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Field studies were conducted to characterize the predominant nonstructural carbohydrates in roots and crowns of purple loosestrife and to determine the seasonal fluctuation of root and crown carbohydrates in three Minnesota wetland habitats. Starch was the primary nonstructural carbohydrate present, and concentrations were consistently higher in roots than in crowns. Starch content decreased following shoot emergence and declined until flower bud formation, at which time levels reached seasonal lows. Following bud initiation, levels of starch increased during flowering, and this trend continued through to plant senescence in late September or October. Sucrose was the predominant soluble sugar in roots and crowns of purple loosestrife, but fructose and glucose were detected. Levels of sucrose in roots and crowns followed the same seasonal trends as starch.
In common milkweed, the development of subterranean root buds on excised root segments, following emergence from the parent root, is characterized by development of nodes and internodes followed by internode expansion. Transverse sections of root buds reveal that bicollateral vascular bundles as well as leaf traces and gaps are well developed in buds from 3-month-old plants. Strands of xylem and phloem connect the parent root and root bud in both inhibited and noninhibited root buds. Pitted primary tracheary elements, characteristic of developmentally advanced primary xylem, are present in these traces. The occurrence of a well-developed vascular system throughout the root bud and between the parent root and bud provides evidence that retardation of growth of inhibited root buds in common milkweed is not caused by anatomical constraints.
Greenhouse studies were conducted to determine the influence of plant density and spray volume on the retention, spray deposition, efficacy, and translocation of the amine salt of triclopyr in purple loosestrife. More spray solution was retained on leaves at 935 Lha−1 than at 94 Lha−1 at populations of 0, 4, or 8 nontarget neighbors. Spray coverage decreased with decreasing height within the plant canopy when spray cards were placed in the top, middle, and soil surface adjacent to the central target plant. Within a population, spray card coverage generally increased as spray volume increased. Regrowth from the crown was affected by spray volume, and uniform spray coverage of the plant was required for adequate control of vegetative regrowth and was achieved with spray volumes of 374 and 935 L ha−1 spray volume. Regrowth of purple loosestrife was greater at 94 Lha−1 at all three plant populations indicating that less herbicide penetrated the canopy to reach the basal portion of the plant. A laboratory experiment was conducted to investigate the translocation of radiolabelled triclopyr to roots and crowns of purple loosestrife. Only 0.3 to 1.4% of absorbed 14C-labelled material was translocated to roots and crowns. Low spray volumes and dense stands of purple loosestrife would likely result in poor control because inadequate amounts of triclopyr reach the basal portion of the plant and translocate to vegetative propagules.
Starch levels, used as a measure of plant stress, were not consistently reduced in root or crown tissue of purple loosestrife plants after 2 yr of severe Galerucella calmariensis or Galerucella pusilla (Coleoptera: Chrysomelidae) defoliation. Early in the season, defoliation from Galerucella spp. approached 100%, but the majority of Lythrum salicaria plants regrew by the end of August, resulting in an average reduction of 81% of the aboveground biomass compared to the control. The stress imposed by Galerucella spp. defoliation was less than that achieved from more severe stress imposed by mechanical shoot clipping at 2- or 4-wk intervals from June to October. Both shoot-clipping treatments killed the majority of plants after one growing season. Galerucella spp. feeding reduced plant stature, which may reduce competitiveness. However, considering the extensive carbohydrate reserves present in the large woody crowns of Lythrum salicaria, it will require in excess of 2 yr of consistent, severe leaf defoliation to cause plant mortality. A combination of stresses, such as winter crown injury, or other biological control agents in addition to Galerucella leaf defoliation may be required for plant mortality.
The effect of shoot feeding by the biocontrol agents, Galerucella calmariensis and Galerucella pusilla (Coleoptera: Chrysomelidae) on purple loosestrife (Lythrum salicaria) seed production and seed germination was quantified in two Minnesota wetlands. In a wet meadow where Galerucella spp. were present on isolated plants, feeding by adults and larvae during shoot elongation resulted in stunting and malformation of shoot tips. There was a subsequent reduction in purple loosestrife inflorescence length and number of flower buds and seed capsules. As Galerucella spp. larvae preferentially fed on shoot meristems, even low levels of feeding on a whole-plant basis (approximately 10% defoliation) reduced seed production. In a sedge meadow wetland with severe feeding damage (a minimum of 70% leaf defoliation), few to no flower buds formed on plants, and subsequently, few to no seed capsules were produced on purple loosestrife plants. Of the few capsules that were produced, number of seeds per capsule and percent germination of seeds did not differ from control plants. In both wetlands, feeding on a main shoot of purple loosestrife did not result in a compensatory increase in the number of axillary inflorescences. Feeding by Galerucella spp. and the subsequent reduction in number of seeds produced on purple loosestrife plants will decrease the number of seeds available for dissemination to new sites. Fewer seeds will enter the seedbank, and over time, feeding by Galerucella spp. will decrease the number of seeds available for seedling recruitment. The benefit of leaf defoliation on purple loosestrife plants caused by Galerucella spp. feeding has been reported. In this study, we have quantified the additional benefits of reduced seed production from Galerucella spp. feeding on purple loosestrife in North America.
Previous studies have characterized the development of the biological control insects, Galerucella calmariensis and Galerucella pusilla on purple loosestrife and on nontarget Lythraceae species, including two species native to Minnesota, winged loosestrife, and swamp loosestrife. The impact of Galerucella spp. on these plants, when grown in outdoor mesocosms that more closely mimics ecological host range, has not been reported. The first objective of this study was to compare the growth and seed capsule production of purple loosestrife, winged loosestrife, and swamp loosestrife, with and without exposure to Galerucella spp. With purple loosestrife, larval feeding on apical and lateral shoot buds resulted in fewer seed capsules, and reduced aboveground biomass and plant height compared to control plants. No measured plant growth or reproductive parameters were reduced as a result of beetle feeding on swamp loosestrife. Presence of Galerucella spp. on winged loosestrife resulted in a reduction of seed capsules in one of 2 yr of study. A second objective of our study was to compare the phenology of the three Lythraceae species in relation to that of Galerucella spp. In the northern United States, flowering and seed development in swamp loosestrife occurred a month later than in purple or winged loosestrife. The delayed flowering of swamp loosestrife resulted in avoidance of shoot meristem feeding damage caused by the first generation of beetles. Laboratory studies might have overestimated the host range of Galerucella spp. on swamp loosestrife with the finding of asynchronous flowering times of purple and swamp loosestrife. Our mesocosm studies confirm that previous laboratory host range testing did accurately predict the ecological host range of winged loosestrife.
In wetlands, drought or managed late-summer drawdowns create exposed mudflats that provide an excellent substrate for germination of purple loosestrife seeds. If late-emerging purple loosestrife seedlings survive the winter, new or expanding populations of purple loosestrife will result. Spring survival was determined for overwintered purple loosestrife seedlings from seeds planted at weekly intervals in late summer or fall of the previous year. Seedlings of purple loosestrife that emerged from late July to early August had the greatest survival rates and the greatest shoot dry weight, and they were the tallest the following spring. However, 37% of purple loosestrife seedlings that emerged in late August, although stunted, generated a crown that was able to overwinter successfully and regrow the following spring. The number of growing degree days accumulated from planting date to October 6 (the average date of first frost for Minneapolis and St. Paul, MN) was 1,424 for seedlings from seeds planted on July 21 but only 219 for seedlings from seeds planted on September 15. Purple loosestrife seedlings that emerge during late summer through early September in Minnesota may survive the winter to create additional purple loosestrife weed problems in wetland mudflats caused by artificial drawdowns or droughts.
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