Pre-release impact assessment of two stem-boring weevils proposed as biological control agents for Alliaria petiolata

Experimental studies can be useful tools to test plant responses to herbivory and to quantify the impact of potential biological control agents prior to their release. We evaluated the per-capita effect of Ceutorhynchus alliariae and C. roberti, two stem-boring weevils currently being investigated as potential biological control agents for garlic mustard, Alliaria petiolata, in North America. Weevils were released at three different densities in individual and mixed-species treatments onto potted plants of A. petiolata. Damage by C. roberti alone and by both weevils combined caused an increase in the numbers of inflorescences produced per plant. Although plants could compensate for low levels of damage, moderate to high levels of damage by both C. alliariae and C. roberti, individually and in combination, caused a decrease in plant height and a reduction in seed output per plant. The damage inflicted by both weevil species is similar so the overall impact of both species combined can be predicted by summing the impact of each species alone. Provided they are sufficiently host specific, both weevils could be released as biocontrol agents. Because reduced seed production is necessary to suppress A. petiolata populations, both species have the potential to contribute to control of A. petiolata in North America.     

Pre-release monitoring of Alliaria petiolata (garlic mustard) invasions and the impacts of extant natural enemies in southern Michigan forests

Monitoring of populations of a target weed species prior to releasing natural enemies has the potential to improve the rigor and safety of biological control and to determine the invader’s impacts on native communities while creating a reference point for evaluating the efficacy of subsequent biocontrol agent releases. Eight populations of garlic mustard, Alliaria petiolata (M. Bieb) Cavara and Grande (Brassicaceae), an invasive weed in southern Michigan, were monitored in anticipation of releases of classical biological control agents. The A. petiolata populations were shown to be expanding with 44.4% of initially uninvaded sampling quadrats becoming invaded after four years. While 88.2% of quadrats with A. petiolata showed evidence of foliar damage from pathogens and browsing by mammals, insects and other invertebrates, levels of damage were low and had little impact on rosette or seedling survival. Contrary to expectations, damage was positively correlated with A. petiolata fecundity (P = 0.0465). Given the continued expansion of A. petiolata and the lack of significant herbivore damage by acquired natural enemies, a biological control program should be considered against this invasive plant. If biological control agents are released, the results of this study will provide a benchmark for evaluating their performance.     

How novel are the chemical weapons of garlic mustard in North American forest understories?

The Novel Weapons Hypothesis predicts that invasive plants excel in their new ranges because they produce novel metabolites to which native species possess little resistance. We examined the novelty of the phytochemistry of the Eurasian invader, Alliaria petiolata, in North America by comparing its phytochemical profile with those of closely related Brassicaceae native to North America. We examined the profile and/or concentrations of glucosinolates, alliarinoside, flavonoids, cyanide, and trypsin inhibitors in cauline leaves of field-collected A. petiolata, Arabis laevigata, Cardamine concatenata, C. bulbosa, and C. douglassii. Cyanide and the glucosinolates and flavonoids produced by A. petiolata were detected only in A. petiolata. Trypsin inhibitor activity was highest in A. laevigata, intermediate in the Cardamine species, and lowest in A. petiolata. The phytochemical profile of A. petiolata was distinct from those of four closely related and/or abundant Brassicaceaeous species native to North America, providing support for the Novel Weapons Hypothesis.     

The effects of arbuscular mycorrhizal (AM) fungal and garlic mustard introductions on native AM fungal diversity

Introduced, non-native organisms are of global concern, because biological invasions can negatively affect local communities. Arbuscular mycorrhizal (AM) fungal communities have not been well studied in this context. AM fungi are abundant in most soils, forming symbiotic root-associations with many plant species. Commercial AM fungal inocula are increasingly spread worldwide, because of potentially beneficial effects on plant growth. In contrast, some invasive plant species, such as the non-mycorrhizal Alliaria petiolata, can negatively influence AM fungi. In a greenhouse study we examined changes in the structure of a local Canadian AM fungal community in response to inoculation by foreign AM fungi and the manipulated presence/absence of A. petiolata. We expected A. petiolata to have a stronger effect on the local AM fungal community than the addition of foreign AM fungal isolates. Molecular analyses indicated that inoculated foreign AM fungi successfully established and decreased molecular diversity of the local AM fungal community in host roots. A. petiolata did not affect molecular diversity, but reduced AM fungal growth in the greenhouse study and in a in vitro assay. Our findings suggest that both introduced plants and exotic AM fungi can have negative impacts on local AM fungi.     

