Lindsey R. Milbrath

Overwintering Survival, Phenology, Voltinism, and Reproduction Among Different Populations of the Leaf Beetle Diorhabda elongata (Coleoptera: Chrysomelidae)

Abstract The classical biological control program for exotic saltcedars (various Tamarix species and hybrids) has involved the assessment of different populations of the leaf beetle Diorhabda elongata (Brullé) s.l. that are promising for release in areas of North America that are located south of 37° N latitude. We report here the overwintering survival, phenology, and voltinism of four D. elongata populations (Tunisia, Crete, Uzbekistan, and Turpan) in eastcentral Texas. In addition, we studied their developmental and reproductive biology, which also included the previously released population from Fukang, China. Overwintering survival of the adult beetles of the Crete and Tunisia populations was 90–99 and 75%, respectively. The Uzbekistan and Turpan beetles had <31% overwintering survival. All D. elongata populations began ovipositing in late March. The Turpan beetle may produce three summer generations and ceased oviposition by September. The Crete beetle produced four summer generations plus a partial fifth generation and ceased ovipositing by mid-October. Both the Tunisia and Uzbekistan beetles produced five summer generations plus an unsuccessful partial sixth generation; oviposition extended into late November. Larval development and survival were generally similar among D. elongata populations. The Turpan and Fukang beetles had a shorter preoviposition period and produced more but smaller egg masses than the other beetle populations. However, this did not alter a female’s lifetime fecundity and generally did not affect the innate capacity for increase compared with other populations. The Crete beetle seems to be the most promising for release in central Texas and points further south.

Survival, Growth, and Fecundity of the Invasive Swallowworts (Vincetoxicum rossicum and V. nigrum) in New York State

Abstract Black and pale swallowwort (BSW and PSW, respectively) are perennial, herbaceous vines in the Apocynaceae that are native to Europe. The species are becoming increasingly abundant in the northeastern United States and southeastern Canada and are difficult to manage. However, we know little about the demographic parameters of these species. We determined the survival, annual rate of vegetative growth, and fecundity of mature clumps of these swallowwort species. We selected four PSW sites (three of which comprised both old-field and forest habitats) in central New York and three BSW old fields in southeastern New York. BSW is largely restricted to higher light habitats in its introduced range. In each habitat, we followed the growth of 30 to 32 randomly selected clumps of similar size (2 to 5 stems clump−1 in the initial year) for 3 to 4 yr. Yearly survival was 99.6 ± 0.3% [mean ± standard error] for PSW and 100 ± 0% for BSW. In old fields, vegetative expansion varied from −0.01 ± 0.1 to 4.6 ± 0.4 stems clump−1 yr−1 for BSW and −0.02 ± 0.2 to 2.1 ± 0.5 stems clump−1 yr−1 for PSW. In forests, PSW growth was lower with vegetative expansion ranging from −0.01 ± 0.1 to 0.8 ± 0.2 stems clump−1 yr−1. Fecundity of PSW in 2007 and 2008 (130 ± 10 viable seeds stem−1 yr−1) was similar to BSW (100 ± 10 viable seeds stem−1 yr−1). Fecundity of PSW in forests was generally lower than PSW in old fields, but it varied greatly among sites (0 to 170 viable seeds stem−1 yr−1). We found that stem growth and fecundity did not vary with clump size (stems per clump). Since vegetative expansion and fecundity rates were high in old-field habitats, but were generally low or nonexistent in forest habitats, we suggest that management of these two invasive vines be focused in higher light environments to reduce overall seed production and its subsequent spread to surrounding areas. Nomenclature: Black (or Louises) swallowwort, Vincetoxicum nigrum (L.) Moench., syn. Cynanchum louiseae Kartesz & Gandhi; Pale (or European) swallowwort, Vincetoxicum rossicum (Kleopow) Barbar., syn. Cynanchum rossicum (Kleopow) Borhidi Interpretative Summary: Black swallowwort (BSW) and pale swallowwort (PSW) are climbing herbaceous perennials related to milkweeds. Although both vines were introduced from Europe into the northeastern United States and southeastern Canada in the latter 1800s, they increasingly have become problematic in the last 20 yr. Many gaps remain in our knowledge of swallowwort field biology, which can vary by plant species and habitat. We found that nearly all mature plants survived during a 4-yr field study in New York in which we tracked individual plants in relatively low-density stands in old fields (BSW and PSW) and forest understories (PSW only). We rarely have observed BSW in heavily shaded forests. Both species annually increased in size through the production of additional stems, and this was generally similar for PSW and BSW in old fields and much less for PSW in forests. BSW and PSW in old fields also produced the same amount of viable seed, but PSW in forests produced less seed. However, both reproduction and vegetative expansion by PSW in forests varied greatly, depending on the amount of light within the forest. Based on our findings in this study, it does not appear that the two species should require different strategies for management. We suggest that management of swallowwort should be focused in higher light environments, where vegetative growth and reproduction is greatest, to reduce seed production and thereby minimize its spread to other habitats.

