Swallow-Worts L. Tewksbury - Department of Plant Sciences, University of Rhode Island, Kingston, Rhode Island, USA,

In: Van Driesche, R., et al., 2002, Biological Control of Invasive Plants in the Eastern United States, USDA Forest Service Publication FHTET-2002-04, 413 p.

Pest Status of Weed

There are three European species of swallow-worts found in North America: Vincetoxicum nigrum (L.) Moench (black swallow-wort), Vincetoxicum rossicum (Kleo.) Barb. (pale swallow-wort or dog strangling-vine), and Vincetoxicum hirundinaria Medik. (white swallow-wort) (Sheeley and Raynal, 1996). Swallow-worts are in the family Asclepiadaceae. Vincetoxicum nigrum and V. rossicum now are naturalized in northeastern North America, and both are invasive in natural areas and abandoned pastures (Lawlor, 2000). Swallow-worts are found in gardens and fields, along fencerows, roadways, grassy slopes, wooded edges, and streambanks. Tangled masses of swallow-wort vines shade and suppress native plants (Sheeley and Raynal, 1996). In Rhode Island, heavy growth of V. nigrum reduces the effectiveness of electric fences around pastures (Minto, pers. comm.). Loss of native plant species reduces habitat value for wildlife (Christensen, 1998). Vincetoxicum hirundinaria occurs sparsely in the northeast (Gleason and Cronquist, 1963), and Sheeley and Raynal (1996) suggest that this species is not well established in North America.

Nature of Damage

In central New York, V. rossicum forms dense monospecific stands in shrubby areas with a history of disturbance, and in the understory of successional woodlands (Lawlor, 2000). Near Windsor, Vermont, Lawlor (2000) observed V. nigrum out competing a population of a federally endangered endemic species, Jesup‘s milkvetch, Astragalus robbinsii (Oakes) Gray var. jesupii Egglest. and Sheldon. She also observed that V. rossicum is overgrowing the federally listed Hart’s tongue fern, Phyllitis scolopendrium (L.) Newman at Split Rock, near Onadaga, New York. Vincetoxicum rossicum is threatening the only New England population of Asclepias viridiflora Raf. in Connecticut, an endangered species in that state (Mehrhoff, pers. comm.).

Loss of native plant species may reduce biodiversity and delay or redirect succession (Lawlor, 2000), as well as reduce the value of habitat to wildlife (Christensen, 1998). Cows and sheep in pastures will eat swallow-worts, and control their growth, but swallow-worts spread rapidly in abandoned pastures in New York (Lawlor, 2000) and Rhode Island (Casagrande, unpubl.). Swallow-worts are important weeds requiring management in tree plantations in New York (Lawlor, 2000), and in Rhode Island nurseries (Casagrande, unpubl.).

Current control measures have not been adequate to alleviate harmful effects of swallow-worts. Lawlor (2000) evaluated a number of control techniques used against V. rossicum. Her recommendations include mowing or hand pulling just as pods are beginning to form to minimize seed production. For herbicidal control, repeated applications are necessary. Christensen (1998) conducted experiments in Ontario to evaluate the effectiveness of herbicide control techniques for V. rossicum. Two applications of glyphosate were necessary (in mid-June and early August) to achieve greater than 90% reduction of V. rossicum. Following treatment with herbicide, another invasive plant, Melilotus alba Medicus (sweet white clover), replaced V. rossicum as the dominant plant. Repeated mowing is not successful in reducing the amount of cover of V. rossicum (Kirk, 1985).

Geographical Distribution Gray (1868) first reported Vincetoxicum nigrum as escaping from gardens in Cambridge, Massachusetts. According to Pringle (1973), Scoggan (1979), and Sheeley and Raynal (1996), the distribution of V. nigrum currently extends west from the Atlantic coast to southeastern Ontario and south to southern Pennsylvania and Missouri. There is a record of V. nigrum from California in the USDA PLANTS Database (Fig. 1). Swallow-worts are commonly found in pastures and natural sites in these areas. Vincetoxicum rossicum is distributed from the Atlantic coast west to southern Michigan and northern Indiana, and from southern Ontario, Canada, south through southern Pennsylvania (Pringle, 1973; Scoggan, 1979; Sheeley and Raynal, 1996) (Fig. 2). Moore (1959) reported that Cynanchum medium had been collected frequently since 1889 in Ontario, Canada. Cynanchum medium is a synonym of V. rossicum (Sheeley and Raynal, 1996). Collections of V. rossicum have come primarily from roadsides and wild areas (Sheeley and Raynal, 1996). Vincetoxicum hirundinaria was first recorded in North America in Gray’s Manual (Robinson and Fernald, 1908) as Cynanchum vincetoxicum. There are records of V. hirundinaria from New York, Michigan, and Montreal. The USDA PLANTS Database has only the New York record for the United States (Fig. 3). However, there are no reports of well-established populations of V. hirundinaria in North America (Sheeley and Raynal, 1996). Background Information On The Pest Plant Taxonomy