Differences in arbuscular mycorrhizal fungal communities associated with sugar maple seedlings in and outside of invaded garlic mustard forest patches

Garlic mustard (Alliaria petiolata) is a Eurasian native that has become invasive in North America. The invasive success of A. petiolata has been partly attributed to its production of allelopathic compounds that can limit the growth of arbuscular mycorrhizal fungi (AMF). Although such effects are well known, specific effects on the richness and community composition of AMF associated with woody species have not been explored. We collected sugar maple (Acer saccharum) seedlings from eight natural forest sites in Ohio and Massachusetts, containing areas either invaded or uninvaded by A. petiolata. We measured AMF root colonization of seedlings, isolated DNA from the roots and performed PCR-TRFLP analysis to assess the richness and community composition of AMF. As expected, we found reduced AMF colonization in A. petiolata invaded patches. A. petiolata did not alter the detected TRF richness, but was associated with significant changes in the composition of AMF communities in half of the sites monitored in each region. Our results suggest that although AMF colonization was reduced in A. petiolata patches, many indigenous AMF communities include AMF that are tolerant to allelopathic effects of A. petiolata.     

Impact of simulated herbivory on Alliaria petiolata survival, growth, and reproduction

In weed biological control, insect damage to target weeds can be simulated in invaded habitats to study potential responses of the plant to introduced natural enemies. In the present study, we investigated the impact of two levels of manual flower-shoot damage (shoots cut at tip or base) on Alliaria petiolata (garlic mustard) survival, size, and reproduction. Experiments were conducted in 2002 and 2003 using invasive field populations of A. petiolata under naturally varying plant densities. Plant survival was recorded, and size and reproduction parameters were measured. Manual flower-shoot damage had a significant effect on plant survival. In both years, fewer plants survived in the basal-cut treatment than in either the control (un-cut) or tip-cut treatment. Plant size and reproductive output were likewise reduced in the basal-cut treatment. In both years, total seed production was significantly lower in the basal-cut treatment than either the control or tip-cut treatment. When combined, increased mortality and reduced seed production of basal-cut plants greatly reduced the contribution these plants made to the seed bank. Plant density did not affect reproduction or plant size. The impacts of cutting were consistent across years and sites with distinct biotic and abiotic conditions, and A. petiolata densities. We anticipate that herbivore damage to A. petiolata populations by introduced biological control agents will likewise remain consistent under varying biotic and abiotic conditions if the agents are equally adapted to these.     

An exotic invader drives the evolution of plant traits that determine mycorrhizal fungal diversity in a native competitor

The symbiosis between land plants and arbuscular mycorrhizal fungi (AMF) is one of the most widespread and ancient mutualisms on the planet. However, relatively little is known about the evolution of these symbiotic plant–fungal interactions in natural communities. In this study, we investigated the symbiotic AMF communities of populations of the native plant species Pilea pumila (Urticaceae) with varying histories of coexistence with a nonmycorrhizal invasive species, Alliaria petiolata (Brassicaceae), known to affect mycorrhizal communities. We found that native populations of P. pumila with a long history of coexistence with the invasive species developed more diverse symbiotic AMF communities. This effect was strongest when A. petiolata plants were actively growing with the natives, and in soils with the longest history of A. petiolata growth. These results suggest that despite the ancient and widespread nature of the plant–AMF symbiosis, the plant traits responsible for symbiotic preferences can, nevertheless, evolve rapidly in response to environmental changes.     