Acceptability and Suitability of Athel, Tamarix aphylla, to the Leaf Beetle Diorhabda elongata (Coleoptera: Chrysomelidae), a Biological Control Agent of Saltcedar (Tamarix spp.)

Abstract The leaf beetle Diorhabda elongata (Brullé) sensu lato has been released in the western United States for the classical biological control of exotic saltcedars (Tamarix species and hybrids). However, athel (T. aphylla [L.] Karsten), an exotic, moderately valued evergreen species in the southwestern United States and northern Mexico, has not been targeted for biological control. All populations of D. elongata previously examined, including those promising for release in southern areas of the saltcedar infestation, develop and oviposit on athel. Therefore, we assessed more fully the acceptability and suitability of athel to three D. elongata populations (Tunisia, Crete, and Uzbekistan). All populations of D. elongata laid similar numbers of eggs on athel and saltcedar in no-choice tests. In multiple- and paired-choice tests, oviposition on saltcedar was generally greater than on athel but with some notable exceptions and inconsistencies within populations. Increasing cage size delayed the colonization of and oviposition on test plants by small groups of adult beetles but did not change the pattern of egg-laying. For Crete beetles, survival and development were similar for larvae fed athel or saltcedar. Adult size was negatively affected by a larval diet of athel. An adult diet of athel did not reduce lifetime fecundity, although it did decrease egg mass size and delayed the start of oviposition. As a result, the innate capacity for increase decreased. The potential for damage to athel by D. elongata may be higher than previously thought; however, this may be offset by the potential for increased invasiveness of athel.

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

ABSTRACT 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 ≈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.

Emergence and Performance of Two Invasive Swallowworts (Vincetoxicum spp.) in Contrasting Soil Types and Soil pH

Abstract The alien invasive vines black and pale swallowwort are currently spreading across eastern North America, invading parklands, old fields, restored forest sites, and other natural areas. These plants spread by wind-borne seed and can form dense stands where they become established. Although their current geographic ranges overlap, there is little known overlap locally. Preliminary observations and anecdotal information have associated black swallowwort with low-pH inceptisols and pale swallowwort with high-pH alfisols. We conducted a common garden field experiment repeated over two years and a growth chamber germination experiment to assess whether seedling emergence and performance of these two swallowworts are affected by soil type and/or soil pH. Soil pH was artificially modified. In the common garden experiment, plants of both species grown on the Onondaga County soil type (an alfisol) produced a smaller root mass than on the Orange County soil (an inceptisol). Also, in one of two years more seedlings emerged and plants produced more follicles on the Onondaga County soil. Soil pH did not affect seedling emergence, although plants grown on low pH soils had a smaller root dry mass compared with plants grown on higher pH soils. Soil pH effects on stem length, stem dry mass, and follicle production were inconsistent among years. Species differences were also evident, with more pale swallowwort seedlings emerging than black swallowwort seedlings, whereas black swallowwort plants mostly had greater biomass and fecundity than pale swallowwort plants. In the growth chamber experiment, final percentage seed germination was greater on the Onondaga County soil than on the Orange County soil. The germination speed index as well as the probability to reach 50% germination for black swallowwort was higher on the Onondaga County soil than the Orange County soil, but only at lower pH levels. The germination speed index of pale swallowwort on the Orange County soil was higher than black swallowwort at low, but not high, pH levels. In contrast, black swallowwort had a higher probability of reaching 50% germination than pale swallowwort on the Orange County soil at higher pH levels. Contrary to our expectations, interactions between the two swallowwort species with their associated soil type or with their presumably preferred soil pH were weak, contradictory, or non-existent in both experiments. This suggests that these two species can colonize and grow well in a relatively wide range of soil pH conditions. From a management perspective, our results suggest that the current range and local overlap of these two species will continue to increase and that early detection rapid response (EDRR) programs should be established in susceptible regions not yet colonized by these two invasive vines. Nomenclature: Black (or Louises) swallowwort, Vincetoxicum nigrum (L.) Moench, syn. Cynanchum louiseae Kartesz & Gandhi, Pale (or European) swallowwort, Vincetoxicum rossicum (Kleopow) Barbar., syn. Cynanchum rossicum (Kleopow) Borhidi Management Implications: Black swallowwort (BSW) and pale swallowwort (PSW) have become an invasive weed problem in northeastern North America. Currently, there is little overlap at specific field sites of the two species. Anecdotal information has associated BSW with low-pH soils and PSW with high-pH soils. Using both a common garden field experiment and growth chamber study, we assessed whether seedling emergence and performance of these two swallowworts were affected by soil type and/or soil pH. We found that seedling emergence of BSW and PSW was generally not affected by varying levels of soil pH, although soil type could influence germination rates. Apart from root mass size, the effect of soil pH or soil type on subsequent growth, survival and reproduction of first-year plants was variable. There were more consistent differences between species, across soil type or pH range, with more PSW seedlings emerging than BSW seedlings, whereas BSW plants mostly grew larger and reproduced to a greater extent than PSW plants. Based on our findings in this study, it appears that these two species can colonize and grow well in a relatively wide range of soil pH conditions. From a management perspective, our results suggest that the current range and local overlap of these two species will continue to increase and that early detection rapid response (EDRR) programs should be established in susceptible regions not yet colonized by these two invasive vines.

