Location: Emerging Pests and Pathogens Research
2017 Annual Report
Objectives
Biological control is considered the only long-term control option for many invasive plants. Enhancing the probability of successful biological control requires the identification of not only host-specific biological control agents (although non-specific agents may be feasible in certain situations) but also effective agents. This in turn depends upon a fundamental knowledge of a target weed’s biology, genetics, and population dynamics to inform agent selection. Long-term monitoring is needed to verify the specificity and efficacy of field-released agents. It is also important to understand why biological control may not be feasible with currently available agents. Our objectives are to:
Objective 1: Evaluate host specificity, efficacy, or other biological traits of biological control agents of invasive plants, especially swallow-wort and eastern knapweeds; and determine the biology, ecology and demography of these weeds.
Subobjective 1a: Assess agents of swallow-wort and knapweed.
Subobjective 1b: Elucidate the genetic structure of knapweed populations in the Northeast.
Subobjective 1c: Determine demographic rates for swallow-wort and knapweed.
Objective 2: Release approved biological control agents and evaluate their establishment, phenology, and impact on weed populations and other flora, for invasive plants such as swallow-wort.
Approach
European swallow-worts (Vincetoxicum spp.) and knapweeds (Centaurea spp.) are invasive weeds of continued or increasing concern, respectively, in the northeastern U.S. They invade a variety of natural and managed terrestrial ecosystems. A biological control program for swallow-worts has been in development since 2004. This project will evaluate remaining potential insect and fungal agents of swallow-wort for host range and/or life history traits and plant impact in laboratory, greenhouse and (as appropriate) field studies, and implement a biological control agent (a defoliating moth that is anticipated for approval) to control pale and black swallow-wort. Long-term monitoring plots, already established at several sites, will help document the released agent’s impact on swallow-wort and resulting responses of invaded plant communities. A remaining knowledge gap of swallow-wort demography (years to first reproduction) will continue to be investigated in a field experiment. The genetics of the fertile hybrid meadow knapweed and demography of northeastern populations of spotted and meadow knapweed will be investigated through field and common garden studies. Initial assessment of a seed-feeding biological control agent on meadow knapweed will be made in the greenhouse to understand its potential impact. Successful implementation of this program should lead to the suppression of the two swallow-wort species in some of the habitats they have invaded, and define future control measures to be implemented against knapweeds.
Progress Report
Objective 1: Evaluate biocontrol agents and biology of swallow-worts and knapweeds. Laboratory studies indicate that different populations of the leaf-feeding moth Abrostola asclepiadis, which was imported from Europe into U.S. quarantine as a potential biological control agent of pale and black swallow-wort, will likely only have one generation per year. A French field study continues with a collaborator for a second year to determine how many generations the moth may have when reared outdoors under naturally-changing daylengths at a latitude similar to the northeastern U.S. Currently, it appears that a single generation occurs in the field. This information will help elucidate the maximum amount of damage the moth can inflict within a growing season.
A greenhouse study examining the combined impact of plant competition and defoliation by a French population of the moth on pale and black swallow-wort is nearing completion. This study complements a previous experiment involving combinations of defoliation and shading. Results of the two studies will inform whether different types of environmental stress can enhance the damage of a leaf-feeding biocontrol agent and thus increase its effectiveness in reducing swallow-wort populations over time.
A seed-destroying fly of swallow-worts is currently being evaluated by Swiss collaborators. We are examining if the adult fly will lay eggs, and larvae successfully develop, on various closely-related plants to the swallow-worts.
A disease of swallow-worts had previously been shown to kill 67-100% of swallow-wort seedlings. On-going laboratory assays with a collaborator indicate that the pathogen is also highly virulent and able to cause disease on adult flowering plants. In separate field studies, data are being collected at a New York site on pathogen infection rates over time of pale swallow-wort seedlings and adult plants and on potential changes in the existing plant community as swallow-wort declines due to the pathogen. An overwintering survival study of the pathogen was delayed due to the retirement of a key collaborator; the study is expected to be implemented later this year.
Data collection continues for long-term field experiments assessing survival and growth of vegetative juveniles of pale and black swallow-wort in different habitats and locations. The time to reproductive maturity may be a critical phase in the life cycle of swallow-worts. To date, individual plants of black swallow-wort begin reproducing at a faster rate (beginning at 3-4 years) compared to pale swallow-wort (6 years or more). Further, few pale swallow-wort individuals have reached a flowering state across several locations, whereas most black swallow-wort juveniles have already done so.
In an effort to document the efficacy of alternative control measures should biocontrol not be available, a long-term mowing study continues for a sixth and final season at a heavily-infested pale swallow-wort field site. Preliminary analyses suggest that a high frequency of mowing (three or six times per season) does not influence pale swallow-wort densities.
