First documentation of adult Trichosirocalus horridus on several non-target native Cirsium species in Tennessee

Releases of Trichosirocalus horridus (Panzer) (Coleoptera: Curculionidae), native to Europe, began in Tennessee in 1989 as part of a biological control program against musk thistle (Carduus nutans L.). In surveys conducted to investigate non-target feeding of T. horridus on native Cirsium thistle species from 2005 to 2008, adults of T. horridus were observed on all five native Cirsium thistles. These adult occurrences are the first documentation of T. horridus occurring on three of these native species [C. carolinianum (Walt.) Fern & Schub., C. horridulum Michx. and C. muticum Michx.], and the first record of T. horridus occurring in the Great Smoky Mountains National Park. While C. carolinianum and C. horridulum did not show symptoms of larval feeding in the meristematic tissues, C. altissimum, C. discolor, and C. muticum all had damaged meristems and possible oviposition scars on the midribs of the leaves. However, the impact of feeding by larvae of T. horridus on the reproductive potential of native plants is uncertain, because even in the target species (musk thistle and other introduced Cirsium species) plant death only sometimes occurs, and seed production continues.     

Individually mark–mass release–resight study elucidates effects of patch characteristics and distance on host patch location by an insect herbivore

1. How organisms locate their hosts is of fundamental importance in a variety of basic and applied ecological fields, including population dynamics, invasive species management and biological control. However, tracking movement of small organisms, such as insects, poses significant logistical challenges. 2. Mass‐release and individual–mark–recapture techniques were combined in an individually mark–mass release–resight (IMMRR) approach to track the movement of over 2000 adult insects in an economically important plant–herbivore system. Despite its widespread use for the biological control of the invasive thistle Carduus nutans, the host‐finding behaviour of the thistle head weevil Rhinocyllus conicus has not previously been studied. Insects were released at different distances from a mosaic of artificially created host patches with different areas and number of plants to assess the ecological determinants of patch finding. 3. The study was able to characterize the within‐season dispersal abilities and between‐patch movement patterns of R. conicus. Weevils found host plant patches over 900 m away. Large patches, with tall plants, situated close to the nearest release point had the highest first R. conicus resights. Patch area and plant density had no effect on the number of weevils resighted per plant; however, R. conicus individuals were more likely to disperse out of small patches and into large patches. 4. By understanding how R. conicus locates host patches of C. nutans, management activities for the control of this invasive thistle can be better informed. A deeper mechanistic understanding of host location will also improve prediction of coupled plant–herbivore spatial dynamics in general.     

Direct and Indirect Effects of a Shoot-Base Boring Weevil and Plant Competition on the Performance of Creeping Thistle, Cirsium arvense

Creeping thistle or Canada thistle, Cirsium arvense (L.) Scop., is considered one of the world's worst weeds and the third most important weed in Europe. Biological control of this indigenous weed in Europe by use of native agents may provide a low-cost alternative to use of chemical or mechanical control measures and contribute to a more sustainable weed management. We investigated the potential of a shoot-base boring weevil, Apion onopordi Kirby (Coleoptera: Apionidae), for biological weed control, in the presence or absence of plant competition by three grass species. Infestation of thistle shoots by A. onopordi at natural infestation levels reduced above- and belowground plant performance after 2 years. Plant competition at natural levels had an overall greater effect than that of herbivory, significantly reducing both above- and belowground thistle performance in both years, thereby slowing the propagation of the weed. Weevil infestation and grass competition had a synergistic effect on C. arvense growth; the combined effects of the two factors was greater than the sum of both single-factor effects. The experiment revealed that A. onopordi promotes systemic infections of the rust fungus Puccinia punctiformis (Str.) Röhl in the year following weevil infestation. Systemically infected thistle shoots died before the end of the growing season. Although the direct effect of A. onopordi may not be sufficient to control creeping thistle, the synergistic interaction with plant competition and the indirect effect via promotion of systemic rust infections makes A. onopordi a promising agent for the biological control of this weed.     