Pale Swallowwort (Vincetoxicum rossicum) Response to Cutting and Herbicides

Abstract Effective control techniques for pale swallowwort (PSW), an invasive herbaceous vine of old fields and forest understories, are limited. We conducted a 3-yr cutting and herbicide study on an adjacent old-field and forest understory site near Ithaca, NY, for control of PSW. Plants in experimental plots were cut in early July and cut again or sprayed in late August for two seasons with the isopropylamine salt of glyphosate , or one of two rates (low or high) of either triclopyr triethylamine salt (i.e., SL, SH) or triclopyr butoxyethyl ester (EL, EH). The herbicide treatments were effective in reducing PSW cover, plant (stem) density, and aboveground biomass in the old-field site, but in several cases, only after 2 yr of cutting plus herbicide application. Only the cutting plus SH treatment did not reduce PSW cover relative to the unmanaged control in the forest understory and no treatment reduced biomass. In general, the cutting plus EH treatment was most effective in reducing PSW stem densities in the forest site. The most effective herbicide treatments differed between sites. Cutting plus EH reduced PSW cover by 84% and stem density (> 5 cm) by 86% in the old-field site. Cutting plus SH effectively decreased long and short (≤ 5 cm) stem densities by 86 and 96%, respectively. Cutting plants twice during each of two seasons increased PSW cover by 301% and density of stems > 5 cm by 73% at this site. In the forest site, cutting plus glyphosate, or cutting plus EH or cutting plus SL and EL resulted in the greatest reductions in PSW cover (80, 76, 66, and 56%, respectively). Cover in plots cut twice per year decreased by 19%. The EH or SL treatments decreased long-stem densities by 78 and 71%, respectively. The EH treatment decreased short-stem density by 37%. These findings suggest that integrated techniques may control PSW but that effective management strategies may be habitat constrained. Nomenclature: Glyphosate, triclopyr butoxyethyl ester, triclopyr triethylamine salt, pale swallowwort, Vincetoxicum rossicum (Kleopow) Barbar., syn. European swallowwort, Cynanchum rossicum (Kleopow) Borhidi Management Implications: Pale swallowwort (PSW) is a highly invasive, difficult-to-control herbaceous vine that thrives in old fields but can also establish in shaded forest understories. This study sought to determine the efficacy of integrated management techniques in open-field and forest understory habitats. When PSW is cut in early July and followed in late August by application of an herbicide, the best control is obtained using different herbicides and rates in old-field compared with forest understory habitats. In the old field, two cuttings a year were ineffective and should be avoided. All herbicides reduced PSW cover, stem densities, and shoot biomass in the old field after 2 yr of combined mechanical and chemical treatments. However, cutting plus high rates of triclopyr, either as a triethylamine salt (4.87 kg ae ha−1) or butoxyethyl ester (4.87 kg ae ha−1), provided the best control. The cutting plus glyphosate treatment also provided effective control of PSW, but the cover of other species was significantly lower relative to three of four cutting plus triclopyr treatments. Regardless, it will take longer than two seasons of cutting plus herbicide applications to achieve total control of PSW in this old-field site. In the forest understory site, where PSW density was initially much lower than the old-field site, most cutting plus herbicide treatments were effective in reducing PSW cover. In general, the high rate of the triclopyr butoxyethyl ester formulation provided the best control of PSW, including a reduction in stem densities and increased cover of other species. Cutting PSW twice in each of two growing seasons was also ineffective as a management option in the forest site. Infestations of PSW in old fields will likely require higher rates of commonly used herbicides (e.g. glyphosate, triclopyr) relative to forest understory infestations largely because of greater densities and more vigorous populations in more favorable high-light old-field habitats.