Concurrent Management of an Exotic Species and Initial Restoration Efforts in Forests

One of the proximate results of forest fragmentation, and a cause of continued microenvironmental change and exacerbation of ecological problems, is increased invasions by weedy plant species. One such example is Alliaria petiolata (Brassicaceae), a serious pest threatening much of eastern North America. Alliaria petiolata impedes mitigation of fragmentation and restoration efforts because it tends to outcompete and possibly extirpate much of the native understory species on localized scales. As part of a strategy to address the problems of fragmented habitats, an experiment was conducted to determine whether Sanguinaria canadensis (Papaveraceae) could outcompete A. petiolata. Using an additive design, I transplanted S. canadensis at densities of 0, 1, 2, 3, 5, 7, 9, 11, 15, and 20 ramets/m² in 1997 and allowed them to interact with initial A. petiolata densities of 128 seedlings and 31 rosettes/m². As of 2000, multivariate analyses of variance with repeated measures and simple analyses of variance indicated that initial S. canadensis densities of as little as 5 ramets/m² suppressed A. petiolata. Initial S. canadensis densities of 9 and 11 ramets/m² resulted in the lowest numbers of late‐spring seedlings, numbers and sizes of year 1 and 2 rosettes, numbers and gross areas of stem leaves, numbers of flowering individuals, number of flowers, number of fruits (siliques), and height at flowering. While it remains to be tested whether this will continue and if the reestablishment of S. canadensis will help reassemble forest ecosystems, the experiments indicated that transplanting S. canadensis was effective at mitigating the spread of A. petiolata.     

Exotic consumers interact with exotic plants to mediate native plant survival in a Midwestern forest herb layer

Consumer-facilitated invasions have been proposed as an alternative mechanism to direct competitive exclusion to explain the replacement of native plants by exotics. In a factorial field experiment manipulating competition from the exotic plant Alliaria petiolata and herbivory by exotic mollusks, we documented that mollusk herbivory significantly reduced the survival of two species of native palatable plants, but found minimal direct herbivore effects on less palatable species, including the invasive A. petiolata. These effects were evident after one growing season on younger juvenile plants of Aster cordifolius, but only after two growing seasons on older transplants of the same species, suggesting a greater vulnerability of young plants. In contrast to our expectations, A. petiolata competition alone had no effect on any of the six native species we tested. However, competition from A. petiolata did affect the survival of the most palatable native plant when mollusks were also present. While not significant for any other single species, this same pattern was observed for three of the five remaining native species tested. The selective grazing on palatable plants that we document provides novel evidence contributing to our understanding of observed shifts in the forest herbaceous layer towards the dominance of exotic plants and unpalatable species. More broadly, our results highlight the importance of the interactive effect of consumers and inter-specific competition in forest understories via its contribution to differential survival among regenerating species.     

PCR–RFLP assays for discerning three weevil stem feeders (Ceutorhynchus spp.) (Col.: Curculionidae) on garlic mustard (Alliaria petiolata)

Polymerase chain reaction–restriction fragment length polymorphism (PCR–RFLP) assays were developed to rapidly and cost-effectively differentiate the morphologically indistinguishable larval stages of three species, Ceutorhynchus alliariae, C. roberti and C. scrobicollis, proposed for the biological control of Alliaria petiolata. A PCR–RFLP using TaqαI restriction enzyme can differentiate the three target species only, while a PCR–RFLP double digestion using the restriction enzymes AluI and TaqαI should enable, based on virtual digestion, distinguishing them from 12 other Ceutorhynchus species associated with A. petiolata.     

Impact of the belowground herbivore and potential biological control agent, Ceutorhynchus scrobicollis, on Alliaria petiolata performance

