THE FACTS |
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There are many kinds of insects in the tallgrass prairie, each with a different niche. Some are grazers, others pollinators or predators, and some are parasites. Among the parasitic insects are those that form a gall or a swelling of plant tissue, either on the leaf or stem, inside of which the insect lives. Galls are formed by the plant in response to an irritation or a chemical that comes from the insect. Sometimes the gall is formed by a female injecting the chemical while she lays eggs. They can also be formed by larvae that hatch on the plant surface, bore into the plant and cause a gall to form. Plants respond differently to each type of parasite and many different kinds of galls are formed. |
Canada goldenrod |
| Two different insects are parasites on Canada Goldenrod (Solidago canadensis) which is a common forb found on the tallgrass prairie. These gall insects are being studied to see whether fire affects their numbers. Galls are convenient for studying the effect of fire on insect parasites because they live inside their host plant and form a structure on the plant which is easy to see. |
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| The Goldenrod Gall Fly (Diptera) forms round or ball-shaped galls with one larva inside. It emerges as an adult in May. The Goldenrod Gall Moth (Lepidoptera) forms elliptical galls nearer the base of the stems of Canada Goldenrod. The adults emerge in the fall. By counting the number of galls, population densities in the tallgrass prairie ecosystem can be estimated. |
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PURPOSE Gall insect populations are studied because they are consumers of plant tissue. They live inside their host plant and could be destroyed by fire. Their numbers can be easily estimated by surveying their host plants for the presence of stem galls. Sampling is conducted in the fall when Canada Goldenrod plants are large and the galls are well formed. The gall will remain on the stem through the winter. |
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METHODS Teams will be assigned to one of the fire demonstration test plots and a series of transects in each plot. You will count all of the Canada Goldenrod stems within the transect and classify which type of gall is on each stem. Some of the stems may have no galls at all. This procedure is repeated until 100 stems are counted within the same test plot. Averages are taken to estimate the number of insect galls found in the given area for a particular plot. Each plot is characterized by a certain burn treatment (annual, biennial, every 4 years, every 20 years or unburned). Knowing the burn treatment of the plot sampled, students can interpret the results of each sampling. Note the year each plot was first burned to understand the effect of the burn interval on the number of galls present. |
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GLOSSARY annual - once a year biennial - once every two years elliptical - oval forb - broad-leaf herbaceous plant; not a grass gall
- a swelling of plant
tissue, either leaf or stem, inside of which the insect host - an organism on or in which a parasite lives to get its food larva
(plural: larvae) - the young stage in the life cycle of some niche - the place and activity for which an organism is best suited parasite
- organism that lives on or in another organism and depends on population
density - the number of a particular species of organism in transect - a straight line on which data is taken. |
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