Figure 1. Distribution of Vincetoxicum nigrum in the United States. (USDA PLANTS Database.) Figure 2. Distribution of Vincetoxicum rossicum in the United States. (USDA PLANTS Database.) Figure 3. Distribution of Vincetoxicum hirundinaria in the United States. (USDA PLANTS Database.)

Swallow-worts are members of the family Asclepiadaceae. The generic placement of the European and North American swallow-wort species is dynamic. The generic names Cynanchum and Vincetoxicum have been applied in North American and European literature. Black swallow-wort has been known as Cynanchum nigrum (L.) Pers. or Vincetoxicum nigrum (L.) Moench. (Kartesz and Gandhi, 1991). We use the scientific nomenclature of Sheeley and Raynal (1996) for the three swallow-wort species introduced into North America: Vincetoxicum nigrum, Vincetoxicum rossicum, and Vincetoxicum hirundinaria. Recent molecular evidence indicates that both Vincetoxicum and Cynanchum are valid genera and that the European species presently in the northeastern United States are all in the genus Vincetoxicum (W. D. Stevens, pers. comm.). These are apparently the only three species of Vincetoxicum present in North America. In Europe, there are 18 native species of Vincetoxicum (with nine subspecies of V. hirundinaria) and one native species of Cynanchum (C. acutum L.) (Heywood, 1972) (Table 1).

Table 1. Native Cynanchum and Vincetoxicum Species in Europe (Heywood, 1972)

Biology

Swallow-worts are herbaceous perennials in the family Asclepiadaceae that grow into twining vines (Fig. 4). In one season a vine can grow 1 to 2 m. In New York, flowering begins in late May, peaks in mid-June and ends in mid-July (Lumer and Yost, 1995). Flowers (Fig. 5), which have the scent of rotting fruit, each remain open for six to eight days. The fruit pods (Fig. 6) release seeds from mid-August to early October (Lumer and Yost, 1995). Vincetoxicum nigrum spreads clonally from deep rhizomes (Lumer and Yost, 1995). Vincetoxicum rossicum moves into new sites by seed (Christensen, 1998). Like other members of the family Asclepiadaceae, these species have parachute-lifted seeds that are carried by wind. Vincetoxicum rossicum is very successful in areas of shallow soil over limestone bedrock (Lawlor, 2000). Swallow-worts growing in shaded areas have thinner stems and tendrils; larger, darker, thinner leaves; fewer roots; and fewer, paler flowers than plants growing in sunny areas (Christensen, 1998).		Figure 4. Vincetoxicum nigrum vine on pasture fence. (Photograph by H. Faubert.)
Figure 5. Vincetoxicum nigrum flower. (Photograph by R.A. Casagrande.)		Figure 6. Vincetoxicum nigrum seed pod. (Photograph by R.A. Casagrande.)

Analysis of Related Native Plants in the Eastern United States

There are no native species in the genus Vincetoxicum in North America. There are fourteen species of native plants listed in the genus Cynanchum in the United States (The Biota of North America Program, 2001) (Table 2). Most have a very limited distribution; nine of the fourteen have been recorded in only one state. One species, Cynanchum laeve (Michx.) Pers., is found in 26 states. The family Asclepiadaceae in North America includes more than 80 species of milkweeds, in the genus Asclepias (USDA, NCRS, 1999). Asclepias syriaca L., common milkweed, is a preferred food source for larvae of the monarch butterfly (Danaus plexippus L.) (Hartzler and Buhler, 2000).