Supplemental risk evaluations and status of Puccinia carduorum for biological control of musk thistle

Tests of seven rare and endangered native North American Cirsium species and four modern artichoke lines were requested in response to a proposal for introduction of Puccinia carduorum into the United States for biological control of musk thistle (Carduus nutans ssp. leiophyllus). These tests were supplemental to an earlier extensive host-range study that established P. carduorum from musk thistle as host specific, useful for biological control, and suitable for limited field tests in Virginia. Test plants in the current study were evaluated in support of a proposal to use the rust in the western United States, and particularly, in California. None of the test plants in this study had been evaluated in previous assessments and all were either rare, endangered or threatened in California. Tests were conducted in both field and greenhouse settings. Field tests were run for two seasons, and test plants were inoculated by natural spread of the pathogen from source plants inside rings of test plants. Greenhouse tests involved direct inoculation under optimal conditions of dew and temperature (18–20 °C, 16 h) for infection. None of the seven Cirsium species or subspecies tested became infected by P. carduorum, either in field or greenhouse tests, compared to infection of 98% of the individual musk thistle plants (n = 102) from all the studies. Modern artichoke cultivars were tested only by direct inoculation under optimal greenhouse conditions. All artichoke plants (n = 115) either were immune (no macroscopic symptoms, n = 69) or at most, resistant (n = 46); pustules on all but two of the resistant plants were very small (0.30 mm diam). Despite infections on artichokes, P. carduorum could not be maintained on artichokes under optimal greenhouse conditions. These results confirm earlier findings from host-range tests and risk assessments of P. carduorum. This information suggests that rare, threatened, or endangered Cirsium spp. and modern artichoke cultivars are not likely to be adversely affected by the use of P. carduorum for biological control of musk thistle. These data have been reviewed by grower groups and regulatory agencies in a proposal for permission to use the rust for musk thistle control throughout the United States.     

Biological control of three floating water weeds,<Emphasis Type="Italic">Eichhornia crassipes,Pistia stratiotes</Emphasis>, and <Emphasis Type="Italic">Salviniamolesta</Emphasis> in the Republic of Congo

Since 1999, four specific weevils (Coleoptera, Curculionidae) were released in the Republic of Congo against three exotic floating water weeds: Neochetina eichhorniae Warner and N. bruchi Hustache against water hyacinth, Neohydronomus affinis Hustache against water lettuce, and Cyrtobagous salviniae Calder and Sands against water fern. Recoveries of exotic weevils were made from all 24 release sites except one, and all four species have established and spread (up to 800 km for water hyacinth weevils). Within a few years of releases, control of water fern and water lettuce was such that fishing and navigation could be resumed, while reductions of water hyacinth populations were only beginning.     

Spatial Segregation of Congeneric Invaders in Central Pennsylvania, USA

Carduus acanthoides and Carduus nutans (plumeless and musk thistles) are among the most noxious weeds in the United States of America, presenting a serious challenge in cropping and pasture systems. Unfortunately, a lack of detailed spatial distribution information hampers both our ability to understand the factors affecting their invasive success, and the effectiveness of monitoring and management efforts. To examine patterns of distribution and co-occurrence at a local level, we sampled a 5000 km² area of central Pennsylvania that cut a transect across known areas of C. acanthoides and C. nutans infestation. A number of potential environmental explanatory variables were recorded and analyzed to examine whether they correlated with observed species distribution patterns. Patterns of forest density and spatial aggregation of the thistles were the primary covariates that significantly impacted both species’ distributions. The survey established that the frequency of sightings for each species diminished as the ranges converged, with only brief overlap: the two species are strongly negatively correlated in space. Understanding environmental correlates of infestation and the pattern of spatial dissociation of these two invasive species is an important step towards an improved understanding of the mechanisms underlying their invasive potential, and hence towards effective weed control.     

Does lignin modification affect feeding preference or growth performance of insect herbivores in transgenic silver birch (<Emphasis Type="Italic">Betula pendula</Emphasis> Roth)?

