Vincetoxicum Rossicum Research Articles

Effects of leaf and root herbivory by potential insect biological control agents on the performance of invasive Vincetoxicum spp.

The European leaf-feeding moth Abrostola asclepiadis and root-feeding beetle Eumolpus asclepiadeus are promising biological control agents for two European swallow-worts ( Vincetoxicum rossicum and Vincetoxicum nigrum) in North America, however, their impact on plant performance is uncertain. Densities of each herbivore were manipulated in a common garden to determine whether leaf and root herbivory affect the performance of these plants. During the second year of the experiment, V. rossicum and V. nigrum unexpectedly became infected with the fungal pathogens Ascochyta sp. and Cercospora sp. (Ascomycota), respectively. Although pathogen infection mainly reduced shoot height and delayed reproduction, herbivore effects on plant growth were still evident. Leaf herbivory by A. asclepiadis had no effect on plant growth 1 year after defoliation. Root herbivory by E. asclepiadeus reduced shoot height and plant biomass and decreased the ability of plants to compensate for pathogen attack. Pathogen infection prevented detection of herbivore effect on reproduction. Due to its substantial impact on plant biomass, E. asclepiadeus should be further evaluated as a biological control agent against Vincetoxicum spp. populations invading open habitats in North America. Further research is needed to evaluate the impact of A. asclepiadis in combination with E. asclepiadeus and plant competition under high and low light conditions.

Establishment of the invasive perennial Vincetoxicum rossicum across a disturbance gradient in New York State, USA

Vincetoxicum rossicum (pale swallow-wort) is a non-native, perennial, herbaceous vine in the Apocynaceae. The species’ abundance is steadily increasing in the northeastern United States and southeastern Canada. Little is known about Vincetoxicum species recruitment and growth. Therefore, we conducted a field experiment in New York State to address this knowledge gap. We determined the establishment, survival, and growth of V. rossicum during the first 2 years after sowing in two old fields subjected to four disturbance regimens. We hypothesized that establishment and survival would be higher in treatments with greater disturbance. At the better-drained location, overall establishment was 15 ± 1% [mean ± standard error] and did not differ among treatments. At the poorly drained location, establishment varied by treatment; mowed and control plots had greater establishment [10 ± 2%] than herbicide + tillage and herbicide-only plots [1.6 ± 0.5%]. Of those seedlings that emerged, overall survival was high at both locations (70–84%). Similarly, total (above + belowground) biomass was greater in herbicide + tillage and herbicide-only plots than in mowed and control plots at both locations. Thus, V. rossicum was successful in establishing and surviving across a range of disturbance regimens particularly relative to other old field species, but growth was greater in more disturbed treatments. The relatively high-establishment rates in old field habitats help explain the invasiveness of this Vincetoxicum species in the northeastern U.S. and southeastern Canada.

Does polyembryony confer a competitive advantage to the invasive perennial vine Vincetoxicum rossicum (Apocynaceae)?

Determining which traits may allow some introduced plant species to become invasive in their new environment continues to be a key question in invasion biology. Vincetoxicum rossicum is an invasive, perennial vine colonizing natural and seminatural habitats primarily in the northeastern United States and southeastern Canada. More than half its seeds exhibit polyembryony, a relatively uncommon condition in which a single seed produces multiple seedlings. For evaluating the potential consequences of polyembryony on invasiveness, V. rossicum plants derived from seeds of three embryonic classes-singlets, doublets, and triplets (one, two, and three seedlings per seed, respectively)-were paired in all combinations intraspecifically and with the co-occurring native herbs Solidago canadensis and Asclepias syriaca in a greenhouse study. Vincetoxicum rossicum biomass was 25-55% greater and follicle production 55-100% greater under intraspecific competition compared with interspecific competition. However, within a competitive environment, follicle production varied little. Regardless of competitive environment, V. rossicum originating from seeds with a greater number of embryos typically performed no better than plants arising from seed with fewer embryos (singlets = doublets = triplets)-except intraspecifically where doublets outperformed singlets, and with S. canadensis where triplets outperformed singlets. Our findings suggest that overall performance and fitness of V. rossicum is higher in monocultures than in mixed stands and that its ability to invade new habitats may not be attributable to the production of polyembryonic seeds.

