Selective herbivory of Platyptilia carduidactyla on Cirsium muticum and C. palustre.

Abstract

Native ecosystems are changing rapidly by the invasion of species, many of which thrive in their new environments because of lack of predators. If predators present in the new location of the invasive attack the invasive, it may be controlled, preventing a population explosion. I studied an infestation of plume moth (Platyptilia carduidactyla) larvae in two species of thistle, one invasive and one native. I also performed experiments to examine larval choice of thistle flowers and consumption of seeds. Samples were collected from Reese's Swamp, Cheboygan County, Michigan. Flowers were opened to determine infestation, and larva were taken to use for the choice experiments. There was no difference in the cumulative infestations of the native thistle Cirsium muticum and the invasive species C. palustre. However, more individual C. palustre than C. muticum plants were infested. Among only the infested plants, C. muticum had a higher average infestation. These results are probably artifacts of the physical plant structure, such as flower density. They cannot be accurately interpreted without information on relative thistle densities in Reese's Swamp. The results of the choice experiments were also inconclusive, because of possible larval bias from their host plant origin. There was no significant difference in larval choice of thistle flowers. Statistically more seeds of C. palustre were consumed. The question of plume moth preference for species of thistle cannot be answered without further research, yet we can draw encouraging general conclusions about the state of thistles in Northern Michigan. The herbivory demonstrated in my study is a hopeful assurance that invasive thistles will not cause the extinction of native species of thistle.