Exploring the Feasibility of Sclerotium rolfsii VrNY as a Potential Bioherbicide for Control of Swallowworts (Vincetoxicum spp.)

Abstract Pale swallowwort (PSW) and black swallowwort (BSW) are two viney milkweeds native to Europe that have increasingly become problematic and noxious weeds in eastern North America. An indigenous fungal isolate, Sclerotium rolfsii VrNY, was discovered causing significant mortality in a dense stand of PSW in a park in upstate New York. Although this fungus is a known pathogen with a broad host range, we evaluated the host potential of S. rolfsii VrNY on a limited range of related and nonrelated U.S. species as a critical first step to assess its suitability as a mycoherbicide for PSW and BSW. In addition, PSW and BSW produce the specific stereoisomer (−)-antofine, a compound with antimicrobial and phytotoxic activity that could inhibit the pathogen. Tests revealed this compound had no effect on S. rolfsii VrNY. This isolate caused significant mortality on all broadleaf plants tested (Asclepias syriaca, Asclepias curassavica, Apocynum cannabinum, Monarda fistulosa, Rudbeckia hirta, PSW, BSW) with the exception of Glycine max, and had no effect on the monocots Schizachyrium scoparium and Zea mays. Although these laboratory studies indicate that most broadleaf vegetation may be susceptible to the pathogen, S. rolfsii might have potential as a mycoherbicide in natural eco-niche environments where invasive PSW and BSW have already become the predominant vegetation. Further laboratory testing of S. rolfsii and limited field testing at the initial discovery site are needed in order to prevent premature rejection of this isolate as a potential management tool against these highly invasive weeds. Nomenclature: Black (or Louises) swallowwort, Vincetoxicum nigrum (L.) Moench, syn. Cynanchum louiseae Kartesz & Gandhi, bloodflower milkweed, Asclepias curassavica L., common milkweed, Asclepias syriaca L, hairy coneflower, Rudbeckia hirta L., hemp dogbane, Apocynum cannabinum L., little bluestem, Schizachyrium scoparium (Michx.) Nash, pale (or European) swallowwort, Vincetoxicum rossicum (Kleopow) Barb., syn. Cynanchum rossicum (Kleopow) Borhidi, wild bergamot, Monarda fistulosa L., corn, Zea mays L., soybean, Glycine max (L.) Merr. Management Implications: Pale swallowwort (PSW) and black swallowwort (BSW) are two viney milkweeds native to Europe that have become invasive in eastern North America. Long term biological control efforts are the best hope for mitigating the spread of these weeds, but mycoherbicides might also offer a complementary approach for integrated pest management. An indigenous fungal isolate, Sclerotium rolfsii VrNY, was discovered casuing significant mortality in a dense stand of PSW in a park in upstate NY. This fungus is known to be a pathogen of a wide range of plants, but the possibility exists for a pathogen strain to occur that has a narrower host range. Even if not, the fungus might still have potential as a mycoherbicide in limited natural environments where PSW and BSW have become the predominant vegetation. In a laboratory study, we tested the ability of S. rolfsii VrNY to infect related and nonrelated (habitat associated) native U.S. species as well as soybean and corn, known hosts in the southern U.S. The isolate caused significant mortality on all broadleaf plants tested (common milkweed, bloodflower milkweed, hemp dogbane, wild bergamot, and hairy coneflower) except soybean, and had no effect on the monocots little bluestem and corn. Further laboratory testing of S. rolfsii and limited field testing at the initial discovery site are needed in order to prevent premature rejection of this isolate as a potential management tool against these highly invasive weeds.