Phytophagous Arthropods of Invasive Swallow-Wort Vines (Vincetoxicumspp.) in New York

Pale swallow-wort (Vincetoxicum rossicum [Kleopow] Barbar.) and black swallow-wort (V. nigrum [L.] Moench), European species of herbaceous, perennial viny milkweeds, have become increasingly invasive in various natural and managed habitats in the northeastern United States and southeastern Canada, including low- and high-light habitats. A classical biological control program is being developed, but almost no information was available on the current arthropod fauna for either species in the invaded range. I conducted quantitative surveys on pale and black swallow-wort at several locations in New York State over 3 yr to identify and compare the seasonal assemblage of phytophagous arthropods that are feeding and developing on the plants in sunny and shaded habitats. Of the approximately 84 nonpredatory species collected, 10 polyphagous, ectophagous species of native and exotic arthropods were identified, exclusively from the leaves or stems, which could develop to the adult stage and in most cases complete at least one generation on one or both species of swallow-wort. However, their densities were low throughout the season and generally did not differ between the sunny and shaded habitats. Very little to no damage was observed on the plants. Exotic swallow-worts seem to have been released from specialized natural enemies and have not accrued a damaging suite of generalist herbivores. This may be a contributing factor in the increasing invasiveness of these weeds, and biological control appears promising for these plants.

Biology and larval feeding impact of Hypena opulenta (Christoph) (Lepidoptera: Noctuidae): A potential biological control agent for Vincetoxicum nigrum and V. rossicum

A classical biological control program has been initiated against the invasive European swallow-worts Vincetoxicum nigrum and Vincetoxicum rossicum in North America. The noctuid moth Hypena opulenta, discovered feeding on V. rossicum in forests of southeastern Ukraine, is currently under evaluation as a potential biological control agent. In this study the life cycle of H. opulenta and factors affecting diapause induction were evaluated. Additionally, larval impact of H. opulenta on Vincetoxicum spp. performance was tested to determine whether future screening is worthwhile. Adults of H. opulenta begin oviposition 2 days after emergence and produce approximately 600 eggs. Larvae develop through five larval instars and overwinter as pupae. Pupal diapause is facultative, resulting in at least two generations per year. Diapause induction is affected by photoperiod and seasonal changes in plant quality as evidenced by increasing diapause induction when larvae are grown under a short day photoperiod and on senescing plants in the fall. Feeding by two larvae per plant caused reductions in aboveground biomass to V. rossicum resulting in decreased reproductive output (flower, seedpod, and seed production). Only flower production of V. nigrum was negatively affected by larval feeding. The results of this study indicate that H. opulenta is a promising agent against forested populations of V. rossicum and warrants completion of host specificity testing and examination of population dynamics of H. opulenta. This practice of conducting impact assessments of herbivores that are apparent specialist feeders prior to conducting the full battery of host specificity tests should improve agent selection by reducing the costs associated with screening ineffective agents.

Growth and reproduction of invasiveVincetoxicum rossicumandV. nigrumunder artificial defoliation and different light environments

The exotic vines Vincetoxicum rossicum (Kleopow) Barbar. and Vincetoxicum nigrum (L.) Moench have become increasingly invasive in low- and high-light habitats in North America, and a biological control program is being developed. These plants experience little damage in North America, so it is unclear how they might respond to introduced herbivores. I conducted an artificial defoliation study on seedlings and mature plants of V. rossicum and V. nigrum grown under different light environments. Under high light, V. nigrum produced more seed and allocated more resources to aboveground tissue (root:shoot ratios < 1), whereas V. rossicum allocated more resources to roots with root:shoot ratios of 1.9 for mature plants and > 3 for seedlings. These differences disappeared with shading. Increasing frequencies of 100% defoliation caused greater reductions in biomass and seed production for both species and plant stages. Shading further reduced biomass and no seed was produced. Defoliation of shaded, but not unshaded, plants caused high mortality. Additional cutting of stem tips increased branching only. Defoliation may be effective against Vincetoxicum plants growing in low-light environments such as forest understories, but appears to be of more limited value in high-light environments unless repeated defoliation occurs.

