Vincetoxicum nigrum
(black swallowwort)

V. nigrum is a herbaceous climbing vine native to south-western Europe (Italy, France, Portugal and Spain). It has spread extensively throughout New England, the lower Hudson River basin and south-eastern Pennsylvania, USA. V. nigrum produces a large number of wind-borne, self-fertile seeds which readily disperse to new sites. Small patches of V. nigrum can coalesce to form large, monospecific stands that outcompete with native vegetation and alter habitats. In Vermont and New Hampshire, USA, the endemic and endangered species, Astragalus robbinsii var. jesupii, is under pressure by an expanding population of V. nigrum. In addition to this V. nigrum may have negative impact on the monarch butterfly, Danaus plexippus.

V. nigrum is native to western European Mediterranean regions such as France, Italy, Spain and Portugal. V. nigrum has been introduced into North America where it is primarily in south-eastern New York, eastern Massachusetts and southeastern Pennsylvania, USA. Smaller populations are scattered in Ontario and Quebec, Canada and in the mid-western USA and southern California.

It is possible that accidental introductions of V. nigrum will continue via agricultural commodities, contaminated soil and vines/seeds caught on equipment. In addition to this, seeds of this species are available online for sale and the attractive flowers and pods invite collection by an uneducated public.

In the USA The state of Vermont, has included V. nigrum in a noxious weed quarantine list. It is included on invasive plant lists for New York State and the New England states, but the lists have no regulatory power.

V. nigrum is found in a wide range of upland habitats including woods and moist sunny places (Gleason and Cronquist, 1991). Habitat in its native western Mediterranean region is slopes, copses and stony dry areas from sea-level to 580 m, for instance in the Pyrenees (Fournier, 1977).

In North America, where it has escaped cultivation, V. nigrum habitat is widely varied including sunny, open areas, shrub habitats and fully shaded forest on substrates ranging from rocky slopes and stony dry areas to riparian terraces, stony shores and garden soils, at altitudes from sea level to at least 300 m. Coastal and roadside infestations are common, indicating a probable tolerance of alkaline soils. It is associated with calcareous soils, but may also be found in granite-based soils.

Genetics
The chromosome number of V. nigrum (Cynanchum nigrum Pers.) is 2n=44.

Physiology and Phenology

The polyembryonic seeds of V. nigrum germinate in autumn and spring, with improved germination after vernalization, yielding 1-4 seedlings (Lumer and Yost, 1995). A fibrous root system radiates from the crown. Seedlings have pointed ovate leaves. First-year growth is juvenile. Plants are clonal with a deep rhizome system (Lumer and Yost, 1995). Perennating buds on the root crown readily sprout after top destruction. In successive years, plants emerge in May and expand rapidly. Twining begins as flower buds begin to enlarge. Flowering occurs from mid to late May until mid-July, with diminished flowering during September. Flowers begin to open just after sunrise, remaining open for six to eight days. Fruit dehisce and release seeds from mid-August until early October in New York State, USA (Lumer and Yost, 1995). The seeds bear a coma (a tuft of silky hairs). The plants senesce after seed dispersal. Dead stems persist through winter and the successive growing season.

Reproductive Biology

Reproduction is amphimictic, by entomophily or self-pollination, and vegetative, by short rhizomes. Flowers are visited mainly by Hymenoptera, Diptera and Coleoptera (Lumer and Yost, 1995). Seed longevity is unknown. Seed production is greatly reduced in low light environments.

Associations

The monarch butterfly, Danaus plexippus, will lay eggs on V. nigrum, but larvae die (Casagrande and Dacy, 2001).

Environmental Requirements

V. nigrum, originally a western European Mediterranean species, has naturalized and spread in the continental climate of eastern North America, withstanding winter lows as extreme as -38°C and uniform annual precipitation over 1100 mm. It has a wide range of light and moisture tolerance. Habitats include meadows, successional shrublands, hedgerows, copses and woodland from rocky shores to more than 300 m altitude. The plant has a preference for lime-derived soils but is also found in granitic-based systems. Shore and roadside occurrences indicate salt tolerance.

One polyphagous Lepidoptera species, Nothris congressariella, is reported in the literature of insects associated with V. nigrum. However, eight other insect species have been reported on species of Vincetoxicum in Europe (Tewksbury et al., 2002) and are being considered for their potential as a biocontrol agent. There are no records of phytophagous insects on V. nigrum in North America. V. nigrum is one of the Vincetoxicum species susceptible to the European rust fungus Cronartium flaccidum, which causes cronartium rust and resin-top in conifers.

V. nigrum may be confused with a number of closely related species in the genus Vincetoxicum.

In North America where their distributions overlap, V. nigrum is often confused with V. rossicum. However, flowers and fruits of V. rossicum tend to be smaller; flowers are 4-6 cm across, glabrous, creamy pale to dark maroon, on peduncles 2-5 cm long.

In Europe, V. hirundinaria has yellow-white flowers and only occasionally twines whereas V. fuscatum has brown flowers with short, erect stems. Lack of taxonomic agreement, numerous subspecies and evidence of hybridization all add to confusion between species.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Cultural Control

There are no reports of cultural control of V. nigrum, however grazing of the closely related V. hirundinaria by sheep has been investigated (Haeggström, 1990) and V. rossicum has been suppressed in cattle pastures in New York State, USA.

Mechanical Control

Mowing, pulling and other top destruction methods have poor long-term success for control of V. nigrum, as there is rapid regrowth from perennating buds. Effective timing of cutting, as fruit begin to enlarge, will prevent maturation of the seed crop and prevent successful recovery to form a new seed crop. Digging must remove the rhizomes and root crown.

Chemical Control

Triclopyr was found to be more effective at reducing the related species V. rossicum than has glyphosate (Lawlor and Raynal, 2002). A systemic herbicide must be used against this perennial plant.

Biological Control

Possibilities for biological control of V. nigrum and V. rossicum are currently being investigated (Tewksbury et al., 2002). Host-range studies at the University of Rhode Island (URI), USA, for Chrysolina aurichalcea ssp. asclepiadis have shown that this species is an unsuitable agent owing to the potential risk of attack on native North American Asclepias and Asteraceae (Weed and Casagrande, 2011).

In contrast, host-specificity tests conducted with more than 80 test plant species showed that the leaf-feeding moths Hypena opulenta and Abrostola asclepiadis are very host specific: successful larval development occurred only on species of Vincetoxicum. In 2014, mature H. opulenta larvae and pupae were successfully released on V. rossicum in Ottawa, Canada and the first release on V. nigrum was made in 2016. In 2016, H. opulenta had not yet been approved for release in the USA. Work on Chrysochus asclepiadeus did not show such a high host specificity level as for the two leaf feeding moths. Comparative host specificity studies are planed with various populations of C. asclepiadeus. Preliminary studies with the fruit fly Euphranta connexa indicate high host specificity. The potential of pathogens for biological control of V. nigrum has not yet been investigated. The chemical signature of Old World Asclepiadaceae may separate Vincetoxicum spp. from Asclepiadaceae species of the New World for effective host specificity.

IPM

Prevention and early eradication are the best control strategies for V. nigrum. Seed crops from upwind infestations must be managed to successfully prevent new infestations.

28/12/2016 Updated by:

André Gassmann, CABI-CH, Switzerland