We studied the influence of the root-crown weevil Ceutorhynchus scrobicollis on its host plant Alliaria petiolata, a European biennial herb that is currently invading much of temperate North America. Varying timing of attack and herbivore densities in a common garden allowed to assess seasonality of plant response, density-dependence of impact, and the effect of intraspecific competition on C. scrobicollis recruitment (number of F₁ generation adults emerged). Data collected in the common garden were compared with data collected at field sites. C. scrobicollis is a common weevil in Europe, frequently attaining high attack levels on its host plant. In the common garden, weevil attack decreased plant survival by up to 43%, reduced plant height by 54%, increased the number of shoots by up to four–fold and delayed seed ripening, but had no significant negative effect on seed production. Plants infested in spring allocated less biomass to aboveground plant parts, and remained smaller than plants attacked in autumn, indicating that the latter were able to partly compensate for weevil attack. Increasing weevil density rarely had an effect on A. petiolata performance, and did not increase F₁ recruitment, suggesting strong intraspecific competition. At field sites, C. scrobicollis attack is spread over a long time period, which probably alleviates intraspecific competition. In summary, attack by the root-crown feeding weevil, C. scrobicollis, can substantially reduce growth and survival of A. petiolata. If introduced as a biological control agent into North America, C. scrobicollis is likely to decrease the fitness and competitive superiority of A. petiolata.     

Intraspecific variation in allelochemistry determines an invasive species’ impact on soil microbial communities

Invasive species can benefit from altered species interactions in their new range, and by interfering with species interactions among native competitors. Since exotic invasions are generally studied at the species level, relatively little is known about intraspecific variation in the traits that determine an invader’s effect on native species. Alliaria petiolata is a widespread and aggressive invader of forest understories that succeeds in part by interfering with mutualistic interactions between native plants and soil fungi. Here, I show that the impact of A. petiolata on soil microbial communities varied among individuals due to variation in their allelochemical concentrations. The differential impacts translated into varied effects on native tree growth, partly because A. petiolata’s allelochemicals preferentially affected the most mutualistic fungal taxa. These results highlight the importance of considering the spatial and temporal variation in an invasive species’ impacts for understanding and managing the invasion process.     

Biology and host specificity of Ceutorhynchus scrobicollis (Curculionidae; Coleoptera), a root-crown mining weevil proposed as biological control agent against Alliaria petiolata in North America

Ceutorhynchus scrobicollis is a root-crown mining weevil proposed for release as biological control agent of Alliaria petiolata (Brassicaceae, Thlaspideae), a European biennial herb, currently invading temperate North America. Using a combination of laboratory, common garden and fieldwork we studied biology, ecology and host range of C. scrobicollis, a univoltine species that oviposits and develops in A. petiolata rosettes in fall and spring. Individual C. scrobicollis can be long-lived (>2 years) and females show a second oviposition period. Weevils did not attack any of 31 test plant species outside the Brassicaceae. Within the Brassicaceae, five species allowed complete larval development under no-choice conditions. In subsequent choice tests, three of these five species (Nasturtium officinale, Peltaria alliacea and Thlaspi arvense; which are of European origin) were attacked. North American Rorippa sinuata was the only native species to be attacked by C. scrobicollis and only under no-choice conditions. Results of subsequent impact experiments showed that C. scrobicollis attack changed plant architecture but had no effect on overall plant vigour and reproductive output of R. sinuata, suggesting lack of impact on demography or population dynamics. A petition for field release of C. scrobicollis in North America has been submitted.     

Comparison of survey methods for an invasive plant at the subwatershed level

Invasive plant survey methods that are practical and economical are needed to describe established colonies and detect nascent invaders. We compared results from random and roadside surveys of Alliaria petiolata (Bieb.) Cavara & Grande across a 5730-ha subwatershed. The random survey included 150 1-ha plots; the roadside survey examined 0.1-mile increments (10-m deep) along paved roads (totaling 1104 0.16-ha plots). In the random survey, agriculture was the dominant land use (49% of sampled area), and most A. petiolata patches were in wooded, shaded riparian, and waste areas (34%, 34%, and 29% of patches, respectively). In the roadside survey, right-of-way land use was dominant (38% of sampled area), and most A. petiolata patches were in right-of-way, wooded, and shaded riparian areas (53%, 22%, and 19% of patches, respectively). According to generalized linear model analysis, survey methods did not differ in the overall probability of finding A. petiolata (P=0.17 and 0.11 for random and roadside surveys, respectively). Shaded riparian, wooded, and mixed-species right-of-way land uses were the dominant habitat for A. petiolata in both surveys, but only the random survey indicated waste areas as significant habitat. Alliaria petiolata occurred mostly as small patches in roadsides, but as large patches in random plots, suggesting faster spread in the roadside. Results indicated that disturbed lands along roadsides were important for invasion and spread of A. petiolata; therefore, the roadside survey was a useful, practical method for detecting nascent invasions and management planning. The random sampling lacked a land use bias, and provided data that could be generalized across the subwatershed; however, this method required at least four times more person hours to complete than the roadside survey for a similar amount of area. Although roadside sampling did not provide a completely reliable assessment of target plant populations within the landscape, it may provide an adequate approximation, depending on the specific goals of the survey. Concurrent surveys would provide the most complete information.     

