Tolerance of Swallowworts (Vincetoxicum spp.) to multiple years of artificial defoliation or clipping

Authors: Milbrath, Lindsey; DITOMMASO, ANTONIO - Cornell University; Biazzo, Jeromy; MORRIS, SCOTT - Cornell University

Submitted to: Invasive Plant Science and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/9/2015
Publication Date: 4/6/2016
Citation: 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.

Interpretive Summary: Pale and black swallowwort are European viney milkweeds that have become invasive in eastern North America. We evaluated different artificial defoliation and cutting treatments over six years in a common garden field experiment for their impact on swallowwort survival, growth, and reproduction. Both species appeared to respond similarly to damage. No plants died after six years of the different damage treatments, and in general they increased in size (root biomass, number of crown buds and stems) or did not change (aboveground biomass, seed per plant) over time. The most severe defoliation treatment (100% twice each year) partially limited growth and reproduction compared to undamaged plants. This result calls into question the potential efficacy of a defoliating insect against field populations of swallowworts. Two clippings per season sometimes prevented seed production and should be considered the minimum frequency of mowing for this purpose. Four clippings per season was the only type of damage that consistently prevented plant growth and eliminated seed production, although it is not clear what the long-term effects of high frequency mowing will be apart from eliminating seed. Pale and black swallowwort display a high tolerance to above-ground tissue loss in high-light environments without plant competition.

Technical Abstract: The European vines pale swallowwort (Vincetoxicum rossicum) and black swallowwort (V. nigrum) are invading various habitats in northeastern North America. It is unclear how these plants might respond to potential biological control agents, as they experience little herbivore damage in North America, or longer durations of mowing given the reported lack of efficacy of mechanical control. We evaluated the effect of six seasons of artificial defoliation (50 percent or 100 percent defoliation once or twice per season) and clipping (once, twice or four times at 8 centimeters above the soil level) on the survival, growth, and reproduction of mature plants of the two species grown in a common garden field experiment. No plants died from damage after six years. Black swallowwort produced more aboveground biomass, whereas pale swallowwort produced more root biomass and root crown buds, compared with its congener species. For most damage treatments, root biomass and the number of crown buds and stems increased over time whereas aboveground biomass and viable seed per plant generally did not change. Substantial overlap among treatments also occurred. The most severe defoliation treatment partially limited growth and reproduction compared to undamaged plants. While two clippings per season sometimes prevented seed production, four clippings per season was the only type of damage that consistently prevented plant growth and eliminated seed production. Pale and black swallowwort display a high tolerance to above-ground tissue loss in high-light environments without plant competition. Their annual increase in plant size calls into question the potential efficacy of a defoliating insect against field populations of swallowworts, and it is not clear what the long-term effects of high frequency mowing will be apart from eliminating seed production.