Table 2. Native Cynanchum Species in North America

Haribal and Renwick (1998) found that female monarch butterflies (Fig. 7) oviposit on black swallow-wort (V. nigrum) in choice tests in the laboratory; however, resulting larvae were not able to complete development on this plant. Dacey and Casagrande (pers. obs.) found that monarchs readily oviposit on V. nigrum in the laboratory and in large cages in the field. In both cases, when monarchs were given a choice of ovipositing on common milkweed (A. syriaca) or black swallow-wort, they laid approximately 25% of their eggs on the swallow-wort, and all of the resulting larvae died. Dacey and Casagrande (pers. obs.) found eggshells as evidence of monarch oviposition on black swallow-wort in uncaged field populations of swallow-wort and milkweed.

Figure 7. Monarch butterfly (Danaus plexippus L.) (Photograph by J. Dacey.)

It is not clear what impact V. nigrum may be having on populations of monarch butterflies in nature. Haribal and Renwick (1998) note that in some locations in the United States, V. nigrum has replaced much of the native vegetation, including the Asclepias spp., hosts for monarch butterfly larvae. Milkweeds are used as host plants by at least eight other native North American insects (Arnett, 1985; Palmer, 1985; McCauley, 1991). There is no literature on whether these native insects are able to successfully use Vincetoxicum species as host plants or whether, as with monarchs, they are threatened by these invasive plants.

History of Biological Control Efforts in the Eastern United States

Area of Origin of Weed

Vincetoxicum nigrum is native to southwestern Europe; V. rossicum is found in the Ukraine and southeast Russia; and V. hirundinaria is distributed throughout Europe (Heywood, 1972).

Areas Surveyed for Natural Enemies in North America

Only one insect, the tarnished plant bug (Lygus lineolaris [Palisot de Beauvois]), has been reported feeding on Vincetoxicum species in North America (Lawlor, 2000), but damage was minimal. Sheeley (1992) noted the lack of herbivores and pathogens in swallow-wort stands in upstate New York. Christensen (1998) also found no evidence of herbivory or disease in swallow-worts in Ontario. Christensen suggests that the toxic substances in swallow-worts repel herbivorous insects. It appears that none of the natural enemies known to be associated with swallow-wort in Europe have been accidentally established in North America.

Natural Enemies Found in Europe

There are several potential biological control agents associated with V. hirundinaria in Europe. In western and central Europe, two chrysomelids, Chrysochus asclepiadeus Pallas and Chrysomela aurichalcea ssp. bohemica Mann, are reported as specialists on V. hirundinaria (Mohr, 1966; Dobler et al., 1998). The Russian literature indicates that there are several other species of chrysomelid beetles that feed on Vincetoxicum species in Russia and central Asia (Izhevski, pers. comm.). Five other species of specialist insects herbivorous on V. hirundinaria have been reported in Europe: the noctuid moth Abrostola asclepiadis (Denis and Schiff.) (Forare, 1995); two gall midges, Contarinia vincetoxici Kieffer and Contarinia asclepiadis (Giraud), which feed, respectively, on the flowerbuds and the pods of V. hirundinaria (Buhr, 1965); and two other seed-feeding species, the tephritid fly Euphranta connexa (Fabricius) (Solbreck and Sillén-Tullberg, 1986) and the lygaeid bug Lygaeus equestris (L.) (Kugelberg, 1977; Solberg and Sillén-Tullberg 1990). One weevil, Otiorhynchus pinastri Herbst, is believed to be monophagous on V. hirundinaria in Europe (Dieckmann, 1980; Kippenberg, 1981). The insect fauna of V. nigrum and V. rossicum in Eastern Europe and in Russia is confounded by the plant synonymy at both the specific and generic level. The potential for finding herbivores of swallow-worts seems great given the number of Vincetoxicum species recorded in Eurasia.

Two pathogens of Vincetoxicum spp. are recorded in the literature; Peridermium pini (Pers.) Lev., a stem rust that alternates between pine and other plants such as Vincetoxicum spp. and Paeonia spp. (Gibbs et al., 1988), and Cronartium asclepiadeum (Willdenau) Fries (Gaumann, 1959).

Biology and Ecology of Key Natural Enemies

Little is known on the biology and ecology of natural enemies of swallow-worts. Some information is available on the following species.

