Date of Award:


Document Type:


Degree Name:

Master of Science (MS)



Committee Chair(s)

Edward W. Evans


Edward W. Evans


John O. Evans


Frank J. Messina


The responses of Canada thistle stems to damage by two types of herbivores were investigated to determine the potential role of these herbivores as biological control agents. Canada thistle stems at a cattle exclosure in Rich County, Utah were cut at one to two centimeters aboveground in June, 1991, to mimic grazing by cattle. Neighboring stems within 30 centimeters were also cut in some treatments to investigate the effects of physiologically connected ramets on the growth and reproductive responses of individual focal stems. I measured plant height, stem diameter, and number of flowerheads for each of the focal stems during the growing season and obtained dry weights in late September. Cut stems had reduced survival, and were shorter, lighter, and less successful at producing flowerheads than were uncut stems. Cutting neighboring stems did not significantly affect survival or growth of the focal stem when the focal stem was also cut. When the focal stem was left intact, however, those stems with uncut neighbors grew significantly taller and produced more flowerheads than did stems with cut neighbors. These results suggest that neighboring Canada thistle stems assist each other by translocating nutrients or by changing the microhabitat in the absence of defoliation.

A stem-mining weevil, Ceutorhynchus litura, was released into eight 4 X 6-m plots at three exclosures in Rich County, Utah in 1990. I compared stem density, height, stem diameter, and flowerhead production of individual stems in weevil-infested versus control plots during the 1991 growing season. The presence of weevils did not affect the density of thistle stems, nor did it affect their growth or reproductive responses in weevil-infested plots. Weevil infestation declined from 1990 to 1991 in release plots but increased slightly in control plots, which suggests that emigration caused reduced infestation in release plots.

These experiments illustrate some of the complexities of Canada thistle's responses to grazing; they also reinforce that biological control is generally a gradual process with rather subtle effects over the short-term.

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