Constraints in implementing biological weed control: A review

Biological weed control is a selective, environment-friendly process, utilizing host-specific control agents towards targeted weeds that prevent damage to non-target crops or native plants. The objective of biological control for weeds is not to eradicate but, rather, to regulate weed populations below levels that cause economic injury. There has been criticism that biological weed control research efforts aim to substitute one purchased input (a bioherbicide) for another (a chemical herbicide). It is essential to remember that 'bioherbicides' and 'biological weed control' are not synonymous. Biological control of weeds involves using any organism to reduce or eliminate the detrimental effects of weed populations, whereas bioherbicides utilize plant pathogens repeatedly. The integration of herbicides, both chemical and biological, into ecologically based weed management is an essential process for the sustainability of agriculture. Throughout this review, the constraints affecting the implementation of biological weed control are described. Greater understanding of the morphology, phenology and genetic diversity of targeted weeds is emphasized and the need to examine all aspects related to biocontrol agents is stressed. Improved technologies, better public support and financial aid, and more scientific interest, will all contribute to the progress of the 'science of biological weed control'.     

我国杂草生防现状及若干问题的讨论

杂草生防就是利用寄主范围较专一的植食性动物(如昆虫、螨类、线虫、鱼类等)或植物病原微生物,将影响人类经济活动的杂草种群控制在为害阈限下。这一门学科已有百年历史,近年来发展很快。生物防治杂草与化学除草相比,具有不污染环境、不产生药害、经济效益高等优点。有时一次成功的天敌引种可一劳永逸地解决草害。对一些除草剂难以奏效的多年生     

Biological control of weeds in crops: a coordinated European research programme (COST-816)

For dominant weed species that are difficult to control by traditional means, the development of new, selective, control methods that can be implemented in integrated pest management (IPM) is essential. Here, biological control can be the appropriate means of control due to its high degree of selectivity and environmental safety (direct control value). The biocontrol strategy is based on a detailed analysis of the crop environment and, thus, provides a fundamental tool for developing sustainable agroecosystems (indirect, heuristic value). The successful application of biological weed control will lead to substantial reductions in pesticide use and, thus, will also contribute to the conservation, augmentation and utilization of biodiversity in agroecosystems, an explicit objective of IPM. Only cooperative and concerted efforts, such as those envisaged by COST, will allow the effective completion of weed biocontrol projects within a reasonable period of time. At present, over 25 institutions from 14 countries are participating in this COST action. The following six objectives have been defined for COST-816: to bring together European institutions, to promote a programme for scientific research and exchange, to draw up a general protocol for biological weed control in Europe, to integrate biological control into general weed management strategies, to establish a protocol to resolve potential conflicts of interest and to establish a list of agricultural weed species in Europe for biological control. Three principal methods of biological weed control are used in COST-816: the inoculative or classical approach, the system management approach and the inundative or microbial herbicide approach. Initially, Amaranthus spp., Convolvulus arvensis/Calystegia sepium, Chenopodium album and Senecio vulgaris were chosen as target weeds, each being the subject of a working group. A fifth working group on the control of Orobanche spp. control is in preparation. This concentration on a few target weed species has greatly stimulated cooperation and facilitated technology transfer between the research groups     

Evolving strategies for biological control of weeds with plant pathogens

Current biological control strategies involve the use of exotic or native biocontrol agents, proven to be host-specific and lethal. This is a successful field of endeavor, attractive lo a large cadre of entomologists and a few plant pathologists. The main limitation is that each weed species must be attacked by a separate host-specific agent and these organisms are not always lethal. Only by using a radical paradigm shift can we reach a new strategy of biocontrol. In this strategy, lethal, broad host-range pathogens are genetically modified to permit their safe release. Either they are rendered host-specific or they are given a chemical dependency that prevents their spread or long-term survival. This genetic-manipulative approach offers numerous and diverse scenarios for biocontrol. Host-specific promoters or toxins, host-dependency by multiple auxotrophy, or mutants dependent on specific environmental conditions are all approaches that lend themselves to genetic modification. Such self-delimiting pathogens may have the advantage of repeated marketability, environmental safely and multiple target weeds.     