Transgenic silver birch (Betula pendula Roth) lines were produced in order to modify lignin biosynthesis. These lines carry COMT (caffeate/5-hydroxyferulate O-methyltransferase) gene from Populus tremuloides driven by constitutive promoter 35S CaMV (cauliflower mosaic virus) or UbB1 (ubiquitin promoter from sunflower). The decreased syringyl/guaiacyl (S/G) ratio was found in stem and leaf lignin of 35S CaMV-PtCOMT transgenic silver birch lines when compared to non-transformed control or UbB1–PtCOMT lines. In controlled feeding experiments the leaves of transgenic birch lines as well as controls were fed to insect herbivores common in boreal environment, i.e., larvae of Aethalura punctulata, Cleora cinctaria and Trichopteryx carpinata (Lepidoptera: Geometridae) as well as the adults of birch leaf-feeding beetles Agelastica alni (Coleoptera: Chrysomelidae) and Phyllobius spp. (Coleoptera: Curculionidae). The feeding preferences of these herbivores differed in some cases among the tested birch lines, but these differences could not be directly associated to lignin modification. They could as well be explained by other characteristics of leaves, either natural or caused by transgene site effects. Growth performance of lepidopteran larvae fed on transgenic or control leaves did not differ significantly.     

Establishment in Australia of Trichosirocalus horridus a Biological Control Agent for Carduus nutans , and Preliminary Assessment of its Impact on Plant Growth and Reproductive Potential

Testing of the crown weevil , Trichosirocalus horridus, against a range of Australian Aster aceae species confirmed its specificity to nodding thistle , Carduus nutans, and permission was granted for its release in Australia . It has established at the release site , and has been redistributed to other areas in eastern Australia . A field experiment designed to study the impact of this weevil on rosette growth and reproductive potential of the thistle is described . Results indicate that the weevil can kill some plants , and reduce rosette growth of survivors by 50% . The reproductive potential of the attacked plants , measured as seeds per plant , was reduced by 67% . The interaction of this biological control agent with two others already established in Australia , the receptacle weevil , Rhinocyllus conicus, and the seed fly , Urophora solstitialis, is discussed .     

Exotic weevil invasion increases floral herbivore community density,function, and impact on a native plant

Consumer communities are being re‐arranged through unprecedented rates of human‐mediated invasions and extinctions. Such changes in consumer diversity potentially alter community function and impact on resource populations. Although insect herbivore invasions are increasingly common, the influence of such species additions on native resident herbivore guilds, along with their individual and combined effects on native plant resources, are rarely investigated. Here, we used site‐to‐site and plant‐to‐plant variation in herbivore composition to examine how the addition of an invasive exotic weevil, Rhinocyllus conicus, combines with a guild of native floral herbivores (tephritid flies, pyralid moths) to influence two key components of herbivore community function – aggregate herbivore densities and cumulative levels of seed destruction – on a native thistle, Cirsium canescens. Invasion of a site by R. conicus more than doubled aggregate herbivore density, resulting in increased levels of seed destruction and a halving of seed production by the native thistle. Further, herbivore function was significantly higher on individual plants attacked by R. conicus, compared to plants attacked only by native herbivores. Insect densities and levels of seed destruction on plants attacked by multiple herbivore taxa never exceeded those observed for plants attacked by R. conicus alone, suggesting that increases in herbivore community function with invasion resulted from the inclusion of a functionally dominant insect rather than any complementarity effects. Some evidence for interference between insects emerged, with a trend towards reduced moth and weevil densities in two and three taxon mixtures compared with plants attacked by each taxon alone. However, density compensation was limited so that, overall, the addition of a novel herbivore to the floral guild was associated with a significant increase in herbivore community function and impact on seed production. The results suggest that invasion of a functionally dominant herbivore into an unsaturated recipient community can augment function within a resource guild.     

