Cross-fertilizing weed science and plant invasion science to improve efficient management: A European challenge

Both weed science and plant invasion science deal with noxious plants. Yet, they have historically developed as two distinct research areas in Europe, with different target species, approaches and management aims, as well as with diverging institutions and researchers involved. We argue that the strengths of these two disciplines can be highly complementary in implementing management strategies and outline how synergies were created in an international, multidisciplinary project to develop efficient and sustainable management of common ragweed, Ambrosia artemisiifolia. Because this species has severe impacts on human health and is also a crop weed in large parts of Europe, common ragweed is one of the economically most important plant invaders in Europe. Our multidisciplinary approach combining expertise from weed science and plant invasion science allowed us (i) to develop a comprehensive plant demographic model to evaluate and compare management tools, such as optimal cutting regimes and biological control for different regions and habitat types, and (ii) to assess benefits and risks of biological control. It further (iii) showed ways to reconcile different stakeholder interests and management objectives (health versus crop yield), and (iv) led to an economic model to assess invader impact across actors and domains, and effectiveness of control measures. (v) It also led to design and implement management strategies in collaboration with the various stakeholder groups affected by noxious weeds, created training opportunities for early stage researchers in the sustainable management of noxious plants, and actively promoted improved decision making regarding the use of exotic biocontrol agents at the national and European level. We critically discuss our achievements and limitations, and list and discuss other potential Old World (Afro-Eurasian) target species that could benefit from applying such an integrative approach, as typical invasive alien plants are increasingly reported from crop fields and native crop weeds are invading adjacent non-crop land, thereby forming new source populations for further spread.     

The population dynamics of an introduced tree, Sesbania punicea, in South Africa, in response to long-term damage caused by different combinations of three species of biological control agents

This paper contributes to the relatively sparse literature on the effects of insect herbivory on the population dynamics of plants and is probably unique in that it reports the long-term effects of combinations of three insect herbivore species on the population densities of a moderately long-lived tree species. The tree is Sesbania punicea, a leguminous perennial from South America that has been the target of a biological control programme in South Africa for almost 20 years. Sixteen infestations of the weed have been monitored for periods of up to 10 years to determine changes in the density of the mature, reproductive plants under the influence of different combinations of three biological control agents (i.e. with one, two or three of the agent species present in the weed infestation). The three biological control agents, all weevil species, include Trichapion lativentre, which primarily destroys the flower-buds, Rhyssomatus marginatus, which destroys the developing seeds, and Neodiplogrammus quadrivittatus, whose larvae bore into the trunk and stems of the plants. While T. lativentre occurs throughout the range of the weed in South Africa, the other two species are less mobile, more recent introductions and are largely confined to the vicinity of selected release sites. There has been a significant decline in the density of mature S. punicea in areas where two or more of the agents are established. The decline of the weed has been most evident where N. quadrivittatus is active and particularly so where both of the other two weevil species are also present. Received: 2 April 1997 / Accepted: 30 November 1997     

Biological control: Premises,ecological input and Mimosa pigra in the wetlands of Australia's Top End

The adverse impacts of weeds on natural ecosystems, together with the inadequate outcomes from treating weeds as a symptom, have escalated interest in finding efficacious control methods. With the aim of protecting wetlands from invasive weeds, this contribution uses the woody shrub Mimosa pigra L. (mimosa) as a case history to examine the methodology of classical biological control and the reasons for the widely accepted 75% failure rate. Overall objectives are for all biocontrol agents to have the opportunity to fully express their potential and to insure that limited resources are spent wisely on attainable weed control. The three main conclusions were that (1) the premises on which biocontrol is based has restricted advancement of this method; (2) monitoring is the logical first step to improving the selection of agents and release sites; and (3) it may be more cost-effective to introduce fewer agents that have undergone agent/plant and host/home range pre- and post-release ecological studies. Weed control may remain elusive unless advantage is taken of every beneficial result. Innovative assistance to agents and the integration of different control methods may preserve a role for weed biological control and has the potential to be of great importance for future weed management. It is proposed that the climate in the Top End of the Northern Territory and the lack of competition on the floodplains has contributed to mimosa's invasiveness. Climate may also underly the difficulties faced by agents. Agents appear unable to impart effective control in the dry season because of low numbers which relates to mimosa's poor growth; nor in the wet season, when the impact from high numbers of agents is outstripped by mimosa's growth.     

