Location: Emerging Pests and Pathogens Research2020 Annual Report
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.
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.
This is the final report for Project 8062-22620-005-00D which will end by November 1, 2020. A new project entitled “Management and Biology of Arthropod Pests and Arthropod-Borne Plant Pathogens” is under OSQR review. Subobjective 1a: Assess agents of swallow-wort and knapweed. The biology and host specificity of four candidate biological control agents of pale and black swallow-wort were assessed over the last five years. In collaboration with Russian and French researchers, it was demonstrated in laboratory and field studies that the leaf-feeding moth Abrostola asclepiadis will only produce one generation per summer if it were to be released into the United States. The ARS researcher also documented that a single defoliation by the larvae has very limited impacts on swallow-worts and insect damage was not enhanced by shading or competition with other plants. The moth A. asclepiadis will therefore be of limited effectiveness in reducing pale and black swallow-wort populations based on previously produced plant models, and it is recommended that this insect be given a low priority for release (two manuscripts published). The seed-destroying fly Euphranta connexa, the second biocontrol agent of swallow-wort, was evaluated for its host specificity by Swiss collaborators. Egg-laying by adult flies into seed pods and successful development of larvae on plant species, besides the normal swallow-wort hosts, was limited to a European species not present in North America. The fly may be suitably host specific but additional testing is required. Testing protocols must still be further refined to ensure high quality seed pods of several plant species are available in large numbers for testing when insects are available. Additional tests have been suspended to focus on a more promising root-feeding beetle. Larvae of the root-feeding beetle Chrysochus asclepiadeus, the third agent, were previously shown in laboratory tests to develop on some North American milkweeds, which are closely related to European swallow-worts. Additional laboratory tests with a French population of the beetle are underway. However, an open-field test by Swiss collaborators showed that adult beetles only fed and laid eggs on swallow-wort plants. Thus, the risk of damage to other plant species appears to be very low under natural field conditions. Follow-up field tests by a French collaborator initiated in 2019, in which beetles only had access to individual species of plants, confirmed that adult beetles did not feed on common milkweed plants although a few larvae were subsequently found on milkweed roots in one of two tests. In contrast, swallow-wort plants were heavily defoliated and had much larger numbers of larvae. Another French open-field test using higher densities of adult beetles is currently in progress. The fungus Sclerotium rolfsii, the fourth agent, was previously discovered killing pale swallow-wort in New York; it was being evaluated as a potential management option for swallow-worts. In collaboration with a plant pathologist, it was shown to be highly virulent to adult flowering plants, similar to previous tests involving seedlings. A two-year overwintering survival study of the pathogen demonstrated that fungal survival by mid-summer was poorer with shallow burial of the fungus compared to fungi on the soil surface. Potential changes in the associated plant community and incidence of disease at the discovery site were documented. The same fungus has recently been documented to cause disease in table beets in New York. Therefore, future studies involving the fungus will need to consider impacts on agricultural crops. A manuscript is in preparation. Subobjective 1b: Elucidate the genetic structure of knapweed populations in the Northeast. Meadow knapweed is widespread in the Northeast and is considered to be a hybrid between black and brown knapweeds (parent species), all of which were introduced from Europe. A manuscript has been published on the population genetics of 10 New York and 10 Vermont populations. Analyses by collaborators at the University of Vermont indicate that three genetic groups are present, extensive mixing among the groups has occurred, and genome size is correlated with genetic ancestry. Additional sampling by collaborators occurred in the Pacific Northwest, where meadow knapweed has long been present, to understand whether this hybrid knapweed resulted from a cross-continental move from Europe or an intracontinental move West to East in the United States. Seeds, leaf and seed head samples were collected from 16 populations distributed across Oregon and Washington. DNA extractions will soon be underway, and morphological measurements of seed heads are planned. In addition, seeds will be used for future common garden experiments comparing quantitative plant traits between western and eastern U.S. populations. Genetic and morphological data will also be compared to European samples collected from six countries. Seeds from the same New York and Vermont