An ecosystem-level perspective of allelopathy

Allelopathy is an interference mechanism by which plants release chemicals which affect other plants; while it has often been proposed as a mechanism for influencing plant populations and communities, its acceptance by plant ecologists has been limited because of methodological problems as well as difficulties of relating the results of bioassays used for testing allelopathy to vegetation patterns in the field. Here we argue that the concept of allelopathy is more appropriately applied at the ecosystem-level, rather than the traditional population/community level of resolution. Firstly, we consider the wide ranging effects of secondary metabolites (widely regarded as allelochemicals) on organisms and processes which regulate ecosystem function, including herbivory, decomposition and nutrient mineralization. It is apparent that plants with allelopathic potential against other organisms induce net changes in ecosystem properties, which may in turn impact upon the plant community in the longer term. We then illustrate these concepts using two contrasting examples of how invasive plant species with allelopathic potential may alter ecosystem properties through the production of secondary metabolites, i.e. Carduus nutans (nodding thistle) in New Zealand pastures and Empetrum hermaphroditum (crowberry) in Swedish boreal forests. In both cases the production of secondary metabolites by the invasive species induces important effects on other organisms and key processes, which help determine how the ecosystem functions and ultimately the structure of the plant community. These examples help demonstrate that the concept of allelopathy is most effectively applied at the ecosystem-level of resolution, rather than at the population-level (i.e. plant-plant interference).     

A Histological Study of the Effect of Feeding bySmicronyxspp. (Coleoptera: Curculionidae) Larvae on Seed Production byStriga hermonthica(Scrophulariaceae)

Experiments were conducted in Burkina Faso to determine the effect of feeding bySmicronyxspp. (Coleoptera: Curculionidae) larvae on the development of fruit capsule and seed production inStriga hermonthica(Scrophulariaceae). In treated plotsSmicronyxadults were released in the cages while in untreated plots (= control) noSmicronyxwere released. TheStrigacapsules were collected, fixed, and prepared for histological examination. UninfestedStrigacapsules developed normally. In infested capsules, three types of galls were distinguished: they reflected the development of capsule and the seeds. There was a synchrony between seed development and larval development in the same capsule. In most instances, one egg or larva was found per gall.Smicronyxlarvae completely destroyedStrigaseeds in the developing capsule by two mechanisms: (1) by directly eating most of the seeds and (2) by eating the placenta that normally feed the seeds.     

Field assessment of a frond-feeding weevil, a successful biological control agent of red waterfern, Azolla filiculoides, in southern Africa

Azolla filiculoides (red waterfern) is a small, floating fern native to South America, that has invaded aquatic habitats, predominantly water resevoirs in southern Africa. A frond-feeding weevil, Stenopelmus rufinasus Gyllenhal (Coleoptera: Curculionidae), was imported from Florida, USA, and released as a biological control agent against this weed in South Africa at the end of 1997. To date, 24,700 weevils have been released, which has resulted in local extinction of red waterfern at 81% of the 112 release sites. The weevil has not failed to control a single site. Several sites were, however, lost due to flooding or drainage of dams. The surface area of weed controlled totalled 203.5 ha. On average, A. filiculoides was controlled in infested sites in 6.9 (±4.3) months. The weed recolonized at 22 of the sites (through either spore germination or dispersal by waterfowl), but the weevils subsequently spread to all of these sites and successfully caused local extinction of the weed at 18 of the sites. Five years after the release of the weevil, the weed no longer poses a threat to aquatic systems in southern Africa. In comparison to other biological control programs of aquatic weeds, the program against A. filiculoides in southern Africa ranks among the most successful cases anywhere in the world.     

Indirect interaction between two native thistles mediated by an invasive exotic floral herbivore

Spatial and temporal variation in insect floral herbivory is common and often important. Yet, the determinants of such variation remain incompletely understood. Using 12 years of flowering data and 4 years of biweekly insect counts, we evaluated four hypotheses to explain variation in damage by the Eurasian flower head weevil, Rhinocyllus conicus, to the native North American wavyleaf thistle, Cirsium undulatum. The four factors hypothesized to influence weevil impact were variations in climate, weevil abundance, phenological synchrony, and number of flower heads available, either on wavyleaf thistle or on the other cooccurring, acquired native host plant (Platte thistle, Cirsium canescens), or on both. Climate did not contribute significantly to an explanation of variation in R. conicus damage to wavyleaf thistle. However, climate did influence weevil synchrony with wavyleaf flower head initiation, and phenological synchrony was important in determining R. conicus oviposition levels on wavyleaf thistle. The earlier R. conicus was active, the less it oviposited on wavyleaf thistle, even when weevils were abundant. Neither weevil abundance nor availability of wavyleaf flower heads predicted R. conicus egg load. Instead, the strongest predictor of R. conicus egg load on wavyleaf thistle was the availability of flower heads on Platte thistle, the more common, earlier flowering native thistle in the sand prairie. Egg load on wavyleaf thistle decreased as the number of Platte thistle flower heads at a site increased. Thus, wavyleaf thistle experienced associational defense in the presence of flowering by its now declining native congener, Platte thistle. These results demonstrate that prediction of damage to a native plant by an exotic insect may require knowledge of both likely phenological synchrony and total resource availability to the herbivore, including resources provided by other nontarget native species.     