Effects of Arbuscular Mycorrhizal Fungi on The Exotic Invasive Vine Pale Swallow-Wort (Vincetoxicum rossicum)

Abstract The ability of arbuscular mycorrhizal fungi (AMF) to influence the performance of nonnative invasive plants in their introduced range has received increasing attention. The dependence of the invasive nonnative vine pale swallow-wort on AMF was studied in three greenhouse experiments. The aims of the present work were to (1) determine AMF colonization levels of field-collected pale swallow-wort plants and several co-occurring native and nonnative plant species, (2) evaluate the growth response of pale swallow-wort to different components of the soil microbial community from an infested site, and (3) determine the growth response of pale swallow-wort when grown with a nonlocal AMF species. AMF root colonization was greater in pale swallow-wort (85, 98, and 50% arbuscules, hyphae, and vesicles, respectively) than in leek (72, 80, and 25%), a species that has been frequently used as a predictor of AMF density in soil. Root colonization of pale swallow-wort in the field was also greater than root colo...

Response of Pale Swallow-wort (Vincetoxicum rossicum) to Triclopyr Application and Clipping

Pale swallow-wort is a nonnative vine, invading natural areas across much of the northeastern United States and southeastern Canada. Control of this clump-forming herbaceous perennial has been difficult. We conducted a 2-yr study (2005–2006) in a heavily infested site in Chaumont, NY to assess the response of swallow-wort to triclopyr applied once as a foliar treatment (1.9 kg ae/ha) (2005 only) alone or in combination with clipping 4 wk later, followed by a single clipping in 2006. We also evaluated the response of swallow-wort to one or two clippings during each of the 2 yr. Two yr after treatments began, swallow-wort cover was lower in plots treated with triclopyr (20 ± 5%) compared with plots subjected to clipping-only (56 ± 6%) or unmanaged controls (76 ± 6%). Stem densities were also lower in triclopyr-treated plots (25 ± 5 stems/m2) than in clipping-only (188 ± 9 stems/m2) and control (178 ± 10 stems/m2) plots across three different sample dates. Seedling densities were lower in triclopyr-treated plots (160 ± 50 seedlings/m2) relative to clipping-only (1,120 ± 180 seedlings/m2) and control (960 ± 50 seedlings/m2) plots after the 2005 growing season. The cover of other plant species was negatively correlated with swallow-wort cover and was higher in triclopyr-treated plots (75 ± 3%) than in clipping-only (5 ± 1%) and control (7 ± 4%) plots in 2006. Across both years, swallow-wort in control and clipped plots produced follicles, but not in triclopyr-treated plots. Regardless of clipping frequency, clipping in June or July was not effective in reducing swallow-wort stem density, cover, or follicle production. Although a single application of triclopyr provided considerable suppression of swallow-wort after two growing seasons, application of triclopyr in subsequent years is likely required to achieve long-term control.Nomenclature: Triclopyr, butoxyethyl ester; Pale Swallow-wort, Vincetoxicum rossicum (Kleopow) Barbar., syn. Cynanchum rossicum (Kleopow) Borhidi.