Weed biological control in California,USA: review of the past and prospects for the future

Weed biological control in California, USA began in 1940 with the release of a native scale insect on native Opuntia spp. on Santa Cruz Island, just offshore from mainland California. Since then, a total 39 weed species have been targets of biological control releases in California. Releases on 11 weed targets were transfer experiments where agents from related weed hosts were released on a new host. Most of the transfer experiment introductions failed but one weed was successfully controlled. Of the other 28 weeds, release sites for three species were destroyed and for six species releases are too recent to score, but for 19 weeds, their level of control was rated as: complete control (three species), substantial control (five species), and partial (six species), and negligible control (five species). Overall, 42% of the projects provide successful control, a result lower than observed in other countries worldwide. Since 1940, 77 species of agents have been released: 54 species established, 12 species failed to establish, six species had their release sites destroyed, and five species are too early to determine. Establishment rate was 82% but the rate differed among taxonomic orders. Individual agents were scored according to level of impact on their host and Coleoptera obtained the highest average impact score and Diptera the lowest. Mean impact scores over time showed a substantial drop in the 1980s but later increased. Future research efforts that emphasize introduction of high impact agents will further support development of this critical weed control method for California.     

Making host specificity testing more efficient: Exploring the use of abridged test plant lists

Testing the specificity of candidate agents is a key component of risk analysis in weed biological control. This step is often time-consuming due to the numerous plant species that need to be tested under quarantine conditions in the invaded country of the weed species. Here, we examined whether an abridged phylogenetically based test list could be used in the weed's native range to quickly screen the host specificity of candidate agents. Ten plant species were used to test the host specificity of a promising candidate for the biological control of Sonchus oleraceus in Australia, the gall midge, Cystiphora sonchi. No-choice and choice tests were carried out in the native Mediterranean range of the midge. The results showed the midge has potential to threaten native Australian species, as those species showed high infestation levels in no-choice tests and produced significantly higher numbers of galls in choice tests. As a result of this approach, C. sonchi was rapidly discarded from the list of agents to be imported into Australian quarantines for further tests. This study demonstrates that testing a few key phylogenetically related species in the native range may save cost and effort in a weed biological control programme.     

The biology and host specificity of the onion weed rust, Puccinia barbeyi, a potentially useful agent for biological control in Australia

Onion weed, Asphodelus fistulosus L., (Liliaceae) a weed of Mediterranean and Middle Eastern origin is widespread in southern Australia where it invades pastures making them unsuitable for grazing. A program of research is underway to discover natural enemies of this plant and to study their possible role in the biological control of onion weed. A rust fungus Puccinia barbeyi (Roum.) Magnus has been found to severely attack A. fistulosus . Observations on the biology of the rust confirmed that it is monoecious and microcyclic and multiplied essentially by aecial and telial stages, although occasionally urediniospores also appeared among teliospores. Several members of the Liliaceae exposed to the aeciospores of the rust remained unattacked indicating that it is most probably specific to Asphodelus spp. and thus its potential for the biological control of A. fistulosus in Australia should be studied further.     

Success and safety in the biological control of environmental weeds in New Zealand

Biological control, using specialist insect herbivores and plant pathogens, can be a self‐sustaining, cost‐effective and low‐risk tool for the management of environmental weeds. Agents have been recorded attacking non‐target plants in New Zealand and elsewhere, but the effects are usually minor and/or transitory. It seems probable that only two cases, worldwide, will result in significant damage to non‐target plants (representing 0.5% of the nearly 400 insect, mite, or fungal species used in classical weed biocontrol). Both of these cases were predictable from host range testing. Negative indirect, or ‘downstream’, ecological effects from specific weed biocontrol agents are difficult to predict and measure. They are probably insignificant compared to the impacts of the invasive plants that the agents are introduced to control. However, it is necessary to balance the risks associated with any introduction against the environmental benefits from controlling a weed to a predicted level. Recent analyses suggest that success rates are better than generally perceived. For New Zealand programmes, where enough time has lapsed to allow assessment, we calculate a full/partial success rate of 83%. Many of the costs associated with environmental weeds are difficult to quantify. Detailed risk assessment will make biological control programmes more expensive and time‐consuming, so that reliance on non‐biological management methods for environmental weeds may actually increase. The costs of biocontrol programmes against some New Zealand weeds can be kept down by using research already carried out in Australia and other countries, and the process is reciprocal. Developing international consortia of sponsors is also a potential way to fund programmes against weeds shared by several countries.     