Evaluation of Puccinia carduorum for biological control of Carduus pycnocephalus in Tunisia

Puccinia carduorum, a rust fungus from Italian thistle in Tunisia, was most aggressive on young growth stages of the weed in greenhouse tests. Repeated inoculations with the fungus significantly reduced weed biomass. Host-range tests suggest the fungus may be a safe biological control agent of Italian thistle in Tunisia.     

Host recognition by Rhinocyllus conicus of floral scents from invasive and threatened thistles

One of the main obstacles of classical biological control is that biological control organisms cannot be recalled once they are released in nature. It is particularly true for the flowerhead weevil, Rhinocyllus conicus Frölich, which was released as a biological control organism for the invasive musk thistle, Carduus nutans L. (MT). While weevils successfully suppressed introduced populations of musk thistles and other invasive thistle species, non-target attacks have been reported on multiple native thistles including federally listed threatened and endangered (T&E) thistle species. To investigate the foraging behavior of female weevils on invasive and native thistles, we examined volatile organic compounds (VOCs) emitted from MT and a T&E plant species, Sacramento Mountains thistle, Cirsium vinaceum Wooton & Standley (SMT) in the Lincoln National Forest, New Mexico. We used a dynamic headspace volatile collection system and gas chromatography-mass spectrometry to compare volatile profiles between MT and SMT. Female weevils reacted to 7 electrophysiologically active chemical compounds in the blends based on gas chromatography-electroantennography. The behavioral response of female weevils was indifferent when VOCs from both thistles were offered in y-tube olfactometry experiments. Yet, they preferred VOCs collected from MT to purified air. The searching time of female weevils was longer to VOCs collected from SMT over controls. Investigating signals during the initial host recognition of released biological control organisms may open new opportunities to reduce non-target attacks on T&E plant species.

     

Fire‐induced colonization of a Flooding Pampa grassland by thistles: Remnant litter and interference effects

Abstract. Winter and spring burnings constitute a traditional management practice of the Flooding Pampa grasslands which are dominated by the tussock grass Paspalum quadrifarium. In addition to increasing the primary productivity and the nutrient quality of the regrowth, this burning favours the establishment of opportunistic species, especially the legume Lotus glaber and the thistles Cirsium vulgare and Carduus acanthoides. The aims of the present study were to assess the effects of burning and those of the remnant litter on L. glaber and thistle recruitment, as well as the effects of L. glaber density on the emergence, survival and flowering of thistles. Two field experiments were carried out: 1. A completely randomized factorial design with occurrence of L. glaber and fire was applied in a mature Paspalum stand; 2. A completely randomized design with four L. glaber seed densities oversown on a burned Paspalum stand, as the treatments. Thistle and L. glaber recruitment within the Paspalum stand was dependent on fire, but their emergence in burnt plots was reduced by the presence of remnant litter. Remnant litter and interference from L. glaber provide complementary mechanisms to reduce fire‐cued colonization by thistles. While seedling emergence of thistles was mostly affected by the presence of remnant litter, seedling survival depended on local density of L. glaber. By the end of the first post‐fire growing season, the survivorship of established thistles was linearly reduced with increasing sowing density and above‐ground biomass of L. glaber. The maintenance of a significant litter coverage and a high density of L. glaber in the pre‐fire seed bank, as well as the oversowing of L. glaber after fire, may be useful tools in fire management of Paspalum stands aimed to improve their forage value with reduced dependence on chemical control of weeds.     

