Chapter 13 Black and Pale Swallow-Wort (Vincetoxicum nigrum and V. rossicum): The Biology and Ecology of Two Perennial, Exotic and Invasive Vines C.H. Douglass, L.A. Weston, and A. DiTommaso Abstract Black and pale swallow-worts are invasive perennial vines that were introduced 100 years ago into North America. Their invasion has been centralized in New York State, with neighboring regions of southern Canada and New England also affected. The two species have typically been more problematic in natural areas, but are increasingly impacting agronomic systems such as horticultural nurs- eries, perennial field crops, and pasturelands. While much of the literature reviewed herein is focused on the biology and management of the swallow-worts, conclu- sions are also presented from research assessing the ecological interactions that occur within communities invaded by the swallow-wort species. In particular, we posit that the role of allelopathy and the relationship between genetic diversity lev- els and environmental characteristics could be significant in explaining the aggres- sive nature of swallow-wort invasion in New York. Findings from the literature suggest that the alteration of community-level interactions by invasive species, in this case the swallow-worts, could play a significant role in the invasion process. Keywords Allelopathy • Genetic diversity • Invasive plants • Swallow-wort spp. • Vincetoxicum spp. Abbreviations AMF: Arbuscular mycorrhizal fungi; BSW: Black swallow-wort; PSW: Pale swallow-wort C.H. Douglass () Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins, CO 80523, USA [email protected] A. DiTommaso Department of Crop and Soil Sciences, Cornell University, Ithaca NY 14853, USA L. A. Weston Charles Sturt University, E.H. Graham Centre for Agricultural Innovation, Wagga Wagga NSW 2678, Australia Inderjit (ed.), Management of Invasive Weeds, 261 © Springer Science + Business Media B.V. 2009 262 C.H. Douglass et al. 13.1 Introduction Black swallow-worts (BSW) and pale swallow-worts (PSW) are invasive, herbaceous perennial vines that were introduced over a century ago into North America. Currently both species are on banned or prohibited plant species lists in Connecticut, Massachusetts, and New Hampshire, and BSW is classified as a “noxious” weed in Vermont (USDA Plants Database 2008). Like many invasive species, the two swal- low-worts exhibit numerous attributes of ideal weeds (Baker 1974). That is, they are strong competitors for available and sometimes scarce resources, are prolific repro- ducers, and can significantly alter invaded habitats (Ernst and Cappuccino 2005; Greipsson and DiTommaso 2006; Smith 2006). The focus of our research on the swallow-wort species has been to better understand the specific similarities and dif- ferences among these two congeneric species and evaluate if there is a physiological or genetic basis for their rapid invasion in regions of North America. This chapter is a summary of the literature that has been presented with respect to both species, and in addition, an overview of our recent work related to their spread, allelopathic poten- tial, and genetic diversity among populations in New York. By developing a broader understanding of a plant species’ biology and ecology in particular locations, we can try to develop more effective strategies for the management of these species and other problematic nonnative invaders, which have thus far evaded effective control. Unlike some more infamous plant invaders, swallow-worts produce small flow- ers and their often prostrate growth habit allows them to easily blend in with resi- dent vegetation. Swallow-worts often persist largely unnoticed by landowners or managers until they are well established and have displaced prior vegetation (Lawlor 2003; West and Fowler 2008). Mature vines can grow to several meters in length, and infestations can often become dense impenetrable thickets of inter- twined vines (hence the common name synonym dog-strangling vine) (DiTommaso et al. 2005b; Sheeley and Raynal 1996). Most significantly, cultural control meth- ods that effectively reduce mature infestations are not available at present. Current recommendations for control are limited to the use of broad-spectrum herbicides and mechanical controls that must be repeated both during the growing season and annually for several years. Unfortunately, even these laborious and expensive strat- egies only provide reliable and sometimes temporary control of smaller satellite populations (Averill et al. 2008; Lawlor and Raynal 2002; Weston et al. 2005). 