Location: Emerging Pests and Pathogens Research
2016 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: Laboratory studies continue to characterize the daylength response of different populations of a species of leaf-feeding moth (Abrostola asclepiadis) imported into U.S. quarantine as a potential biological control agent of pale and black swallow-wort. A companion field study involving a French population of the moth has been established in France to determine the moth’s response under naturally-changing daylengths. This information will help elucidate the potential number of generations the moth may have per year and thus how much defoliation damage it may cause.
A study examining the impact of defoliation by a Russian population of the moth on pale and black swallow-wort under different light levels has been completed. Results have been incorporated into a previously completed swallow-wort population model for projections of the moth’s ability to control different swallow-wort infestations. Swallow-wort infestations in forests and a few open fields may be controlled with two generations of the moth during the summer. However, the moth’s effectiveness will decrease if it only produces one generation per year.
A seed-destroying fly is currently being evaluated by Swiss collaborators. They are examining if the adult fly will lay eggs, and larvae successfully develop, on various closely-related plants in addition to the swallow-worts.
Research on a disease that can kill swallow-worts has been postponed due to a change in collaborator.
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 studies have been established at three spotted knapweed and four meadow knapweed field sites. Information on plant phenology, seed viability, population densities, and seed numbers in the soil seed bank have been collected. In addition, the presence of biological control agents that were previously released in the region is being documented.
Leaf, seed, and flower head samples were collected from 10 New York and 10 Vermont populations of meadow knapweed, which is a fertile hybrid between brown and black knapweed. Vermont collaborators have extracted DNA from all samples and genetic libraries are being created. Seeds from all sampled populations have been grown in a common greenhouse study and various plant traits were measured from the resulting plants. Correlations between these physical traits, genetic identities, and known invasive traits will be conducted by the Vermont collaborators.
Data collection continues for a long-term field experiment assessing survival and growth of vegetative juveniles of pale and black swallow-wort in different habitats which may be a critical phase in the life cycle of swallow-worts. Apart from the first year (2008), black swallow-wort survival has been greater than pale swallow-wort across the three habitats (old field, forest edge, forest understory). Low plant survival in recent years (<10%) may be due to both a dense cover of resident vegetation in the old field habitat and leaf litter in the forest as well as wetter conditions at the site than swallow-wort tolerates. Black swallow-wort has been confirmed to have a shorter juvenile phase than pale swallow-wort, although several remaining plants of both species have yet to flower and produce seed after eight years.
A multi-year seed bank study of swallow-wort has been completed and a manuscript is in preparation. The seed bank dynamics for these invasive species was poorly known. The majority of seedling emergence occurred during the first year (92% in 2012), and no new seedlings emerged after the second year. Also, no buried, viable seed was recovered after the second year. Both pale and black swallow-wort appear to have a short-lived seed bank. In addition, although many seedlings died before reaching the soil surface when sown at 5 or 10 cm depths, one pale swallow-wort and many black swallow-wort seedlings did successfully emerge from 10 cm which was not expected. Manuscripts are in preparation for a plant population modelling study of pale and black swallow-wort.
A long-term mowing study continues for a fifth season at a heavily-infested pale swallow-wort field site in an effort to document the efficacy of alternative control measures should biocontrol not be a viable option. Preliminary analyses have yet to show a change in pale swallow-wort densities from high frequency mowing (three or six times per season).
Objective 2: Long-term monitoring of vegetation plots will continue next season at several sites slated for the future release of swallow-wort biological control agents. A petition submitted by colleagues for U.S. release of a moth biological control agent remains under review but is expected to be approved. 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 researchers in Ithaca, New York indicate this moth could be an effective agent against swallow-wort in forests but may only have a limited impact on infestations in fields.
Accomplishments
1. Biological control potential of a moth against swallow-wort in forests. Pale and black swallow-wort, European twining vines, have invaded forest and field habitats in northeastern North America. A moth (Hypena opulenta) collected from the Ukraine, whose larvae eat the leaves of swallow-wort, has been released in Canada and may be released in the U.S. but no information existed on its impact on swallow-wort plants under low light conditions typical for forest understories. ARS researchers in Ithaca, New York determined in a greenhouse study that increasing amounts of defoliation and shading, but not necessarily frequency of defoliation, reduced plant biomass and seed production of both species of swallow-wort. However, even severe defoliation generally caused little mortality of seedlings. Land managers are being advised that Hypena opulenta must completely and repeatedly defoliate swallow-wort every year for this biological control agent to suppress forest infestations of swallow-wort.
2. Swallow-worts are highly tolerant to multiple years of damage. The European vines pale and black swallow-wort are invading various habitats in northeastern North America. Different control tactics have been investigated in studies of one to two year’s duration, but it was unclear how these plants might respond to multiple years of defoliation by a biological control agent or different frequencies of mowing. ARS researchers and university collaborators in Ithaca, New York determined in a field experiment that swallow-wort plants continued to grow larger and in most cases produced abundant seed every year for six years despite artificially removing leaves or clipping plants to varying degrees. Clipping plants four times every season was the only type of experimental damage that prevented plant growth and seed production, but it did not kill the plants. The limited effect of the severest defoliation treatment (complete leaf removal twice per season) calls into question the potential efficacy of a defoliating insect against swallow-wort populations growing under full sun conditions. The only benefit of a long-term mowing program, which needs to occur at least twice per season, will be to eliminate seed production.
3. Swallow-wort chemical unlikely to harm other plants in the field. Antofine is a chemical produced by the invasive weeds pale and black swallow-wort that can inhibit the growth of other plants in laboratory studies giving swallow-wort a potential competitive advantage. ARS researchers in Ithaca, New York have found substantial amounts of antofine in seeds, seedlings, and adult plants but it was not present in detectible amounts in soil samples collected over two growing seasons at four different field sites infested with pale or black swallow-wort. Further study showed that swallow-wort roots exude very low levels of antofine and that it readily breaks down into inactive compounds. Antofine does not appear to promote the invasion of swallow-worts into plant communities through the inhibition of other plants under natural conditions.
None.
Review Publications
Milbrath, L.R., Ditommaso, A., Biazzo, J., Morris, S.H. 2016. Tolerance of Swallowworts (Vincetoxicum spp.) to multiple years of artificial defoliation or clipping. Invasive Plant Science and Management. 9:1-11.
Milbrath, L.R., Biazzo, J. 2016. Impact of the defoliating moth Hypena opulenta on invasive swallow-worts (Vincetoxicum species) under different light environments. Biological Control. 97:1-12.
