet, calculate the percent of gall insects that were eaten by vertebrate predators.

L.  List the different kinds of insects from least abundant to most abundant on your data sheet.

CONCLUSIONS:

2. The first link in a food web is a producer that manufactures its own food by photosynthesis.  What is the producer organism in the goldenrod plant and insect food web?

3. The second link in goldenrod food web is a parasitic insect that feeds on the goldenrod plant, but does not kill it.  The parasitic insect is usually the most abundant animal species.  According to class data, what insect species is probably the parasite in the goldenrod food web?

4. Except where a bird eats a predator insect, predator insects are the highest links in the goldenrod food web.  Predators are usually less abundant than their prey, the parasitic insects they eat.  According to the class data, what insect species are probably the predators in the goldenrod food web?

5. Use your answers to questions 2-4 to draw a possible goldenrod food web on your data sheet.

6.Use class data and your answers to questions 2-4 to describe how the following populations actually compared in size:

Goldenrod Plant Gall Insect

Population > =  < Population

Predator Insect Gall Parasite Insect

Population > = < Population

DISCUSSION:

7.Does your answer to question 6 support or contradict your predictions in question 1?

8.Your teacher will show you a goldenrod food web based on extensive population surveys and an analysis of the feeding habits of the gall insects.  Is the food web you drew based on class data different from the actual food web?  If so, give one possible explanation.  (Hint:  Each predator insect consumes one parasite insect.  Increase the parasite insect population estimate in the class data to account for the individuals who were eaten.)

9. How do you think insect parasites affect the size of the goldenrod plant population?  Use class data on the percent of goldenrod plants parasitized by insects to support your ideas.

10.How do you think vertebrate predators affect the size of the goldenrod insect populations?  Use class data on the percent of galls opened by vertebrate predators to support your ideas.         

11.What is the most likely vertebrate predator on the goldenrod insects?  Use your observations of the goldenrod plants and the opened galls to support your ideas.  How could you find out what the predator is?

Name_____________________________

 GOLDENROD DATA TABLE:  Use tally marks (////) to record the number of insects you find as you open the galls. 

INSECT ABUNDANCE CHART

Most Abundant: _______________________

  _______________________

  _______________________

  _______________________

Least Abundant: _______________________

1a.  Round gall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  . . . . . . 2

1b.  Long, narrow gall. . . . . . . Gnorimoschema gallaesolidaginis (gall moth or one of its                                                                                              
                                                                            predator insects)

2a.  Chamber of round gall contains an insect . . . . . . . . . . . . . .  . . . . . . . . 3

2b.  Chamber of round gall is empty . . . . .  . . . . . . . . . . . . . . . . . . . . . . 6

3a.  White or tan larva which may move when gently touched . . . . . . . . . . . . . . . . 4

3b.  Brown pupa which does not move when gently touched . . . . . . . . . . . . . . .  . . 5

4a.  Large white larva (4 mm thick, 8 mm long) (gall chamber has an exit tunnel, no black 
                       pellets, and is not shiny). . . . . . . Eurosta solidaginis (gall fly)

4b.  Small white or tan larva (2 mm thick, 5 mm long) (gall chamber has no exit tunnel,
            has black pellets, and is not shiny) . . . . . . . Eurytoma gigantea (giant wasp)

5a.  Large brown pupa (4 mm thick, 8 mm long) (gall chamber has exit tunnel, is not shiny, 
              and has no black pellets). . . . . . . . . . . . Eurosta solidaginis (gall fly) 

5b.  Small brown pupa case (2 mm thick, 7 mm long) (gall chamber has no exit tunnel, is shiny, 
                     and has no black pellets). . . . . .Eurytoma obtusiventris (obtuse wasp)

6a.  Empty chamber has exit tunnel. . . . . . . . . . . . . . . Eurosta solidaginis (gall fly)

6b.  Empty chamber has no exit tunnel . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

7a.  Empty chamber is shiny . . . . . . . . . . . . . . .Eurytoma obtusiventris (obtuse wasp)

7b.  Empty chamber is not shiny . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

8a.  Empty chamber contains black pellets . . . . . . . . . . . Eurytoma gigantea (giant wasp)

8b.  Empty chamber contains no black pellets. . . . . . . . . . . . . . . . . . . . . . . . 9

9a.  Empty chamber contains  green or gray sawdust. . . . . . . . . . .Mordellistena unicolor
                                                                           (mordellid beetle)

9b.  Empty chamber contains no green or gray sawdust . . . . . . . . Former Occupant Unknown