Survival and performance of the invasive vine Vincetoxicum rossicum (Apocynaceae) from seeds of different embryo number under two light environments

The nonnative vine Vincetoxicum rossicum threatens several ecosystems in the Lower Great Lakes Basin of North America. One feature that may contribute to its invasiveness is the production of some seeds with multiple embryos (polyembryony), which may be beneficial as a bet-hedging strategy in variable environments. However, lower seed reserves per embryo in polyembryonic seeds may entail costs in low-light environments. The effect of seed from three embryonic classes (1, 2, or 3 embryos/seed) on V. rossicum survival and growth was studied under two forest understory light environments: full canopy (shade) or canopy gaps (light) in New York state. Two seedling cohorts were planted, in May 2004 and in May 2005. The survival and growth of seedlings was monitored biweekly for two (2005 cohort) or three (2004 cohort) seasons. For both cohorts, plants grown in canopy shade had reduced survival and growth compared with those grown in gaps. Contrary to expectations, seed embryo number had no effect on the final height, survival, or dry mass of plants in either habitat. Our results suggest that any fitness advantage provided by polyembryony may be habitat (light) dependent and not a general trait that affords V. rossicum a benefit in all habitats colonized.

Tests of the antibiotic properties of the invasive vine Vincetoxicum rossicum against bacteria, fungi and insects

We tested extracts of Vincetoxicum rossicum for inhibition of bacterial and fungal growth and for anti-insect activities that may account for the invasive characteristics of this introduced species in North America. Bioassay-guided fractionation was used to identify (−)-antofine as the principle inhibitor of bacteria and fungi in root extracts. This compound had especially pronounced antifungal activity, inhibiting the growth of diverse taxa that include yeast-like and filamentous fungi and, notably, broad-host-range plant pathogens. A second compound(s), that is as yet uncharacterized but distinct from (−)-antofine, was detected as having antifeedant activity against a larval hymenopteran, rose sawfly ( Allantus cinctus), and toxicity to two larval lepidopterans, the masked birch caterpillar ( Drepana arcuata) and the European corn borer ( Ostrinia nubilalis). That V. rossicum contains potent inhibitors of plant pathogenic fungi, diverse bacteria, and herbivorous insects likely contributes to its success as an invasive species.

Monarch Butterfly Oviposition on Swallow-Worts (<I>Vincetoxicum</I> spp.)

As black swallow-wort, Vincetoxicum nigrum L. Moench, and pale swallow-wort, V. rossicum (Kleopow) Barbar., spread throughout the northeastern United States and southern Canada, there is concern about the impact of these invasive plants on populations of the native North American monarch butterfly Danaus plexippus L. Recent laboratory studies in New York and Ontario, Canada, have found little or no oviposition by monarch butterflies on the two Vincetoxicum species. In Rhode Island, we found 10.5-21.7% oviposition on Vincetoxicum species relative to common milkweed Asclepias syriaca L. in choice tests and 11.9-20.3% in no-choice tests in 2 yr of laboratory testing. These results were supported by field cage trials where monarchs given a choice between V. nigrum and A. syriaca laid 24.5% of their eggs on V. nigrum. In surveys of three pasture fields in Rhode Island where relative coverage of A. syriaca exceeded that of V. nigrum by a 0.77:0.23 ratio, 15.4% of monarch eggs were found on V. nigrum plants. In V. nigrum stands with very little A. syriaca (6.25 stems/ha), monarch egg density on V. nigrum was found to be over five times greater than in the three mixed pasture fields. In none of our laboratory or field evaluations was there any survival of monarch larvae on Vincetoxicum species. It seems that in Rhode Island, Vincetoxicum species serve as an oviposition sink for monarch butterflies. These findings suggest that East Coast butterflies may differ in host selection from those in central New York and southern Ontario, Canada.

Invasive Non-native Plants Alter the Occurrence of Arbuscular Mycorrhizal Fungi and Benefit from This Association