Abrostola asclepiadis Schiff. (Lepidoptera: Noctuidae)

In Finland, Sweden, and Denmark, the larva of the noctuid moth A. asclepiadis is monophagous on V. hirundinaria (Forare, 1995). Adults fly in June and July. Development from egg to pupa takes about six weeks, leaving time for only one generation per year in Sweden. Further south in Europe, more than one generation occurs (Forare, 1995). Female moths can lay at least 255 eggs on the undersurface of V. hirundinaria leaves. Small shaded patches of host plants receive higher egg densities than large exposed ones (Forare, 1995). First and second instar larvae feed day and night, moving very little from where the eggs were deposited. Larger larvae are nocturnal and actively search for foliage, feeding mainly at the top of plants. Larvae complete development in August or September and pupate in the soil.

Generalist predators and parasitoids cause most of the mortality of A. asclepiadis eggs in Scandinavia. Ants are the primary generalist predators, but chrysopid larvae, anthocorid bugs, and mites also have been observed feeding on A. asclepiadis eggs (Forare, 1995). Species of Trichogramma (Trichogrammatidae) and Telenomus (Scelionidae) parasitize A. asclepiadis eggs. First and second instars are attacked by the same predators as the eggs. Ants and predaceous pentatomid bugs attack larger larvae. Egg and larval predation are important in reducing the population size of A. asclepiadis (Forare, 1995). This is the primary reason why this insect has very little effect on its host plant population, although occasionally there are small outbreaks of the species, which may cause local defoliation (Forare, 1995).

Euphranta connexa (Fabr.) (Diptera: Tephritidae)

Larvae of the fly, Euphranta connexa, feed on the seeds within the developing pods of V. hirundinaria, attacking 50 to 100% of the pods. Pods attacked by E. connexa usually have most seeds destroyed and even undamaged seeds are more susceptible to fungal pathogens. Euphranta connexa is monophagous on V. hirundinaria and only has one generation per year. The polyphagous ichneumonid Scambus brevicornis (Gravenhorst) parasitizes E. connexa in Sweden, but does not appear to regulate its density (Solbreck and Sillén-Tullberg, 1986).

Lygaeus equestris (L.) (Heteroptera: Lygaeidae)

Lygaeus equestris is a seed-feeding bug that feeds on a number of plant species, but prefers V. hirundinaria. Both nymphs and adults of L. equestris feed on seeds of V. hirundinaria. Lygaeus equestris adults hibernate in crevices in rock walls and buildings from late August through late April or May (Solbreck and Sillén-Tullberg, 1990). In Sweden, it appears to have no serious natural enemies. Weather conditions and food resources limit the populations of L. equestris (Kugelberg, 1977).

Natural Enemies Subjected to Host Range Tests

No natural enemies of Vincetoxicum species have been subjected to host range screening.

Releases Made

No natural enemies of Vincetoxicum species have been released in North America.

Recommendations for Future Work

It is commonly recognized that invasive non-indigenous plants can displace native plants, sometimes also affecting populations of native animals that use these plants for food or shelter. Swallow-worts, which often grow as vines on other plants, are very effective competitors, often warranting control measures on this basis alone. Black swallow-wort is apparently unusual, however, in effectively serving as a sink for monarch butterfly eggs. These important native insects are stimulated to oviposit on swallow-wort, but larvae cannot survive. It is not known how many other native North American insects might also unsuccessfully (or successfully) use this new plant as a host. Laboratory and field cage studies, followed by field surveys in eastern United States, are needed to determine if this plant has the potential to harm populations of other native insects in addition to the monarch butterfly. Such research would also provide information on the effects of native insects on swallow-worts.

European research will lead to a better understanding of the potential for classical biological control of swallow-worts in North America. Swallow-worts are not considered weeds in Europe, presumably because of the complex of insects that feed upon them. There is a high probability that European herbivores can be found with the genus-level host specificity that would be required for consideration for North American introduction. European research will involve detailed literature and herbarium survey for distribution and taxonomy of Vincetoxicum herbivore records. Surveys for insects herbivorous on Vincetoxicum species should be conducted in western Europe, the Balkans, and southern Russia. All natural enemies found should be identified and evaluated. Once promising natural enemies are identified, biological studies and preliminary host range testing with critical North American Asclepiadaceae species will be needed. Host use evolution has been shown in the Chrysochus milkweed beetles in behavioral studies and population genetics (Dobler and Farrell, 1999). Parallel studies could be applied to the chrysomelids and other natural enemies associated with Vincetoxicum in Europe.

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