Tolerance of Swallowworts (Vincetoxicum spp.) to Multiple Years of Artificial Defoliation and Clipping

The European vines pale swallowwort and black swallowwort 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 or 100% defoliation once or twice per season) and clipping (once, twice, or four times at 8 cm 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 6 yr. 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 seeds per plant generally did not change. Substantial overlap in plant size and seed production occurred among damage treatments and species. The most severe defoliation treatment did not substantially limit growth and reproduction compared with 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 aboveground tissue loss in high-light environments without plant competition. The annual increase in plant size calls into question the potential efficacy of a defoliating insect against field populations of swallowworts, and it seems likely the only benefits of a long-term mowing regime will be to eliminate seed production. Nomenclature:  Black (or Louises) swallowwort, Vincetoxicum nigrum (L.) Moench, syn. Cynanchum louiseae Kartesz & Gandhi; pale (or European) swallowwort; Vincetoxicum rossicum (Kleopow) Barbarich, syn. Cynanchum rossicum (Kleopow) Borhidi. Management Implications: Pale swallowwort and black swallowwort are European viney milkweeds that have become invasive in eastern North America since the 1980s. Mechanical control is considered ineffective, but previous studies have only been conducted for 1 or 2 yr. Biological control is also being developed, but it is unclear how these plants might respond to damage such as defoliation over a period of years. We evaluated different artificial defoliation and cutting treatments over 6 yr in a common garden field experiment in Ithaca, NY, for their effect on swallowwort survival, growth, and reproduction. Black swallowwort produced more aboveground biomass, whereas pale swallowwort produced more root biomass and root crown buds, compared with the other species. However, they appeared to respond similarly to damage. No plants died after 6 yr 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, seeds per plant) over time. The most severe defoliation treatment (100% twice each year) did not substantially limit growth and reproduction compared with 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 seeds. Pale and black swallowwort display a high tolerance to aboveground tissue loss in high-light environments without plant competition.

Demography of Invasive Black and Pale Swallow-Wort Populations in New York

Abstract Vincetoxicum nigrum (Black Swallow-wort) and Vincetoxicum rossicum (Pale Swallow-wort) are perennial, twining vines introduced from Europe. Both species have become invasive in northeastern North America in a variety of habitats. To develop parameters for a population model for evaluating the control of swallow-worts, including biological control, we collected data from 5 life stages on 20 different demographic rates involving fecundity, germination, survival, and growth. We monitored 2 field and 2 forest populations of Pale Swallow-wort, and 2 field populations of Black Swallow-wort in New York State using a combination of marked individuals and sowing plots. Both species showed moderate to high rates of seed germination and high survival of seedlings, with the primary exception of a heavily shaded forest population. Survival generally continued to remain high postestablishment, although transitions to different life stages varied by species, location, and habitat. Black Swallow-wort became reproductive more quickly than Pale Swallow-wort. These data add to the knowledge of swallow-wort demography and may offer insights into the continued expansion and control of these invasive plants.

Seedbank Dynamics of Two Swallowwort (Vincetoxicum) Species

Pale swallowwort and black swallowwort are European viny milkweeds that have become invasive in many habitats in the northeastern United States and southeastern Canada. A multiyear seedbank study was initiated in fall 2011 to assess annual emergence of seedlings and longevity of seeds of pale swallowwort and black swallowwort at four different burial depths (0, 1, 5, and 10 cm) over 4 yr. One hundred swallowwort seeds were sown in seed pans buried in individual pots, and emerged seedlings were counted and removed from May through September each year. A subset of seed pans was retrieved annually in October, and recovered seeds were counted and tested for viability. The majority of seedling emergence occurred during the first year (92% in 2012), and no new seedlings emerged in the third (2014) or fourth (2015) years. Pale swallowwort had relatively poor emergence at sowing depths of 0 cm (11%), 5 cm (6%), and 10 cm (0.05%—only one seedling), while 37% of pale swallowwort seeds emerged at 1 cm. The larger-seeded black swallowwort was more successful, with two-thirds of all sown seeds emerging at depths of 1 cm (71%) and 5 cm (66%), and 26% emerging at 10 cm. Only 16% of the surface-sown black swallowwort emerged. A large portion of the seeds that germinated at 10 cm, as well as at 5 cm for pale swallowwort, died before reaching the soil surface. Of filled seeds that were recovered in 2012 (black swallowwort at the 0-cm depth), 66% were viable. No viable seeds were recovered after the second growing season. Seeds recovered following the third year had become too deteriorated to accurately assess. Swallowwort seeds do not appear to survive more than 2 yr in the soil, at least in our experiment, suggesting that the elimination of seed production over 3 yr will exhaust the local seedbank. Seeds would need to be buried at least 10 cm for pale swallowwort but more than 10 cm for black swallowwort to prevent seedling emergence. Burial of swallowwort seeds as a management strategy may, however, only be practical in natural areas where high swallowwort densities occur. Nomenclature: Black swallowwort, Vincetoxicum nigrum (L.) Moench [Cynanchum louiseae Kartesz & Gandhi]; pale (or European) swallowwort, V. rossicum (Kleopow) Barbar. [Cynanchum rossicum (Kleopow) Borhidi].