Location: Emerging Pests and Pathogens Research2018 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.
Objective 1: The leaf-feeding moth Abrostola asclepiadis has been shown by a French collaborator to produce only one generation per summer when reared outdoors for two years under naturally-changing daylengths and at a latitude similar to New York State. This result complements previously-completed laboratory studies on diapause in the United States. Thus, as a candidate biological control agent, the moth will have a limited impact on pale and black swallow-wort because of the plants ability to regrow following insect damage. A manuscript is under review that reports on the feeding impact on pale and black swallow-wort of the moth Abrostola asclepiadis both alone and in combination with different levels of shading or plant competition. Shading or competition alone reduced plant performance, in some cases comparable to insect damage. However, the different sources of stress did not appear to interact and therefore did not enhance the effectiveness of the moth. A seed-destroying fly of swallow-worts continues to be evaluated by Swiss collaborators. Testing protocols are being refined to ensure insects are mated in order to document if female flies will lay eggs, and larvae will successfully develop, on various plant species closely related to the swallow-worts. Swiss collaborators are also conducting an open-field test of a root-feeding beetle to determine if female beetles will or will not colonize and lay eggs on North American species of milkweeds. Depending on the results, additional screening of the insects may occur overseas or in quarantine in the U.S. A two-year study is in progress to document the survival in the soil of a disease-causing fungus of swallow-worts that was discovered in New York. Fungal survival by mid-summer is poor with shallow burial of the fungus, whereas fungal survival on the soil surface is around 50%. Additional host-range tests of the fungus were delayed due to the retirement of a key collaborator; the tests may be implemented next year. Data collection has ended for long-term field experiments assessing survival and time to reproductive maturity 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. This result confirms previous greenhouse and outdoor experiments on the rapid maturation rate of black swallow-wort. In an effort to document the efficacy of alternative control measures should biocontrol be of limited availability, a final year of data is being collected for a long-term mowing study at a heavily-infested pale swallow-wort field site. Preliminary analyses suggest that high-frequency mowing (three or six times per season) may reduce pale swallow-wort densities to a limited extent but only after several years of mowing. In addition, a two-year herbicide trial is being established 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. A plant population modelling study of pale and black swallow-wort, including projected outcomes of biological control efforts, and a swallow-wort seed dispersal study have been published. Plant population data, including survival of different life stages and seed production, are in the process of being collected for a second year 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, knapweed seed bank and seedling emergence studies will be established later this summer to determine how many years seeds will take to germinate at different burial depths as well as the longevity of buried seeds. A manuscript will be submitted on the population genetics of 10 New York and 10 Vermont populations of what had been initially identified as meadow knapweed. Meadow knapweed is considered to be a hybrid between black and brown knapweeds. Analyses indicate that three genetic lineages are present, although this varied among the sampled populations, suggesting that the hybrid and forms very similar to the parental species still exist in the field. Extensive mixing among the groups has occurred. In addition, genome size and a subset of flower head traits are correlated with the genetic ancestry. Various plant growth traits measured in a greenhouse study of the different plant lineages, which are known to be associated with invasiveness, also showed strong correlations with the hybrid forms. A second manuscript is in preparation regarding the greenhouse study. Objective 2: A biological control agent, the moth Hypena opulenta, was approved for field release in September 2017. Releases of larvae, which feed on the leaves of swallow-worts, have occurred in June 2018 at two locations in New York State. Additional releases are planned with collaborators for other locations in 2018. A laboratory colony is also being maintained for more extensive releases in 2019.
Milbrath, L.R., Davis, A.S., Biazzo, J. 2018. Identifying critical life stage transitions for biological control of long-lived perennial Vincetoxicum species. Journal of Applied Ecology. 55:1465-1475. https://doi.org/10.1111/1365-2664.13065.
Ditommaso, A., Stokes, C.A., Cordeau, S., Milbrath, L.R., Whitlow, T.H. 2018. Seed dispersal ability of the invasive perennial vines Vincetoxicum nigrum and Vincetoxicum rossicum. Invasive Plant Science and Management. 11:10-19. https://doi.org/10.1017/inp.2018.8.