Weed management in organic agriculture: are we addressing the right issues?

Summary Despite the serious threat which weeds offer to organic crop production, relatively little attention has so far been paid to research on weed management in organic agriculture, an issue that is often approached from a reductionist perspective. This paper aims to outline why and how this problem should instead be tackled from a system perspective. Compared with conventional agriculture, in organic agriculture the effects of cultural practices (e.g. fertilization and direct weed control) on crop:weed interactions usually manifest themselves more slowly. It follows that weed management should be tackled in an extended time domain and needs deep integration with the other cultural practices, aiming to optimize the whole cropping system rather than weed control per se . In this respect, cover crop management is an important issue because of its implications for soil, nutrient, pest and weed management. It is stressed that direct (physical) weed control can only be successful where preventive and cultural weed management is applied to reduce weed emergence (e.g. through appropriate choice of crop sequence, tillage, smother/cover crops) and improve crop competitive ability (e.g. through appropriate choice of crop genotype, sowing/planting pattern and fertilization strategy). Two examples of system-oriented weed management systems designed for organic agriculture are illustrated as well as future perspectives and problems.     

Concepts in weed control -- how does biocontrol fit in?

Up until now there have only been twoprincipal concepts in weed control: control at any cost, and theuse of economic thresholds. Control at any cost is characteristicof situations where no effective control methods are available.The use of economic thresholds has evolved alongside chemicalcontrol, enabling weeds to be controlled effectively and at areasonable price. A future concept may be that of ecologicalthresholds, which takes into account not only the costs but alsothe benefits of weeds. The role that weed control methods shouldfulfil in arable crops, and how biocontrol methods can meet theserequirements, is discussed     

Advancing cover cropping in temperate integrated weed management

The effects of cover crops on weeds and the underlying mechanisms of competition, physical control and allelopathy are not fully understood. Current knowledge reveals great potential for using cover crops as a preventive method in integrated weed management. Cover crops are able to suppress 70–95% of weeds and volunteer crops in the fall‐to‐spring period between two main crops. In addition, cover crop residues can reduce weed emergence during early development of the following cash crop by presenting a physical barrier and releasing allelopathic compounds into the soil solution. Therefore, cover crops can partly replace the weed suppressive function of stubble‐tillage operations and non‐selective chemical weed control in the fall‐to‐spring season. This review describes methods to quantify the competitive and allelopathic effects of cover crops. Insight obtained through such analysis is useful for mixing competitive and allelopathic cover crop species with maximal total weed suppression ability. It seems that cover crops produce and release more allelochemicals when plants are exposed to stress or physical damage. Avena strigose, for example, showed stronger weed suppression under dry conditions than during a moist autumn. These findings raise the question of whether allelopathy can be induced artificially. © 2019 Society of Chemical Industry     

Development of chemical weed control and integrated weed management in China

More than 200 species of weeds are infesting main crop fields in China, among which approximately 30 species are major weeds causing great crop yield losses. About 35.8 million hectares of crop fields are heavily infested by weeds and the annual reduction of crop yields is 12.3–16.5% (weighted average). Along with rural economic development, approximately 50% of the main crop fields undergo herbicide application. Chemical weed control has changed cultural practices to save weeding labor in rice, wheat, maize, soybeans and cotton. At the same time, continuous use of the same herbicides has caused weed shift problems and weed resistance to herbicides. Consequently, integrated weed management in main crops is being developed.     

Turfgrass management and weed control without pesticides on football pitches in Denmark

Management of turfgrass on football pitches without pesticides involves a considerable challenge for weed control. By improving conditions for grass growth by cultural practices, weeds may be repressed by stronger competition from the grass. A 3-year field trial on 37 football pitches investigated the effects of various cultural management practices on percentage grass, weed and bare ground, respectively. The trial included twelve different treatments, comprising different combinations of fertiliser level and cultural methods, including two types of spring-tine harrow, vertical cutting, over-seeding, and top-dressing. Some treatments resulted in significant differences in percentage grass and weed cover, but not in percentage bare ground. However, other factors, such as locality of the football pitch, zone on the pitch, month, year, playing frequency, cutting frequency and, particularly, ground cover at the beginning of the trial, also contributed considerably to the variation in ground cover, indicating that established weeds are difficult to eradicate.     

