2013 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. All beetle species have been rejected as potential agents. Screening of two leaf-feeding moths continues including efforts to produce disease-free colonies and identify a population that has two generations per summer. A plant impact study is in process with a third moth species. Initial host range tests of the fungal pathogen Sclerotium rolfsii indicate that it has the potential to cause disease in native milkweeds and other plants with the exception of little bluestem grass and maize. A field observational trial of the pathogen is in progress. The pathogen is not affected by antofine, a SW-produced chemical with antimicrobial properties (Objective 1). Analysis of soil samples is in progress for suspected SW allelochemical and its breakdown products to determine their concentrations in the field over time. Field data collection is complete 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 juvenile SW survival, growth, and time to first reproduction. As of the fifth season of growth (2012), survival of black SW was greater than pale SW. Juvenile black SW remained twice as tall as pale SW plants. To date only black SW in the field habitat have produced seed after 4-5 years. A long-term field experiment examining the effects of different types and frequencies of foliage removal on SW performance continues. The plants have increased annually in size regardless of the damage they have received except for plants clipped four times each season which have remained the same size. A two-year herbicide and mowing study for pale SW in an old field and adjacent forest understory is nearing publication. A multi-year seed bank study continues for assessing the longevity of seed of pale and black SW at different burial depths. Pale SW emergence in the first year was generally poor except for seed buried 1-cm deep, whereas black SW emergence was generally greater. Both species emerged from 10-cm depths. A greenhouse experiment was conducted to supplement a previous study of the photosynthetic abilities of pale and black SW. Results indicate that black SW may be more adapted to high light environments than pale SW, whereas pale SW may be more adapted to shade than black SW. A long-term mowing study that includes mowing three or six times per season continues at a heavily-infested pale SW site (Objective 2). Long-term monitoring of vegetation plots continued for a sixth year at three sites and a second year at three other sites. One new long-term monitoring site (black SW in a forest) was added this year (Objective 3).
Short-term control measures for pale swallow-wort. Effective control techniques for pale swallow-wort, an invasive herbaceous vine of fields and forest understories, are limited but needed while a biological control program is in development. ARS scientist at Ithaca, New York in collaboration with Cornell University resarchers conducted a 3-year cutting and herbicide control study of pale swallow-wort on an adjacent old-field and forest understory site. Most herbicide-plus-cutting treatments were effective in controlling pale swallow-wort in either habitat although it may take 2 years of treatment to achieve satisfactory control. Cutting alone was less efficacious than herbicides in the forest and greatly increased pale swallow-wort cover and densities in the old-field. These findings suggest that integrated techniques may control pale swallow-wort but that effective management strategies may depend on the particular habitat invaded.
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 pale and black swallow-wort in laboratory studies. Traditional host-specificity studies indicate that it has broad disease potential for multiple milkweeds and other native plants found within the site area, with the exception of little bluestem grass and maize. Interestingly, this native grass has rapidly colonized the area in which the pathogen was first detected, and other native vegetation is also repopulating the area while pale swallow-wort is greatly diminished.
Magidow, L.C., Ditommaso, A., Ketterings, Q.M., Mohler, C.L., Milbrath, L.R. 2013. Emergence and performance of two invasive Swallow-worts (Vincetoxicum spp.) in contrasting soil types and soil pH. Journal of Invasive Plant Science and Management. 6:281-291.