We investigated the influence of three aggressive non-native invasive plants (pale swallowwort [Vincetoxicum rossicum], kudzu [Pueraria lobata], and Chinese privet [Ligustrum sinense]) on arbuscular mycorrhizal fungi (AMF). Our findings from central New York and southeastern Alabama forest habitats confirm that each species formed symbiotic associations with native AMF populations. Mycorrhizal dependency of kudzu was high when grown in invaded (85 percent) and non-invaded (83 percent) soil. The results indicate that invasion into new areas by non-native plant species may alter the occurrence of AMF in the resident soil. Evidence for a possible alteration in the AMF community was obtained largely from a Mycorrhiza Infection Potential bioassay. The bioassay indicated that root colonization levels by AMF of bait plants grown in soil collected from areas where the non-native plants formed pure stands were in general significantly greater than root colonization levels in soils collected from adjacent areas where the invasive plants were not present. Furthermore, the number of AMF spores in soil collected from areas invaded by pale swallow-wort was significantly higher than for nearby non-invaded soil. High colonization levels of pale swallow-wort roots with hyphae were observed during most of the growing season. The presence of arbuscules in pale swallow-wort roots was most evident in July and corresponds with the pre-flowering period of this species.

Growth and reproductive potential of the invasive exotic vine Vincetoxicum rossicum in northern New York State

The alien invasive vine Vincetoxicum rossicum (Kleopow) Barbar. is problematic in the Lower Great Lakes Basin of North America. The lack of effective control strategies exacerbates the need for a better understanding of the growth and reproductive potential of V. rossicum. Thus, a 2-year field experiment was performed in a field site in 2003 and 2004 that was densely colonized by V. rossicum. Mean height of V. rossicum stems was 20% and 45% greater in naturally shaded plots compared with plots receiving full sun in 2003 and 2004, respectively. During the 2-year study, the density of stems 10 cm or taller averaged 134 stems·m–2, substantially greater than reported in previous field studies for this species. The high densities of stems and seedlings indicate that this field site may be near or at carrying capacity. Each stem produced, on average, 17 mature follicles with some stems producing as many as 100 follicles. Each follicle yielded an average of 15 seeds, each composed of an average of two viable embryos. A typical V. rossicum stand in this site produced as many as 54 000 seedlings·m–2 annually. More flowers (2400 flowers·m–2), follicles (2250 follicles·m–2), and total seeds (32 000 seeds·m–2) were produced in plots receiving intermediate light compared with plants in full sun (1650 flowers·m–2, 1600 follicles·m–2, and 25 000 seeds·m–2) or shaded (1600 flowers·m–2, 2000 follicles·m–2, and 28 000 seeds·m–2) plots. Seeds harvested from follicles in the low light plots were 41% more likely to be dormant than seeds collected from follicles in full sun plots. Seeds collected from plants in full sun plots had the lowest frequency (∼36%) of single embryo seeds and the highest frequency (∼64%) of multiple embryo seeds. These findings suggest that V. rossicum growth and reproductive output is influenced by light environment. While competitive strategies differ between the light environments, the decrease in viability and germination in shaded sites may not be large enough to affect population growth over multiple generations. The substantially greater growth and reproduction of V. rossicum reported in this study relative to previous research may explain the increasing abundance and range expansion of this species in many Lower Great Lakes regions of Ontario and New York State.

A Review of the Biology and Ecology of Three Invasive Perennials in New York State: Japanese Knotweed (Polygonum cuspidatum), Mugwort (Artemisia vulgaris) and Pale Swallow-wort (Vincetoxicum rossicum)

Terrestrial weeds continue to evolve in association with the rapid global changes in our land-use systems, due to their regenerative strategies, their adaptability to change, and their inherent diversity. Currently, invasive weeds are estimated to cost the United States’ economy up to $35 billion per year, and this total is rapidly increasing, as greater numbers of invasive species become naturalized. Several invasive exotic species have more recently established across New York State, creating difficulties for agricultural producers, roadside and natural areas managers, and homeowners and resulting in millions of dollars expended annually for their control. Three perennials that have become particularly problematic in New York State in recent years in both agricultural and roadside settings include Japanese knotweed (Polygonum cuspidatum Sieb. & Zucc.), mugwort (Artemisia vulgaris L.), and pale swallow-wort (Vincetoxicum rossicum (Kleop.) Barbar.). This review describes their history, biology, ecology, and potential for management and focuses on key characteristics contributing to their spread in New York State and adjacent regions.