Glyphosate-resistant soybean as a weed management tool: Opportunities and challenges

Transgenic soybean, resistant to glyphosate, represents a revolutionary breakthrough in weed control technology. Transgenic soybean is the most dominant among all transgenic crops grown commercially in the world. In 2000, glyphosate-resistant (GR) soybean was planted to 25.8 million hectares globally, which amounts to 58% of the total global transgenic crop area. The United States soybean area planted with GR soybean has increased from 2% in 1996 to 68% in 2001. Glyphosate-resistant soybean as a weed management tool has provided farmers with the opportunity and flexibility to manage a broad spectrum of weeds. The use of glyphosate in GR soybean offers another alternative to manage weeds that are resistant to other herbicides. The rapid increase in GR soybean area is caused by the simplicity of using only one herbicide and a lower cost for weed control. Adoption of GR soybean has resulted in a dramatic decrease in the area treated with other herbicides. Glyphosphate-resistant soybean should not be relied on solely to the exclusion of other weed control methods, and should be used within integrated weed management systems. Over-reliance on GR soybean could lead to problems such as shifts in weed species and population, and the development of glyphosate-resistant weeds. The challenge is for soybean farmers to understand these problems, and for weed scientists to communicate with farmers that continuous use of glyphosate may diminish the opportunity of GR soybean as a weed management tool in the future.     

The European Biological Control Laboratory: an existing infrastructure for biological control of weeds in Europe

The Agricultural Research Service of the United States Department of Agriculture has maintained a continuous European effort in classical biological control of exotic pests in the USA. The European Parasite Laboratory was established in France in 1919, while the European Weed Laboratory began operations in Rome in 1958. The two laboratories were merged in Montpellier in 1991 as the European Biological Control Laboratory (EBCL), becoming the primary overseas biocontrol laboratory of the USDA. The management of weeds, insect pests, and pathogens is an important feature of agricultural research programmes worldwide. These invasive species can lead to vast financial losses for countries engaged in agriculture. The overall goal of research at EBCL is to develop biological control technologies to be used to suppress invading weeds and insect pests. This is done through expeditions to find natural enemies (insects, mites and pathogens), or phytophagous, parasitoid or predator species. These are characterized in careful experimentation in quarantine facilities and eventually developed as biological control agents. Current weed projects include studies on the Centaurea spp., Arundo donax , Vincetoxicum spp., Isatis tinctoria , Taeniatherum sp. and Dipsacus sp. Insect projects research Lygus bugs, the olive fruit fly, the vine mealybug, and the Asian long-horned beetle. The EBCL team is international and interdisciplinary. Entomology, plant and insect pathology, molecular biology and ecology are the main approaches of our biocontrol research. The team cooperates with universities and agencies worldwide.     

土壤厌氧消毒技术、作用机理及其防治效果的影响因素

土壤厌氧消毒(anaerobic soil disinfestation,ASD)技术作为一种环境友好的非化学土壤消毒技术能有效防治真菌、细菌、线虫和杂草等引起的土传病害。该技术已在多个国家广泛应用并被证实可显著增加作物产量。ASD技术基本流程为向土壤中添加适量有机碳源、补充足够水分和用塑料薄膜覆膜密闭2～15周。ASD技术对土传病害防控的作用机理为有机碳源分解产生的挥发性有机物对土传病原菌有抑制作用,土壤理化指标的改变(如pH降低、有机质增加等)能提升土壤质量,从而提高作物抗逆能力,土壤厌氧条件促进土壤微生物群落结构及多样性的改变,通过生物竞争和结构重建改善土壤微生态环境,降低土传病害发生风险。ASD技术对土传病害的防治效果也因有机碳源类别、土壤理化性质及覆盖薄膜的不同而存在差异。因此,优化ASD技术试验条件、深入探索ASD技术作用机理、挖掘更多有灭菌活性的挥发性物质或有益生防菌是未来ASD技术的研究重点。同时,ASD技术与其他土壤消毒技术结合使用,有助于提高其防治效果,扩大应用范围。     

Biology,ecology and management of the invasive parthenium weed (Parthenium hysterophorus L.)

