PURPOSE:  To investigate the ecology and behavior of goldenrod gall fly larvae and their predators.

MATERIAL: Field -- plastic bag.  Lab -- 25 round goldenrod galls, hand pruner or paring knife with a glove, dissecting pan.  Optional: dissection needle, hand lens.

INTRODUCTION:  The round swellings on the stems of some goldenrod plants are caused by the gall fly (Eurosta solidaginis).  The gall fly is a parasite on the goldenrod plant, because it eats plant tissue but does not kill the host plant.  This insect-plant relationship provides an example of host specificity since the female gall fly injects her eggs on the terminal bud of the Tall Goldenrod (Solidago altissima), and less frequently on other species.

When the gall fly larva hatches, it eats its way to the meristem, the growth tissue of the plant.  Mechanical injury and fluids secreted by the larva cause the plant to produce the ball-shaped swelling around the larva.  In autumn, the larva matures and makes an exit tunnel through the gall wall to within one cell layer of the outside.  The larva spends the winter inside the gall in a resting state.  In the spring, it forms a pupa, and emerges from the gall as an adult in May or early June.  To complete the cycle, the female must find a mate and young goldenrods into which to inject her eggs.

Two different species of wasp predators eat the gall fly larvae.  The obtuse wasp (Eurytoma obtusiventris) lays its egg in the larva before the gall is formed; the giant wasp (Eurytoma gigantea) lays its egg inside the cavity of the developing gall.  The wasp larvae live inside the developing gall fly like a parasite, but the wasp always kills its prey like a true predator.  Gall fly larvae are also eaten by bird predators like woodpeckers and chickadees.  The birds peck holes into the galls to get their prey.

In this investigation you will dissect goldenrod galls to determine the locations of the gall fly larvae exit tunnels and the bird-pecked holes.  Then you will calculate the percent of wasp and bird predation on the gall fly larvae.

PREDICTIONS: Use what you know about insect and bird behavior to make predictions.

1.  Do gall fly larvae distinguish up from down?  Do they usually make their exit tunnels in the upper or lower hemisphere of the gall, or is tunnel orientation random?

2.  Do you think that birds can locate the insect's exit tunnel?  Do they peck there more often than they do on other areas of the gall?

3.  Do you think that many or only a few gall fly larvae are eaten by wasps and birds?

FIELD PROCEDURE:

A.  Go to an open field where the winter remnants of goldenrods are abundant.  Look for galls on the stems.  You will probably find some that are unopened and others that have been pecked open by birds.

B.  Collect a random sample of galls by picking the first 25 galls you see; don't pay attention to whether they are opened or unopened.  Make sure to leave enough of the stem attached so you can tell the upper from the lower hemisphere of the gall.

LAB PROCEDURE:

C.  Separate the unopened galls from the bird-pecked galls.  Record the number of each kind on the data sheet.

D. Put the UNOPENED GALLS into a dissecting pan.  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 a gall along its equator.  Twist the knife until the gall pops open.

E.  For each gall, use tally marks (////) to record your data as follows:

(1) If a gall fly larva is present, it will be white/tan and will move when gently touched.  Determine whether the tunnel was made in the upper or lower hemisphere.

(2) If a wasp pupa is present, it will look reddish brown, and will not move when touched.  An empty chamber is also evidence that a wasp consumed the larva.

(3) Discard the galls when you are finished.

G.  Put the remaining BIRD-PECKED GALLS into the dissecting pan.  Use tally marks (////) to record your data as follows:

(1) For each gall, record the hemisphere in which the hole was pecked.

(2) Cut open the gall and record whether or not the hole was on tunnel.

(3)  Discard the galls when you are finished.

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

CONCLUSIONS:  Use class data to answer the following questions.

4. In which hemisphere of the gall do gall fly larvae tend to make tunnels?

5. Were birds able to detect the location of the tunnels made by the gall fly larvae?  What is your evidence?

6. What percent of the gall fly larvae in the intact galls were eaten by wasps?

7. How do your results in questions 4-6 compare to your predictions in questions 1-3?

DISCUSSION:

8. Do you think that the percent predation by wasps might change as winter advances into spring?  Explain your ideas.

9. Do you think that the percent predation by birds might change as winter advances into spring?  Explain your ideas.

10. How might birds detect the location of insect tunnels in the gall?  What is the advantage to pecking on the tunnel?

11. Do you think predation by wasps and birds has a big effect on the population size of gall flies?  Explain your ideas using class data.

12. What do you think is the survival advantage to the gall fly larvae of making an exit tunnel?  Is there an advantage of tunneling in one hemisphere rather than the other?

Name ____________________________

Data Sheet

Number Unopened Galls _________

Number Bird-Pecked Galls ______

GOING FURTHER:  Your study of goldenrod galls answered some questions about the ecology of the gall fly, but left other questions unanswered.  You may wish to consult a reference book in the library, express an opinion, propose a hypothesis, or suggest an experiment to investigate the following questions.

1. How is the adult gall fly able to distinguish the Tall Goldenrod from all other species?

2. How tall were the goldenrod plants when the gall fly deposited her eggs on them?

3. Why do some goldenrod plants have more than one gall?

4. Does the presence of galls affect the health or reproductive capacity of the goldenrod plant?

5. What other kinds of insects can be found in goldenrod galls?

6. What other kinds of plants have galls?

7. Are the gall larvae a preferred food or a survival food for bird predators?

8. Why do you suppose woodpeckers and chickadees feed on gall fly larvae but not blue jays and sparrows?

9. What other animals might use the larvae as a winter food supply?

10. Could the location of the tunnel reflect the direction of the sun or prevailing winds?  What experimental evidence could you gather to answer this question?

11. Why can you describe the gall fly larva as both a parasite and an herbivore?  What is the difference between the two terms?

12. What is the difference between a parasite and a true predator?  In what ways does a wasp act like both?