Ecological niche modelling of an invasive alien plant and its potential biological control agents

Invasive alien plants are of concern in South Africa. Pompom weed (Campuloclinium macrocephalum) is currently invading the Grassland and Savannah biomes of South Africa and is likely to continue spreading in the southern African sub- region. Two possible biological control agents (Liothrips tractabilis and Cochylis campuloclinium) have been identified for control of pompom weed. We used ecological niche modelling to predict which areas in southern Africa are likely to be suitable for pompom weed and the two potential biological control agents. The overlap between areas predicted to be highly suitable for pompom weed and areas suitable for the biological control agents was assessed. Methods of reducing sampling bias in a data set used for calibrating models were also compared. Finally, the performance of models calibrated using only native range data, only invaded range data and both were also compared. Models indicate that pompom weed is likely to spread across a greater region of southern Africa than it currently occupies, with the Savannah and Grassland biomes being at greatest risk of invasion. Poor overlap was found between the areas predicted to be highly suitable for pompom weed and those areas predicted to be suitable for the biological control agents. However, models of the potential distribution of the biological control agents are interpreted with caution due to the very small sample size of the data set used to calibrate the models. Models calibrated using both native range and invaded range data were found to perform best whilst models calibrated using only native range data performed the worst. There was little difference found between models that were calibrated using spatially reduced (selecting only one record per 30 min grid cell) and randomly reduced (randomly selecting 50% of available records) biased data sets.     

杂草生防作用物对本土生物群落的影响

传统生物防治是治理外来入侵杂草危害切实可行的有效策略和途径,近来对传统生物防治的批评主要集注于,引进的生防作用物攻击威胁本土非靶标生物。引进的生防作用物可能对本土非靶标生物产生直接和间接影响,这类影响通过不同营养级生物之间的取食关系,以及通过同一营养级内生物间的竞争关系,而影响本土非靶标生物群落。列举若干杂草生物防治案例对以上影响方式及其发生途径进行了评述。就防范杂草生防作用物对非靶标生物的负面影响,提出了以下对策:(i)把引进天敌防治外来入侵生物作为最后的有效手段;(ii)适当增加对非靶标生物潜在影响的生态学评估;(iii)选择寄主专一性强而且能有效控制靶标杂草的天敌;(iv)加强对杂草传统生物防治的生态学研究。     

Biological control of Parkinsonia aculeata: what are we trying to achieve?

Abstract  The rangeland shrub, Parkinsonia aculeata (parkinsonia), is considered to be a serious pastoral and environmental weed in Australia. Existing biological control agents have been ineffective and native-range surveying is currently being undertaken to find potential new agents. In this paper I develop unambiguous performance criteria to assist in prioritising potential biological control agents, and against which the success of our biological control program can be judged. These include a reduction of patch density (to below 30% cover) and size (<0.1 ha), a reduction in rates of spread and in-fill (by reducing seed production and understorey seed bank densities to <100 viable seeds m⁻²), and the explicit targeting of regions most at risk from parkinsonia impacts. The development of explicit, a priori performance criteria for biological control programs should help biological control develop into a more rigorous and predictive science.     

Control of Pseudophilothrips ichini (Thysanoptera: Phlaeothripidae) with acephate to exclude a biological control agent of Schinus terebinthifolia

Post-release monitoring of biological control agents to determine impact on the target weed has recently received increased priority. Several methods are available to measure the impact of a biological control agent by manipulating the agent population while measuring fitness of the weed. Brazilian peppertree, Schinus terebinthifolia is one of the most damaging weeds in subtropical areas of Florida and Hawaii. A biological control agent, the thrips, Pseudophilothrips ichini is a sap-feeder that shows high levels of specificity and causes severe distortion of leaf tips of the weed. Thrips populations of this species and a generalist thrips, the red banded thrips Selenothrips rubrocinctus were experimentally manipulated by applications of the systemic insecticide acephate by both foliar applications and by an inserted encapsulated formulation. Foliar applications protected plants against red banded thrips for 29 days and against the biological control thrips, P. ichini for 22 days after treatment. Control with inserts were initially low but was achieved after 60 days and this control continued for 182 days after treatment. Manipulation of these biological control thrips populations with foliar or inserted formulations will assist in the determination of biological control agent impact.     