13.2 Taxonomy PSW [Vincetoxicum rossicum (Kleopow) Babar. = Cynanchum rossicum (Kleopow) Borhidi] and BSW [Vincetoxicum nigrum (L.) Moench = Cynanchum nigrum (L.) Pers. = Cynanchum louiseae (L.) Kartesz & Gandhi] are generally placed in the Asclepiadaceae (Gleason and Cronquist 1991; USDA Plants Database 2008). Recent work, however, suggests that the species should more accurately be placed in the 13 The Biology and Ecology of Two Perennial, Exotic and Invasive Vines 263 Apocynaceae (S.J. Darbyshire, personal communication). In Europe there is evi- dence of successful hybridization between PSW and another relative, white swal- low-wort (V. hirundinaria Medik.) (Lauvanger and Borgen 1998). This evidence not only contributes to taxonomic confusion regarding identification of swallow-worts, but also points to the potential for pale and black wallow-worts to hybridize (DiTommaso et al. 2005b). There remains a great deal of confusion as to the correct taxonomic placement of the invasive swallow-wort species, with some taxonomists placing them under the genus Cynanchum and others in the genus Vincetoxicum. Given that there are a number of native North American plants in the genus Cynanchum [22 according to the USDA Plant Database (2008)], DiTommaso et al. (2005b) proposed that Vincetoxicum should be used solely for the alien species in order to denote their old world origins. This view is shared by Liede (1996), who, primarily on the basis of the presence of unique alka- loids and glycosides, separates Vincetoxicum from Cynanchum and places PSWs and BSWs within the genus Vincetoxicum (also Liede and Tauber 2002). From a review of the literature, no studies have been performed to determine the ploidy level or chromosome numbers for swallow-wort species or populations in the USA, and other evaluations that have been performed were limited in their geo- graphic focus. A chromosome count of 2n = 22 was reported for PSW plants in Ottawa, ON, Canada (Moore 1959), while chromosome counts for BSW plants vary from n = 11 in Spain (Diosdado et al. 1993) to 2n = 22 and 2n = 44 for two populations in Italy (Aparicio and Silvestre 1985; Moore 1959). At this time, we have no information on chromosome numbers of swallow-wort populations in the USA, which could provide valuable information to more fully describe the geno- typic relationships among and between these two species. PSW is native to eastern regions of the Ukraine and southwestern portions of Russia north of the Black Sea and Caucasus; BSW (V. nigrum) is endemic to south- western Europe, particularly regions of the Iberian Peninsula, southern France, and northern Italy (DiTommaso et al. 2005b). In their native ranges, the two species are relatively rare and do not overlap, with scattered patches of 3–15 stems of BSWs found to be typical of native populations in southern France (DiTommaso et al. 2005b; L.R. Milbrath, personal communication; Tewksbury et al. 2002). While PSW has been reported as invasive in one case in Norway, the third Vincetoxicum species, V. hirundinaria, is actually much more widespread and has greater invasive potential in Europe than the two species that are problematic in North America (Lauvanger and Borgen 1998). 13.3 Reproductive Biology and Phenology Flowering can begin as early as mid to late May for PSW populations in Central New York and will peak several weeks later in early to mid June (Sheeley 1992). Floral development can be delayed by 10 days in populations farther north in New York, and by up to 4 weeks for populations in Ontario and other regions of 264 C.H. Douglass et al. southern Canada (DiTommaso et al. 2005b; Lawlor 2000). BSW flowering tends to peak in mid to late June, though in shadier sites it can be delayed by up to a month (DiTommaso et al. 2005b). The swallow-wort species are self-compatible, but are also insect pollinated by a variety of fly, ant, bee, wasp, and beetle species (DiTommaso et al. 2005b; Lumer and Yost 1995). Fruit development for PSW typically begins in early June in Central New York, with maturity occurring 4–5 weeks after flowering, and finally dehiscence peaking at the end of July; development in BSW is normally 2–4 weeks slower (DiTommaso et al. 2005b; Lawlor 2000). There appears to be a physiological dormancy require- ment for PSW seeds, and this likely applies as well to the black species. Though minimal germination is often found experimentally with PSW without subjecting seeds to a cold stratification period, greater germination occurs in seeds subjected to a stratification treatment (Cappuccino et al. 2002; DiTommaso et al. 2005a). While some studies have found a significant positive correlation between seed size and the probability of germination in PSW (DiTommaso et al. 2005a), other studies have reported no correlation between seed size and germinability (Cappuccino et al. 2002; C.H. Douglass, unpublished data). Cappuccino et al. (2002), however, did find that seed size in PSW was positively correlated with final dispersal dis- tance, especially for seeds that had been subjected to a stratification period of 3 months. This effect was weaker in a later study (Ladd and Cappuccino 2005) though the positive trend did generally hold true. While these authors also found that larger seeds tended to produce taller seedlings during the first growing sea- son, they concluded that there was a nominal advantage in survivorship associated with a greater initial seed weight during three growing seasons. Swallow-wort species produce polyembryonic seed, and estimates suggest that 45–75% of PSW seeds are polyembryonic (Sheeley 1992; St.