How to find a needle in a haystack – host plant finding of the weevil Ceratapion onopordi

The weevil Ceratapion onopordi Kirby (Coleoptera: Apionidae) shows a mutualistic interaction with the rust fungus Puccinia punctiformis (Str.) Röhl (Pucciniaceae). One of the weevil’s host plants, the thistle Cirsium arvense (L.) Scop. (Asteraceae), is also the host of the rust fungus. It has been argued that weevils prefer rust‐infected thistle shoots for egg deposition and consequently should be able to detect such shoots. Olfactory, visual, and gustatory orientation was tested using a four‐chamber olfactometer, a visual testing arena, and feeding choice tests. Whereas the weevils used olfactory cues to find their host plants, visual orientation does not seem to be important. Rust‐infected thistle shoots were not preferred over healthy shoots in any of the tests. We conclude that host plants infected with rust fungi, which are rather rare in the field, appear to be found more or less by chance.     

Zooplankton characteristics in monitoring of Lake Ladoga

Water lettuce (Pistia stratiotes L. (Araceae)) is alien to Africa and a declared weed in South Africa. In large perennial rivers it is effectively controlled by its biological control agent, Neohydronomus affinis (Coleoptera: Curculionidae). On those shallow isolated water bodies which are regularly subjected to alternate wet and dry regimes that become infested with water lettuce, chemical control is necessary to prevent further spread of the weed and to facilitate access to water. This paper discusses the short-term chemical control and the long-term biological control of water lettuce. The need for further research is outlined.     

Integrating multiple disturbance aspects: management of an invasive thistle,Carduus nutans

Background and Aims

Disturbances occur in most ecological systems, and play an important role in biological invasions. We delimit five key disturbance aspects: intensity, frequency, timing, duration and extent. Few studies address more than one of these aspects, yet interactions and interdependence between aspects may lead to complex outcomes.

Methods

In a two-cohort experimental study, we examined how multiple aspects (intensity, frequency and timing) of a mowing disturbance regime affect the survival, phenology, growth and reproduction of an invasive thistle Carduus nutans (musk thistle).

Key Results

Our results show that high intensity and late timing strongly delay flowering phenology and reduce plant survival, capitulum production and plant height. A significant interaction between intensity and timing further magnifies the main effects. Unexpectedly, high frequency alone did not effectively reduce reproduction. However, a study examining only frequency and intensity, and not timing, would have erroneously attributed the importance of timing to frequency.

Conclusions

We used management of an invasive species as an example to demonstrate the importance of a multiple-aspect disturbance framework. Failure to consider possible interactions, and the inherent interdependence of certain aspects, could result in misinterpretation and inappropriate management efforts. This framework can be broadly applied to improve our understanding of disturbance effects on individual responses, population dynamics and community composition.     

Classical biological control of Cirsium arvense: Lessons from the past

Cirsium arvense (L.) Scop. is a perennial herb indigenous to Eurasia that is now present throughout temperate regions of the world where it is considered one of the worst weeds of pastoral and agricultural systems. Classical biological control has been attempted in both North America (NA) and New Zealand (NZ). However, nearly 50 years after the first agent releases there are no indications of successful control. We review the status of the five agents deliberately released for control of C. arvense in NA and NZ, plus the species unintentionally introduced, and the occurrence of insects native to NA on C. arvense. We retrospectively evaluate C. arvense as a target weed, critique the agents selected for release, and contrast the different situations in NA and NZ. In retrospect, we see justification for the agents released in NA, but it is evident that these agents would not meet the more stringent host specificity requirements necessary to be released today. The failure of the program in NA is attributed to compromised safety, and lack of impact. Non-target impacts by one of the released agents, Rhinocyllus conicus, have raised safety concerns for native thistle plants. The other released agents either failed to establish, or if established, had no impact on the weed. In contrast, the situation in NZ is quite different because there are no related native thistles (Cardueae), and thus little chance of non-target impacts. Thus far, failure in NZ is attributed to lack of effectiveness due to non-establishment, or no impact, of released agents. In the past, the same agents that were released in NA were subsequently released in NZ, without considering whether or not these were the best choices. Thus, the past failure in NZ might be due to the previous lack of a NZ-specific approach to biocontrol of thistles in general and C. arvense in particular. A new approach taking into consideration the absence of native Cardueae has resulted in the release of agents more likely to be effective, and has potentially set NZ on track towards successful biological control of C. arvense, and other thistles.