populations were germinated and grown in a common greenhouse study by University of Vermont collaborators to correlate various plant and genetic traits with known invasive traits. A second generation experiment was also conducted involving offspring created by selectively crossing parents of different genetic lineages. Plant traits thought to be related to invasiveness showed significant heritability and a strong correlation with the hybrid. A manuscript is in preparation. Subobjective 1c: Determine demographic rates for swallow-wort and knapweed. The time to reproductive maturity may be a critical phase in the life cycle of swallow-worts. Data were collected from a long-term field experiment assessing survival and years to flowering of vegetative juveniles of pale and black swallow-wort in different habitats and locations. Individual plants of black swallow-wort began reproducing at a faster rate (beginning at 3-4 years) compared to pale swallow-wort (6-8 years). Many plants, especially pale swallow-wort, took up to 10 years to flower (manuscript in preparation). If swallow-wort biological control is of limited availability, alternative control measures will continue to be needed by land managers. A manuscript was published on a multi-year mowing study of pale swallow-wort. Mowing at least three times within a season prevented seed production beginning the first year but did not reduce pale swallow-wort densities and cover until the fifth year of mowing. Mowing six times per season did not enhance control. Land managers are being advised that they must mow at least three times annually to prevent the production and spread of seeds, and that it will take several years to reduce but not eliminate existing stands of pale swallow-wort. Also, a three-year herbicide trial will be concluded this summer to assess the efficacy of different herbicides in combination with mowing against black swallow-wort. Currently, no data exist on herbicidal control of black swallow-wort. Spotted and meadow knapweed invade a variety of agricultural fields and natural areas, and they are a potentially new and emerging weed problem for the Northeast. Plant population data, including survival of different life stages and seed production, and information on plant phenology were collected over three years from three spotted knapweed and four meadow knapweed field sites in New York. In addition, the presence of biological control agents that were previously released in the region was documented (manuscript in press). A collaborator (University of Vermont) is currently analyzing these data using a plant population modeling approach. Initial results show that six of the seven populations are increasing in size, and that disruption of the seedling and flowering stages may be important for controlling knapweed populations. Knapweed seed bank and seedling emergence studies were established in fall 2018. Data are being collected with a collaborator to determine how many years buried seeds of spotted and meadow knapweed survive and how successfully knapweed seedlings emerge from different burial depths over time. Deeply buried seeds generally did not germinate and had greater than 90% survival after one year. In contrast, most seeds on the soil surface or that were slightly buried germinated within the first year. 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. Long-term monitoring of vegetation plots occurred prior to release of a biological control agent at several sites in New York State. At some sites, the presence of swallow-wort was correlated with a decline in plant diversity and abundance over time (manuscript in preparation). The biological control agent Hypena opulenta, a moth whose larvae defoliate swallow-worts, was first released in New York State in 2018 at two locations. Two generations per summer were likely based on observations of caged insects. Plant population models developed by us indicated 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. Despite some evidence of successful overwintering in spring 2019, the populations of the moth died out during summer 2019. It was simultaneously discovered by ARS scientists at Ithaca that all colonies of the moth in North America were infected with a pathogen. This prevented additional releases in 2019. A new moth colony has been established which is currently being reared for releases this summer.
Milbrath, L.R., Dolgovskaya, M., Volkovitsh, M., Sforza, R.F., Biazzo, J. 2019. Photoperiodic response of Abrostola asclepiadis (Lepidoptera: Noctuidae), a candidate biological control agent for swallow-worts (Vincetoxicum, Apocynaceae). Great Lakes Entomologist. 52(2).
Biazzo, J., Milbrath, L.R. 2019. Response of pale swallowwort (Vincetoxicum rossicum) to multiple years of mowing. Invasive Plant Science and Management. 12(3):169-175. https://doi.org/10.1017/inp.2019.22.
Lachmuth, S., Molofsky, J., Milbrath, L.R., Suda, J., Keller, S.R. 2019. Associations between genomic ancestry, genome size and capitula morphology in the invasive meadow knapweed hybrid complex (Centaurea ×moncktonii) in eastern North America. AoBP (Annals of Botany PLANTS). 11(5). https://doi.org/10.1093/aobpla/plz055.