To enhance student motivation, you may wish to do "What's Inside Goldenrod Galls?" prior to this activity.
TOPICS: Ecology, insects
LEVEL: High school
TIME: 15 min for prediction discussion; 45 min for field work which may be done as homework; 45 min for lab work.
ADVANCE PREPARATION: Collect 2 intact goldenrod plants, one with and one without a gall, and several galls to open for the class. Show them to your class while discussing predictions. You may wish to collect galls 3-4 weeks in advance and store in a jar at room temperature to allow adult insects to emerge. (This works from January -April but not in the autumn.)
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MATERIALS:
DO NOT USE RAZOR BLADES for this activity. They break too easily. We suggest double-edged hand pruners. Thes cut into galls allowing students to safely break the gall open without cutting the animal inside, or their fingers. The other alternatives include sturdy pocket knives or one-piece scalpels used with gloves. See SAFETY NOTE below
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PREDICTIONS:
In class discussion, encourage creative, logical, and critical thinking. We recommend that you introduce the following ideas if they are not suggested by students: In general, insects are capable of responding to gravity; behavior depends on the species and situation. The tunnel location is sometimes detectable by sight or touch to humans; how do bird senses compare to humans? Birds often tap, especially downy woodpeckers, the gall to determine the exit tunnel. Downies are able to find the exit tunnel and probe through the epidermis to remove the larva. Predator populations are usually smaller than prey populations to avoid local extinction of both populations.
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FIELD PROCEDURE
may be done as a homework assignment.
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SAFETY NOTE:
If a knife and glove is used. Galls are round and hard, and may cause the knife to slip aside suddenly. Practice opening the galls yourself, and then demonstrate proper techniques to students. We strongly recommend the use of hand pruners.
QUESTIONS 8-12 require higher-order thinking skills. We recommend that you discuss these questions with your class before having students write out their answers
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4.
Typical results indicate that more larvae tunnel into the upper hemisphere
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5.
Percents will vary among different samples, ranging from 2-50%
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6.
Birds tend to peck in the upper hemisphere, and typically on tunnel
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7.
Answers will vary among students
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8.
Percent predation by wasps would not be expected to change from winter to spring since infestation by the wasps occurs during the previous summer
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9.
Cumulative percent predation by birds is expected to increase from autumn to spring since birds feed on the larvae over winter. Actual monthly predation rates should decrease over the winter, as the number of available larvae decrease
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10.
Perhaps the birds detect the tunnel location visually, or by touch or sound, using their beaks. Peck marks indicate they tap galls - touch or sound
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11.
Predators always reduce the prey population size; the relative effect will vary depending on class predation rate data
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12.
The gall fly is physically able to make an exit tunnel when it is in the larval stage, but not as a pupa or adult. The advantage is easy access to the outside. Escape through the upper hemisphere may be an advantage to flying insects.
GOING FURTHER
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1.
The recognition could occur by chemical or visual detection. Students should be able to suggest several experiments with appropriate control groups to distinguish between these hypotheses
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2.
It approximates it since cell elongation does occur after oviposition. So galls are a bit higher than egg deposition height
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3.
Each gall indicates infestation by one larva
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4.
Available evidence indicates that there is a decrease in the quantity and quality of the seeds produced by infected plants, and they produce fewer new rhizomes affecting vegetative propagation. Differences in stem height between infected and non-infected plants was found to be insignificant
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5.
Elliptical galls on goldenrod, caused by the gall moth, may contain wasp parasitoid or a beetle. The larva of species of beetle can also be found in the ball-shaped goldenrod galls usually living in a separate chamber from the gall fly larva
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6.
Other plants that produce galls include black cherry, hackberry, willow, alder, jewelweed, and oak
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7.
The evidence is inconclusive. When food supplies become scarce in the winter, the larva and pupa of the gall fly may become an important food source to the downy woodpecker and chickadee. Low predation rates in autumn indicate that the larvae are not a preferred food
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8.
Downy woodpeckers and chickadees have beaks that are capable of pecking open the galls. They are relatively small, acrobatic birds. Blue jays may be too large to land on the plant. Sparrows are seed eaters
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9.
Mice and other rodents may gnaw into galls on plants toppled by wind or snow.
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10.
This is possible, since the tunnels tend to be at an angle rather than vertical or horizontal. To test this, the galls could be marked with a compass orientation. Wind direction and exposure to sun during the day could be recorded
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11.
A parasite, by definition, lives off a living host without killing it. An herbivore, by definition feeds on plant material. Parasitism is a biotic relationship in which the parasite benefits and the host is harmed; herbivory described the energy source in a trophic level
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12.
A parasite weakens but does not kill its host. A true predator kills its prey before, during, or after it ingests it. The wasp slowly consumes the gall fly larva from within. The wasp is technically classified as as a parasitoid, which is a type of predator; a parasitoid depends on an individual organism for food like a parasite, but eventually kills its host like a true predator.