Response of Pale Swallow-wort, <em>Vincetoxicum rossicum</em>, following Aboveground Tissue Loss: Implications for the Timing of Mechanical Control

The growth and reproduction of Pale Swallow-wort, Vincetoxicum rossicum, were investigated following differentially timed above-ground tissue loss by clipping throughout the growing season in Ottawa, Canada. If clipping occurred early in the growing season (before June), mature plants were able to compensate in height but not in biomass or reproductive output when compared to uncut controls. This compensation in height did not seem to come at the expense of below-ground storage tissue; there was no significant difference in root mass among the control and treatment conditions. Final plant height, mass, and reproduction declined as plants were cut later in the season. Pale Swallow-wort seedlings clipped before June were able to compensate in height when compared to uncut controls. Unlike mature plants, this compensation in height did come at the expense of root mass. As the clipping treatment was performed later into the growing season, both final plant height and shoot mass decreased, while root mass increased. The best time for controlling the population spread of Pale Swallow-wort using a single cutting treatment was on or near 26 June. Plants cut earlier than this date were able resprout and produce seeds; those cut after had already produced seeds that appeared viable and may be able to germinate if the cut stems were left in the field. When a single cutting or mowing treatment is to be employed for controlling Pale Swallow-wort, we recommend cutting after the first fruits are produced but before they are fully developed.

A field study of seed dispersal and seedling performance in the invasive exotic vine Vincetoxicum rossicum

The exotic vine Vincetoxicum rossicum (Kleopow) Barbar. (Asclepiadaceae) is a major natural-areas pest throughout the Great Lakes Basin. Colonization of new areas by this herbaceous perennial occurs by comose wind-dispersed seeds. Previous experiments suggested a trade-off between seed dispersability and seed quality: smaller seeds dispersed farther but were less likely to emerge and seedlings were smaller when grown in competition with turf grasses in the greenhouse. We examined dispersability by trapping seeds at distances of 0–60 m from a seed source, and we assessed seedling performance by sowing seeds of known weight in an old field. Smaller seeds travelled significantly farther than larger seeds; however, the relationship between weight and distance dispersed was weak (r2 = 0.043). Large seeds were significantly more likely to emerge, and the seedlings survive and grow taller; however, the relationships between seed size and these performance variables were also weak (r2 ≤ 0.02). The dispersability–quality tradeoff is unlikely to play an important role in V. rossicum spread at the local scale, as even large seeds are competent dispersers and even seedlings from small seeds are capable of becoming established. Seedlings from polyembryonic seeds were more likely to successfully establish than seeds from which a single seedling emerged. The high seedling emergence (50% for seeds planted above ground; 72% for those buried to a depth of 1 cm) and survivorship (71%–100%, depending on embryony) are likely to contribute to the success of this species.

The effect of an invasive alien vine, Vincetoxicum rossicum (Asclepiadaceae), on arthropod populations in Ontario old fields

Vincetoxicum rossicum is an invasive alien vine introduced from Europe in the late 1800s that is now an emerging pest in upstate New York and eastern Ontario. The plant can form dense, monotypic stands in woodlots and old fields, and may be displacing native vegetation. As a consequence, V. rossicum may be displacing arthropod fauna associated with native vegetation. In June and August 2002, we sampled V. rossicum and three other old field plants (Asclepias syriaca, Solidago altissima, and mixed graminoids) for arthropods using pitfall traps and by sampling individual plants. A total of 7868 arthropods were counted on plants and 18,195 individuals were trapped; these were sorted by feeding guild. Overall, stands of V. rossicum supported the lowest abundance of both stem- and ground-dwelling individuals, as well as the lowest number of arthropods in most phytophagous guilds. Some feeding guilds are entirely absent: V. rossicum stands are completely devoid of gall-makers and miners, and support few pollinators. This study suggests that arthropod diversity will decline if V. rossicum displaces native old-field plants.