A scientific approach to agent selection

Abstract  The prioritisation of potential agents on the basis of likely efficacy is an important step in biological control because it can increase the probability of a successful biocontrol program, and reduce risks and costs. In this introductory paper we define success in biological control, review how agent selection has been approached historically, and outline the approach to agent selection that underpins the structure of this special issue on agent selection. Developing criteria by which to judge the success of a biocontrol agent (or program) provides the basis for agent selection decisions. Criteria will depend on the weed, on the ecological and management context in which that weed occurs, and on the negative impacts that biocontrol is seeking to redress. Predicting which potential agents are most likely to be successful poses enormous scientific challenges. 'Rules of thumb', 'scoring systems' and various conceptual and quantitative modelling approaches have been proposed to aid agent selection. However, most attempts have met with limited success due to the diversity and complexity of the systems in question. This special issue presents a series of papers that deconstruct the question of agent choice with the aim of progressively improving the success rate of biological control. Specifically they ask: (i) what potential agents are available and what should we know about them? (ii) what type, timing and degree of damage is required to achieve success? and (iii) which potential agent will reach the necessary density, at the right time, to exert the required damage in the target environment?     

Effects of Hydrilla verticillata (L.f.) Royle (Hydrocharitaceae) growing conditions and nutrient content on the performance of a leaf-mining fly,Hydrellia purcelli Deeming (Diptera: Ephydridae)

Host plant quality for insects used in weed biological control influences their performance and hence their ability to suppress target host populations. Determining the specific response of these insects to the quality of their host is important because phytophagous insects have variable tolerances of the different constituents of host plant quality, most notably dietary nitrogen (N), but also other physical and chemical components. The invasive aquatic weed Hydrilla verticillata (L.f.) Royle (Hydrocharitaceae) was cultivated under varying nutrient conditions to determine the influence of plant quality on immature survival, development, larval mining, reproductive output and adult longevity of a leaf-mining fly Hydrellia purcelli Deeming (Diptera: Ephydridae). Additionally, field-collected H. verticillata was included in the investigations to assess the potential performance of H. purcelli in the field. Variation in plant tissue N and phosphorus (P) concentrations had no effects on larval survival, female fecundity, or adult longevity, but high levels of N and P were associated with reduced immature development times and higher body mass of females. Overall, plant quality factors not measured in this study appeared to have a greater impact on the performance of the fly, rather than dietary N and P. The results provided insights into optimal mass-rearing conditions for H. purcelli and the potential performance of the fly in the field in South Africa. Furthermore, the results demonstrate the importance of considering other aspects of plant quality for insect agents, in addition to dietary N and P, when developing mass-rearing protocols or predicting their potential impact in the field.     

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.     

The effect of targeted high‐threat weed control on wet forest understorey vegetation in the Central Highlands region,Victoria

The effective control of highly invasive weeds in Australia is an important conservation management action. In this study, we monitored the outcome of herbicide control on high‐threat weeds in the wet forests of the Central Highlands of Victoria. Twenty‐two control (no weed control) and 32 treatment (weed control) plots were surveyed annually over 24 months. Initial results show that weed cover and frequency decreased substantially in response to weed control; however, it is too early to determine the response of native species. We recommend that herbicide control and the associated monitoring programme be continued, and depending on the outcomes, data should be used to develop a more integrated management strategy.     

杂草科学管理——理论基础与实施途径

Scientific management of weeds is theoretically based on ecology.The implementation methods incladeIntensifying the weed biology and ecology research, especially those of the heavy weeds;Intensifying the research of developing competition between the crop and the weed;Utilizing a allelopathy the gene engineering and breeding against the weeds;Utilizing allelopathy between the crop and the weed, and utilizing biological and agricultural measurements to control the weeds.     