Flexible N uptake and assimilation mechanisms may assist biological invasion by Alliaria petiolata

Although nitrogen has historically limited terrestrial plant productivity in the northern hemisphere, accelerated industrial activity is changing the availability of N, with consequences for ecosystem properties including altered susceptibility to biological invasion. Alliaria petiolata (Bieb.) Cavara & Grande is an increasingly problematic invader in forests of eastern North America. Population growth rate of this species is especially high in N-rich habitats, and it produces a variety of N-based compounds that have been shown to interfere with the growth and reproduction of native plants. To investigate how increases and shifts in forms of N will impact A. petiolata, seedlings were transplanted to the greenhouse from the field and grown in sand culture. We applied three concentrations of N (0.25, 1 and 2 mM) using five different ratios of NH₄ ⁺ and NO₃ ⁻ (100/0, 75/25, 50/50, 25/75, 0/100) in a crossed design to yield fifteen different treatments. Plants were measured throughout the growing season and a final harvest yielded measures of biomass and tissue quality. Plant growth increased significantly in response to increased concentration of total N. These increases were similar for all combinations of N. This flexibility in uptake ability may facilitate the invasion of this species, not only by increasing the range of habitats A. petiolata can occupy but also by enhancing N uptake that can lead to the production of secondary compounds disrupting other species’ belowground mutualisms. We suggest that this species’ ability to respond rapidly to changes in N availability, regardless of its form, may modify competitive interactions with natives and intensify its negative impacts.     

Biogeography and phenology of oviposition preference and larval performance of <Emphasis Type="Italic">Pieris virginiensis</Emphasis> butterflies on native and invasive host plants

In invaded environments, formerly reliable cues might no longer be associated with adaptive outcomes and organisms can become trapped by their evolved responses. The invasion of Alliaria petiolata (garlic mustard) into the native habitat of Pieris virginiensis (West Virginia White) is one such example. Female butterflies oviposit on the invasive plant because it is related to their preferred native host plant Cardamine diphylla (toothwort), but larvae are unable to complete development. We have studied the impact of the A. petiolata invasion on P. virginiensis butterflies in the Southeastern USA by comparing oviposition preference and larval survival on both plants in North Carolina (NC) populations without A. petiolata and West Virginia (WV) populations where A. petiolata is present. Larval survival to the 3rd instar was equally low in both populations when raised on A. petiolata. Mean oviposition preference on the two plants also did not differ between populations. However, we found a seasonal effect on preference between early and late season flights within WV populations. Late season females laid 99% of total eggs on A. petiolata while early season females utilized both host plants. Late season females were also less likely to lay eggs than early season females. This change in preference toward A. petiolata could be driven by the early senescence of C. diphylla and suggests a seasonal component to the impact of A. petiolata. Therefore, the already short flight season of P. virginiensis could become further constrained in invaded populations.     

Before, During and After: The Need for Long-term Monitoring in Invasive Plant Species Management

The invasion of non-indigenous plants is considered one of the primary threats to rare and endangered species as well as to the integrity and function of North American ecosystems. However, many of the suspected negative ecosystem impacts are based on anecdotal evidence. For example, there is almost unanimous agreement among natural resource managers of the detrimental ecological impacts of species such as Lythrum salicaria (purple loosestrife), Phragmites australis (common reed) and Alliaria petiolata (garlic mustard) but convincing documentation is scarce. Experimental and theoretical ecology predicts large ecosystem impacts of the most widespread invasive species. However, it is difficult to prioritize control of species that occur at intermediate densities. Long-term monitoring before and during the invasion as well as before, during and after any control attempts can provide valuable ecological information. In particular, it is important to understand how changes in the abundance of species influence ecosystem properties and processes which, in turn, will help guide management decisions. Ideally, this monitoring has to go beyond 'simple impacts on plant communities, involve cross-disciplinary teams of scientists and should incorporate many different taxa and their interactions. Monitoring design and data collection should be sophisticated enough to allow statistically sound data analysis. The available information will be paramount in (1) developing new political and scientific guidelines in invasive species management, (2) helping resolve potential conflicts of interest and (3) helping change public attitudes regarding growth, sale, and control of non-indigenous species.     