The Biology of Invasive Alien Plants in Canada. 2. Cynanchum rossicum (Kleopow) Borhidi [= Vincetoxicum rossicum (Kleopow) Barbar.] and Cynanchum louiseae (L.) Kartesz & Gandhi [= Vincetoxicum nigrum (L.) Moench

Cynanchum rossicum (dog-strangling vine) and C. louiseae (black dog-strangling vine) are introduced, perennial herbs or small twining vines in the Milkweed family (Asclepiadaceae). Generic placement of these two species has been problematic, but are here treated as members of Cynanchum subgenus Vincetoxicum. The species occur primarily in natural upland areas, including understories of woodlands, pastures, old fields, shores, flood plains and ruderal areas of southern Ontario and Quebec and the northeastern United States. Ecosystems on well-drained, stony soils are often densely colonized, but both species can tolerate a wide moisture regime. Plants establish in full sun or under forest canopies and may form monospecific stands in all light conditions. They often form dense colonies which smother other vegetation and reduce invertebrate and vertebrate biodiversity. Reproduction is by polyembryonic, wind-dispersed seeds. Effective control is primarily by herbicides, since mechanical control is difficult and no biological control agents have been developed. A third European species, C. vincetoxicum, has been reported as an occasional garden escape in southern Ontario and the northeastern United States, but has not yet become naturalized. Information on this species is included because of its close relationship with C. rossicum and its better known biology. Key words: Dog-strangling vine, swallow-wort, Cynanchum, Vincetoxicum, CYKNI, invasive plant, weed biology

Seed characteristics of the invasive alien vine Vincetoxicum rossicum are affected by site, harvest date, and storage duration

The alien vine Vincetoxicum rossicum (Kleopow) Barbar. (Asclepiadaceae) is a major concern in natural areas of the Lower Great Lakes Basin. Reproduction in this perennial occurs largely by polyembryonic seeds that mature from August to November. Many seeds produced in autumn germinate the following spring, but the germinability of seeds produced in late summer is unknown. The influence of parent plant light environment on seed characteristics in this species is also poorly understood. We examined the characteristics of V. rossicum seeds harvested in late August and early November 2002 from three habitats in central New York State, USA, differing in light availability. Site had a significant effect on seed weight and germination percentage, but not on the number of embryos germinating per seed or radicle length. Seeds produced in the shaded habitat in August weighed significantly more and had lower germin ability than seeds produced in an adjacent open habitat, but this was not observed in a second open habitat. The probability of germination decreased with seed size for seeds ranging from 2 to 8 mg. For the open sites, fresh seeds produced in August had approximately double the germination percentages of those produced in November. However, for all sites, germination following 18 weeks of cold storage was greatest for seeds harvested in November. The probability of multiple embryos germinating was influenced by seed weight. The nature of this relationship varied by site, but not by harvest date or storage duration. Larger seed size resulted in longer total radicle length after 7 d, regardless of site.Key words: Cynanchum rossicum, maternal environment, pale swallowwort, polyembryony, resource allocation, seed dormancy.

Allee effect in an invasive alien plant, pale swallow‐wort Vincetoxicum rossicum (Asclepiadaceae)

I tested for the presence of an Allee effect in pale swallow‐wort Vincetoxicum rossicum, an invasive alien vine, by planting 1‐, 9‐ and 81‐plant patches in an old field. Biomass and seed set of central "target" plants were significantly higher in the 81‐plant patches than in the 1‐ and 9‐plant patches. Biomass of roots did not differ significantly among treatments. The nine central plants of the 81‐plant patches also had higher biomass and produced more seeds than the nine plants of the 9‐plant treatment. Unlike most other examples of Allee effects in plants, the effect in V. rossicum was not due to differences in pollinator visitation rates. Instead, V. rossicum appears to suppress background vegetation more effectively when growing in larger patches. Like many other invasives, V. rossicum has been present in North America for years before it attained pest status. The Allee effect is one of several mechanisms that may contribute to such a latency period.