Prioritising potential guilds of specialist herbivores as biological control agents for prickly acacia through simulated herbivory

Understanding plant response to herbivory facilitates the prioritisation of guilds of specialist herbivores as biological control agents based on their potential impacts. Prickly acacia ( Acacia nilotica ssp. indica ) is a weed of national significance in Australia and is a target for biological control. Information on the susceptibility of prickly acacia to herbivory is limited, and there is no information available on the plant organ (i.e. leaf, shoot and root in isolation or in combination) most susceptible to herbivory. We evaluated the ability of prickly acacia seedlings, to respond to different types of simulated herbivory (defoliation, shoot damage, root damage and combinations), at varying frequencies (no herbivory, single, two and three events of herbivory) to identify the type and frequency of herbivory that will be required to reduce the growth and vigour. Defoliation and shoot damage, individually, had a significant negative impact on prickly acacia seedlings. For the defoliation to be effective, more than two defoliation events were required, whereas a single bout of shoot damage was enough to cause a significant reduction in plant vigour. A combination of defoliation + shoot damage had the greatest negative impact. The study highlights the need to prioritise specialist leaf and shoot herbivores as potential biological control agents for prickly acacia.     

Molecular biology of weed control

The vast commercial effort to utilize chemical and molecular tools to solve weed control problems has had a major impact on the basic biological sciences as well as benefits to agriculture, and the first generation of transgenic products has been successful, while somewhat crude. More sophisticated products are envisaged and expected. Biotechnologically-derived herbicide-resistant crops have been a considerable benefit, yet in some cases there is a risk that the same useful transgenes may introgress into related weeds, specifically the weeds that are hardest to control without such transgenic crops. Biotechnology can also be used to mitigate the risks. Molecular tools should be considered for weed control without the use of, or with less chemicals, whether by enhancing crop competitiveness with weeds for light, nutrients and water, or via allelochemicals. Biocontrol agents may become more effective as well as more safe when rendered hypervirulent yet non-spreading by biotechnology. There might be ways to disperse deleterious transposons throughout weed populations, obviating the need to modify the crops. This revised version was published online in August 2006 with corrections to the Cover Date.     

Avoiding conflicts between insect and weed biological control: selection of non-target species to assess host specificity of cabbage seedpod weevil parasitoids

Abstract:  Classical biological control of insect pests and weeds may lead to potential conflicts, where insect pests are closely related to weed biological control agents. Such a conflict may occur in the classical biological control of the cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) in North America, which belongs to the same subfamily, Ceutorhynchinae, as a number of agents introduced or proposed for introduction against non-indigenous invasive weed species. We propose a step-by-step procedure to select non-target species and thereby to develop a non-target species test list for screening candidate entomophagous biological control agents of a herbivore pest insect in a way that would simultaneously evaluate non-target potential on weed biological control agents and other non-target species. Using these recommendations, we developed a non-target test list for host specificity evaluations in the area of origin (Europe) and the area of introduction (North America) for cabbage seedpod weevil parasitoids. Scientifically based predictions on expected host–parasitoid interactions and ecological information about the ecological host range in the area of origin can help avoid conflicts, while still allowing the introduction of safe and effective agents against both insect pests and weeds.     

Plant-mediated interactions: Considerations for agent selection in weed biological control programs

Plant-mediated indirect interactions among herbivores (arthropods and pathogens) are common and extensively reported in the ecological literature. However, they are not well-documented with respect to weed biological control. Such interactions between biological control agents can have net positive or negative impacts on total weed suppression depending on the strength of the interaction(s), the relative importance of the agent indirectly impacted, and the combined weed suppression that results. A better understanding of plant-mediated interactions may improve decision-making about which agents to introduce in classical biological control programs for greatest impact on invasive weeds. This paper reviews the subject, including examples from the biological control literature; outlines the need for research on indirect effects of herbivores on other herbivores; discusses how such knowledge may strengthen classical biological control programs for invasive weeds; and provides recommendations for the kind of studies that should be done and how information about plant-mediated interactions could be integrated into agent evaluation protocols, to assist in decision-making about agents for importation and release.