2012 Annual Report
1a.Objectives (from AD-416):
1: Evaluate the risk posed by candidate biological control agents of swallow-worts.
2: Discover biological attributes of swallow-worts that contribute to their invasiveness and identify points of attack for enhancing biological control efficacy.
2a: Delineate the potential contributions of swallow-worts’ phytotoxic constituents to its invasiveness.
2b: Determine demographic rates for pale and black swallow-wort.
2c: Determine how swallow-wort responds to multiple seasons of repeated clipping/artificial defoliation.
3: Release and evaluate establishment and initial impact of biological control agents of swallow-worts.
3a: Document pre- and post-release densities of swallow-wort and assess changes in associated vegetation.
3b: Assess agent establishment.
1b.Approach (from AD-416):
Foreign exploration will identify damaging natural enemies of swallow-worts in their native range. The fundamental host range of candidate biological control agents will be determined overseas and in quarantine. Swallow-wort tissue extracts will be assessed for allelopathic activity, a possible mechanism of invasion. Field-based plant demography studies will identify life stage transitions that strongly affect population growth rates of these invasive plants. Knowledge of these critical transitions will be used to guide the selection of potentially effective agents, a new approach for weed biological control in the U.S. Long-term defoliation/clipping studies will be conducted to document the rate of plant decline. Comparative surveys of areas infested and uninfested by pale swallow-wort will document its impact on plant density, cover and species richness of native ecosystems both before and after the release of approved biological control agents. Additional permanent monitoring sites will document changes in pale and black swallow-wort densities before and after release of agents. Establishment success of newly released agents will be evaluated.
This report documents research toward the protection of natural and managed ecosystems from swallow-worts (SW), terrestrial weeds. Foreign surveys and host range testing of potential insect biological control agents of SW continue in collaboration with French, Russian, and Chinese scientists. Specifically, screening of root-feeding beetles and leaf-feeding moths continues. The fungal pathogen Sclerotium rolfsii was identified from a declining field population of pale SW in New York. It causes extensive seedling mortality in both SW species in laboratory studies (Objective 1). A protocol is in place for extraction of a suspected allelochemical and its breakdown products from soil samples to determine their concentrations in the field over time. A fourth season of field data is being collected for the pale and black SW life-history model including four pale SW and two black SW populations in both field and forest habitats. Analyses will be conducted to identify potential weak links in the plants’ life cycles and thereby guide the selection of effective biological control agents. Data collection continues for a field study of the effect of habitat (field, forest edge, and forest interior) on immature SW survival, growth, and time to first flower. After the fourth season of growth (2011), survival of black SW was greater than pale SW. Juvenile black SW remained twice as tall as pale SW plants. Nine black SW in the field habitat flowered in 2011 with seven producing seeds. Low survival is attributed in part to wetter field conditions than SW tolerates. A multi-year seed bank study was initiated in fall 2011 to assess the longevity of seed of pale and black SW at different burial depths. Data are currently being collected for the first year of the study. A long-term field experiment examining the effects of different types and frequencies of foliage removal on SW performance continues. Data analysis will commence after the 2012 season of the experiment is finished. A two-year herbicide and mowing study for pale SW in an old field and adjacent forest understory near Ithaca, NY is nearing submission. Findings suggest that integrated techniques may control pale SW but that effective management strategies are habitat dependent. A separate long-term mowing study has been initiated at a heavily-infested pale SW site, to include intensive mowing three or six times per season. A greenhouse experiment is being conducted to supplement a previous study of the photosynthetic abilities of pale and black SW. Specifically, this experiment will measure the ability of black SW to photosynthesize under very low light conditions, similar to that found in heavily-shaded forest understories. Black SW has not been observed infesting such habitats normally (Objective 2). Long-term monitoring of vegetation plots continued for a fifth year and three new long-term monitoring sites (two pale SW, one black SW) were added this year (Objective 3).
Fungal pathogen identified for pale swallow-wort. Based on a field observation of a declining natural population, ARS researchers at Ithaca, New York subsequently identified a fungus, Sclerotium rolfsii, capable of causing extensive mortality in both swallow-wort species in laboratory studies. Although we do not yet know the potential utility of this fungus for weed control, this isolate may offer potential as a bio-herbicide for invasive swallow-worts in natural ecosystems if it can be demonstrated that this isolate has a restricted host range and could be approved for environmental releases.
Gibson, D.M., Krasnoff, S., Biazzo, J., Milbrath, L.R. 2011. Phytotoxicity of antofine from invasive swallow-worts. Journal of Chemical Ecology. 37:871-879.
Gibson, D.M., Castrillo, L.A., Donzelli, B.G., Milbrath, L.R. 2012. First report of blight caused by Sclerotium rolfsii on the invasive exotic weed, Vincetoxicum rossicum (pale swallow-wort) in western New York. Plant Disease. 96:456.
Milbrath, L.R., Biazzo, J. 2012. Development and reproduction of the foxglove aphid (Hemiptera: Aphididae) on invasive swallow-worts (Vincetoxicum spp.). Environmental Entomology. 41:665-668. DOI: http://dx.doi.org/10.1603/EN11239.