Location: Emerging Pests and Pathogens Research
2019 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
Subobjective 1a: Assess agents of swallow-wort and knapweed. The leaf-feeding moth Abrostola asclepiadis appears to specifically feed and develop on pale and black swallow-wort, and thus could be petitioned for release as a biological control agent. However, laboratory evaluations of French and Russian populations of the moth with Russian collaborators, as well as outdoor rearing of the French moth by a French collaborator, have shown this species will only produce one generation per summer when exposed to day lengths comparable to those of the northeastern United States (manuscript in press). Additional studies of the impact of larval feeding on plants showed that a single defoliation has very limited effects on swallow-worts (manuscript published). Thus, A. asclepiadis will have limited efficacy against pale and black swallow-wort populations, and it should be given a low priority for release.
The seed-destroying fly Euphranta connexa continues to be evaluated by Swiss collaborators as a candidate agent for swallow-wort control. Testing protocols continue to be refined to ensure flies are mated and laying eggs in order to generate accurate results. Egg-laying by adult flies and larval development in seed pods are currently being tested on some North American plant species of concern.
Previous laboratory tests of the root-feeding beetle Chrysochus asclepiadeus showed that larvae could develop on some North American milkweeds, which are closely related to European swallow-worts. However, adult beetles can be more selective of host plants in the open field. Swiss collaborators conducted an open-field test to determine if adult beetles will or will not colonize and lay eggs on transplanted North American species of milkweeds under natural field conditions. No feeding damage or larvae were found on milkweed plants, but feeding damage and larvae were found on swallow-wort plants. A second open-field test involving common milkweed that has naturalized in France is underway with a French collaborator to more rigorously test the beetle’s host preferences.
The fungus Sclerotium rolfsii was previously discovered killing pale swallow-wort in New York. It is being evaluated as a potential management option. Changes in the associated plant community and incidence of disease at the discovery site are being documented with a collaborator at Cornell University. A separate two-year field study will be completed later this year to document the survival of the fungus from autumn to the following summer. Results to date indicate fungal survival by mid-summer is poorer with shallow burial of the fungus compared to fungi on the soil surface. 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.
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 is nearing publication on the population genetics of 10 New York and 10 Vermont populations. Analyses by collaborators at the University of Vermont revealed two genetic lineages, similar to but not necessarily the two parent species, whose presence varied among the sampled populations. In addition, many hybrid individuals exist as expected and extensive mixing of hybrids has occurred. Populations will be sampled this summer 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.
Greenhouse experiments have been conducted by University of Vermont collaborators to correlate traits such as plant size, time to flowering, and number of flower heads with their genetic ancestry. A previous experiment in which plants were grown from field-collected seeds suggested hybrids had greater vigor in plant traits. A second generation experiment has recently been completed involving offspring created by selectively crossing parents of different genetic lineages. Plant traits thought to be related to invasiveness showed significant heritability. A manuscript of the greenhouse experiments is in preparation.
Subobjective 1c: Determine demographic rates for swallow-wort and knapweed. Plant population data, including survival of different life stages and seed production, are in the process of being collected for a third and final year from three spotted knapweed and four meadow knapweed field sites in New York. A collaborator (University of Vermont) is currently analyzing these data using a plant population modeling approach. Initial results show that most populations are increasing. Key points in the knapweed life cycle will be identified that should be targeted for disruption in order to more effectively control these problematic weeds.
Knapweed seed bank and seedling emergence studies were established in fall 2018. Data are being collected 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.
If swallow-wort biological control is of limited availability, alternative control measures will continue to be needed by land managers. We conducted a seven-year mowing study at a heavily-infested pale swallow-wort field site. Mowing three times per season caused significant reductions in pale swallow-wort densities and percent cover, but generally not until the fifth year of mowing. Seed production was eliminated in all years of the study. Mowing six times per season did not enhance control. Repeated mowing over several years is necessary to reduce, but not eliminate, existing stands of pale swallow-wort (manuscript in revision). In addition, a three-year herbicide trial is in process 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.
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. 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 are likely based on initial observations of caged insects. Successful overwintering has been confirmed from at least one location in 2019. Due to known difficulties mass rearing the moth, additional releases have been delayed to later in the summer. Continued issues with the moth culture may necessitate sourcing moths from field populations already established and increasing in Ontario.
Accomplishments
1. Moth will have limited efficacy as a swallow-wort biocontrol agent. The European vines pale and black swallow-wort are invading various habitats in northeastern North America. The leaf-feeding moth Abrostola asclepiadis appears to specifically feed and develop on swallow-worts, and thus could be a useful biological control agent. However, it is also important to estimate its efficacy as an agent. Russian and French researchers in collaboration with an ARS researcher at Ithaca, New York, investigated the moth’s response to spring and summer day lengths comparable to those experienced in the northeastern United States. The moth was shown to only produce one generation per summer. The ARS researcher also documented that a single defoliation by the larvae has very limited impacts on swallow-worts. 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.
2. Controlling swallow-worts by mowing. The European perennial plants pale swallow-wort and black swallow-wort have become invasive in eastern North America. Mowing is generally considered an ineffective management tool for introduced swallow-worts, but long-term experiments had not been conducted. An ARS researcher in Ithaca, New York, investigated the response of a severe pale swallow-wort infestation to mowing three or six times per year over six years. 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. Similar results would likely apply to other perennial non-woody weed species.