A report of potential resistance of pale swallow-wort to a commonly used herbicide in New York was not supported in a laboratory assay, suggesting that issues with the application of the herbicide may be a more likely explanation for lack of herbicidal control. Thus, herbicides are still a viable control option.
Manuscripts for a plant population modelling study of pale and black swallow-wort have been published or are under review. A multi-year seed bank study of swallow-wort has been published, and a manuscript on a previously conducted swallow-wort seed dispersal study is in preparation.
Spotted and meadow knapweed are potentially new and emerging weed problems in agricultural fields and natural areas of the Northeast. Plant population data, including survival of different plant stages and seed production, are currently being collected from three spotted knapweed and four meadow knapweed field sites in New York. These data will be used in plant population models to better define effective control programs. In addition, the presence of knapweed biological control agents that were previously released in the region is being documented.
Preliminary knapweed seed bank and seedling emergence studies were established. Results will help determine the methods needed when implementing the final experiments in 2018.
Genetic libraries have been created by Vermont collaborators from leaf samples previously collected from 10 New York and 10 Vermont populations of meadow knapweed. Population genetic analyses are in process. Initial results indicate that three genetic groups are present, extensive mixing among the groups has occurred, and genome size is correlated with genetic ancestry. Seeds, collected from the same New York and Vermont populations, were germinated and grown in a common greenhouse study, and various plant traits were measured in order to correlate these physical traits, the plants’ genetic identities, and known invasive traits. Crosses were also made between these greenhouse grown plants, and the resulting seeds were germinated and grown in a second common greenhouse study. Physical traits were again measured and leaves were sampled for genetic analyses. Manuscripts are in preparation for the first greenhouse study and genetic analyses.
Objective 2. Monitor released biocontrol agents. Long-term monitoring of vegetation plots has been completed at several sites slated for the release of swallow-wort biological control agents. A petition submitted by colleagues for U.S. release of a moth biological control agent (Hypena opulenta) remains under review but is expected to be approved later this year. Open field releases of the moth are expected to occur in spring 2018. Final permissions are being obtained from local land managers for New York releases pending receipt of federal and state permits. Plant population models developed by ARS researchers in Ithaca, New York indicate this moth could be an effective agent against slow-growing swallow-wort populations (typically forests), but it may only have a limited impact on faster-growing infestations in fields.
Accomplishments
1. Swallow-wort population model helps guide biocontrol. The European perennial plants pale swallow-wort and black swallow-wort have become invasive in eastern North America. For biological control to be successful as a management tool, biocontrol agents must be capable of reducing swallow-wort populations to low levels. ARS researchers at Ithaca, New York and Urbana, Illinois developed plant population models of the two species that identified key points of the plants’ life cycles to be targeted and how much damage an agent must inflict for successful control to occur. Leaf-feeding moths or seed-destroying flies are promising for controlling slow-growing populations of the weeds (forest and some field) and are recommended for screening and eventual release as biological control agents. However, fast-growing field populations will not be controlled with these insects. Instead, it is recommended that land managers integrate other control tactics for successful management of these weeds.
2. Swallow-wort seeds are short-lived in soil. Weed control by land managers and farmers can be hampered by new plants emerging from long-lived seeds in the soil (seed bank). A multi-year seed bank study of pale and black swallow-wort, two invasive species of the Northeast, showed that buried seeds do not survive more than 2-3 years. Also, if practical, burial of seeds up to 10 cm deep for pale swallow-wort, and more than 10 cm deep for black swallow-wort, will prevent seedling emergence. If land managers focus on limiting swallow-wort seed production and seedling establishment for at least three years, restoration of affected areas will have a much greater likelihood of success.
Review Publications
Ditommaso, A., Milbrath, L.R., Morris, S.H., Mohler, C.H., Biazzo, J. 2017. Seedbank dynamics of two swallowwort (Vincetoxicum) species. Invasive Plant Science and Management. 10(2):136-142. doi: 10.1017/inp.2017.10.
Milbrath, L.R., Davis, A.S., Biazzo, J. 2017. Demography of invasive black and pale swallow-wort populations in New York. Northeastern Naturalist. 24:37-53.
Dolgovskaya, M., Volkovitsh, M., Reznik, S., Moseyko, A.G., Milbrath, L.R. 2016. Host specificity of Asian Chrysochus Chevr. in Dej. (Coleoptera, Chrysomelidae, Eumolpinae) and their potential use for biological control of invasive Vincetoxicum species. Entomological Review (Entomol Rev). 96:826-830.
Milbrath, L.R., Nechols, J.R. 2014. Plant-mediated interactions: considerations for agent selection in weed biological control programs. Biological Control. 72:80-90.
Ditommaso, A., Milbrath, L.R., Bittner, T., Wesley, F.R. 2013. Pale Swallowwort (Vincetoxicum rossicum) response to cutting and herbicides. Journal of Invasive Plant Science and Management. 6:381-390.
