Biological control of purple loosestrife Lythrum salicaria (Lythraceae) : stability of control and integration with wetland management practices Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/gq67jv26k

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  • Purple loosestrife is a wetland perennial that came to the eastern seaboard of North America in ship ballast and raw wool in the 1800's. Since coming to the United States, it has spread across the country, forming dense monospecific stands in wetlands. Mechanical and chemical control was expensive and ineffective, and a biological control program was initiated in the 1980's. Four biological control agents were released to control purple loosestrife: two leaf-feeding beetles Galerucella calmariensis and G. pusilla (Coleoptera: Chrysomelidae), a rootmining weevil Hylobius transversovittatus (Coleoptera: Curculionidae), and a flower-feeding weevil Nanophyes marmoratus (Coleoptera: Curculionidae). The leaf-feeding beetles are the most widespread. Most biological control programs monitor the release and establishment of control agents, but neglect to measure control agent spread, suppression of the weed, and community succession. In this thesis I report on the suppression of purple loosestrife, the spread of the beetles in relation to a spreading purple loosestrife population, the potential to integrate biological control of purple loosestrife with traditional management practices, and community succession following purple loosestrife suppression in western Oregon. In the first part of my study I monitored weed suppression and subsequent community changes associated with. biological control at two sites in western Oregon. Two biological control agents, Galerucella calmariensis and G. pusilla were released at Baskett Slough National Wildlife Refuge and Aquatic Gardens in western Oregon in 1992 to control purple loosestrife. An intensive quantitative monitoring program began in 1994. My objectives were to compare the outcome of biological control at two sites, determine the stability and long-term pattern of biological control, and to evaluate the ability of control agents to keep up with the spread of purple loosestrife. I used estimates of Galerucella feeding damage, purple loosestrife biomass, and purple loosestrife stem density to evaluate the presence and density of beetles and suppression of purple loosestrife. Beetle damage varied within and between years at both sites, but increased faster at Baskett Slough than at Aquatic Gardens. Damage rates reached 100% at least one year earlier at Baskett Slough. Initial purple loosestrife density was higher at Aquatic Gardens (48 stems m2) than Baskett Slough (32 stems m2), but final densities converged to less than 12 stems m2. At Baskett Slough, purple loosestrife biomass varied over time. Galerucella beetle spread lagged 676 meters behind the spread of purple loosestrife. As purple loosestrife declined, biomass of forbs and grasses increased, and when purple loosestrife increased again, abundance of forbs decreased. To summarize the results of the first study, control developed one to two years earlier at Aquatic Gardens than Baskett Slough over an eight-year study period. Purple loosestrife biomass was suppressed to 10% of pre-control abundance and seeds in the seedbank declined 55.2% over three years. Forbs and did not vary over time, but reed canary grass was higher in 1998 than in the other four years of the study. In the second part of my study I evaluated how biological control of purple loosestrife and the community responded to mowing, tilling, herbicide, and burning to control reed canary grass using a randomized block experiment. Although there have been several studies examining the effects of herbicides on biological control agents, few studies have evaluated the potential to integrate biological control with mowing, tilling, and burning. I evaluated the responses of our focal species and purple loosestrife - Galerucella interactions for two years after the application of treatments by measuring the biomass of each species in the community and Galerucella damage to purple loosestrife. My experiment detected no effect of treatments on colonization of treatment plots by purple loosestrife or biological control agents. Galerucella beetles colonized all plots, and we detected no difference in insect damage levels among treatments in either year. Tilling and herbicide treatments decreased reed canary grass biomass the first year after treatment, increased purple loosestnfe biomass the second year after treatment, and increased plant diversity both years. I detected no differences in biomass or diversity measures in mow, burn or control treatments either year. To summarize the results of the second part of my study, tilling and herbicide treatments reduced dominance of reed canary grass and increased purple loosestrife biomass and plant diversity indices. The reduced dominance of reed canary grass in tilling and herbicide treatments allowed purple loosestrife and other plant species to successfully compete with reed canary grass and grow in treated plots. The treatments did not interfere with the ability of biological control agents to re-colonize and completely defoliate purple loosestrife in treated areas in this small scale experiment. The third part of my study used an observational study to evaluate plant community changes accompanying biological control of purple loosestrife growing along an elevation-moisture gradient on a lakeshore in western Oregon. Prior literature documents the tendency of vegetation to form distinct bands along lakeshores, and indicates little correspondence between the growing community and the abundance and distribution of seeds in the seed-bank. Prior studies also document the ability of insects to affect plant succession and zonation by feeding selectively on preferred species. Weed management requires controlling specific weeds as well as succession to control a weed and encourage desired vegetation. I evaluated plant community changes following biological control of purple loosestrife along an elevation gradient over three years using estimates of biomass of species along the gradient, buried seed density, and Galerucella feeding damage. The dominant species in this study formed bands along the lake-shore, and the buried seeds were more broadly distributed than the growing plants. This agrees with prior literature showing that the actual range inhabited by growing plants is narrower than the potential range indicated by the buried seeds. Reed canary grass was found only at the high and dry end of the gradient. Purple loosestrife was predominately found in the low and wet end of the gradient and was uncommon in the reed canary grass zone, except in 1999, the only year that beetle feeding did not cause 100% defoliation across the entire gradient. In 1999 defoliation levels increased linearly with elevation. To summarize the third part of this study, zonation occurred in the dominant species along the gradient. Purple loosestrife was uncommon in the reed canary grass zone except in 1999, when Galerucella feeding damage was low, suggesting that the beetles indirectly affect purple loosestrife - reed canary grass interactions. Overall, biological control of purple loosestrife was successful at two sites in western Oregon. The Galerucella beetles were able to control purple loosestrife under a variety of conditions despite annual variation in beetle population. Further studies are necessary to determine the direct effects of treatments to Galerucella beetles, and to clarify the effects of Galerucella beetles on reed canary grass - purple loosestrife interactions.
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