Genome structure and evolution in the cruciferous tribe Thlaspideae (Brassicaceae)

Whole-genome duplications (WGDs) and chromosome rearrangements (CRs) play the key role in driving the diversification and evolution of plant lineages. Although the direct link between WGDs and plant diversification is well documented, relatively few studies focus on the evolutionary significance of CRs. The cruciferous tribe Thlaspideae represents an ideal model system to address the role of large-scale chromosome alterations in genome evolution, as most Thlaspideae species share the same diploid chromosome number (2n = 2x = 14). Here we constructed the genome structure in 12 Thlaspideae species, including field pennycress (Thlaspi arvense) and garlic mustard (Alliaria petiolata). We detected and precisely characterized genus- and species-specific CRs, mostly pericentric inversions, as the main genome-diversifying drivers in the tribe. We reconstructed the structure of seven chromosomes of an ancestral Thlaspideae genome, identified evolutionary stable chromosomes versus chromosomes prone to CRs, estimated the rate of CRs, and uncovered an allohexaploid origin of garlic mustard from diploid taxa closely related to A. petiolata and Parlatoria cakiloidea. Furthermore, we performed detailed bioinformatic analysis of the Thlaspideae repeatomes, and identified repetitive elements applicable as unique species- and genus-specific barcodes and chromosome landmarks. This study deepens our general understanding of the evolutionary role of CRs, particularly pericentric inversions, in plant genome diversification, and provides a robust base for follow-up whole-genome sequencing efforts.     

重楼属植物遗传多样性的SCOT标记

为评估重楼属植物的基因资源开发前景,初步探讨了SCOT标记技术在重楼属植物遗传多样性研究上的应用。采用目标起始密码子多态性(Start codon targeted polymorphism,SCOT)技术,对重楼属9个种40份材料进行基因组DNA多态性分析。结果表明:四川地区重楼属植物具有丰富的遗传多样性,40份供试材料可聚为4类:狭叶重楼单独聚为第一类;球药隔重楼、卵叶重楼、金线重楼及滇重楼聚为第二类;黑籽重楼与七叶一枝花聚为第三类;五指莲与毛重楼聚为第四类。说明SCOT标记能够对重楼属植物进行准确的分子鉴定,为重楼属植物的种类鉴定和种间的分类地位提供分子生物学依据,同时也为深入探究重楼皂苷合成关键酶基因定位研究提供重要参考。     

Seed survival and periodicity of seedling emergence in eight species of Cruciferae

Seeds of eight species of Cruciferae were collected, usually in each of 3 years, and mixed with the top 7-5 cm of sterilised soil confined in cylinders sunk in the ground outdoors and cultivated three times yearly. The seedlings emerging were recorded for 5 yr and the numbers of viable seeds remaining then determined. Emergence of Alliaria petiolata was almost entirely restricted to February and March. That of Erysimum cheiranthoides, Lepidium campestre, Sinapis arvensis and Raphanus raphanistrum took place mainly in March and April, but seedlings continued to appear until late autumn. Some seedlings of Arabidopsis thaliana, Cardamine hirsuta and Sisymbrium officinale appeared in spring, but most did so in summer (C. hirsuta and S. officinale) or early autumn (A. thaliana). Seed survival in A. petiolata was of short duration and there were few seedlings after the second year, whereas 18% of the seeds of R. raphanistrum were still present and viable after 5 yr. Seed survival at this time for the other six species ranged from 1 -6% (C. hirsuta) to 6-1% (S. arvensis).