Parthenium weed (Parthenium hysterophorus L.) is one of the most aggressive invasive weeds, threatening natural ecosystems and agroecosystems in over 30 countries worldwide. Parthenium weed causes losses of crops and pastures, degrading the biodiversity of natural plant communities, causing human and animal health hazards and resulting in serious economic losses to people and their interests in many countries around the globe. Several of its biological and ecological attributes contribute towards its invasiveness. Various management approaches (namely cultural, mechanical, chemical and biological control) have been used to minimise losses caused by this weed, but most of these approaches are ineffective and uneconomical and/or have limitations. Although chemical control using herbicides and biological control utilising exotic insects and pathogens have been found to contribute to the management of the weed, the weed nevertheless remains a significant problem. An integrated management approach is proposed here for the effective management of parthenium weed on a sustainable basis. © 2014 Society of Chemical Industry     

Effect of stale seedbed preparations and subsequent weed control in lettuce (cv. Iceboll) on weed densities

The effects of stale seedbed preparations and several weed control methods on the emergence of weeds in lettuce were studied. The specific goal was to evaluate the use of a stale seedbed in combination with chemical or mechanical weed control methods in the field. Depending on location and year, stale seedbed preparations followed by weed control prior to planting reduced the amount of weeds during crop growth by 43–83%. Control of the emerged seedlings after a stale seedbed preparation was more effective with glyphosate than with a rotary harrow. Covering the rotary harrow during control to prevent light reaching the soil improved its effect on the weed density during crop growth in two of 3 years. Radiation with far red light (FR) did not reduce the number of emerging weeds in this study. Mechanical control by finger weeder, torsion weeder and hoe was applied without stale seedbed preparations. These measures reduced the weed densities by 88–99%, compared with the untreated control and were more effective than chemical weed control with carbetamide and chlorpropham. The results show that the stale seedbed technique in combination with mechanical control of emerging weeds can reduce the weed population during crop growth as effectively as chemical control. The technique may therefore help reduce the use of herbicides in lettuce crops in the future.     

Non-chemical approaches to weed control in horticulture

Concern about herbicide safety has renewed interest in non-chemical systems of weed control. Alternative approaches for dealing with weeds can be broadly grouped into cultural and biological methods. Some of the options available for controlling weeds without the use of herbicides are discussed.     

空心莲子草的分布、危害与防除对策

空心莲子草是一种重要的外来入侵恶性杂草,20世纪80年代以来该草自然扩展成为水域和陆地两个生态类型,形成单一优势种群使入侵地的生物多样性遭到破坏。在水域的空心莲子草可通过人工打捞和利用其天敌控制;在旱地除利用天敌外,还需通过除草剂、物种竞争、化感作用、植物病原菌和人工拔除等措施进行防除。本文就其在我国分布、危害及其防治对策进行了综述。     

The potential for control of bracken in the UK using introduced herbivorous insects

Bracken (Pteridium aquilinum (L.) Kuhn) is an invasive native weed in the UK causing problems in upland agriculture and in land of amenity and conservation value; it may represent a risk to human health. Existing control methods such as cutting or herbicide use are subject to practical, economic or environmental constraints in many areas of the UK. Classical biological control of bracken would involve the introduction of specialist bracken-feeding herbivores from other parts of the world. Classical biological weed control has a reasonable record of success in other parts of the world and an exemplary safety record, but remains untried in the UK. The typical development of a classical weed biocontrol programme is presented using the UK bracken programme as an example. Finally the current position of this classical biocontrol programme is reviewed with an assessment of the prospects for the future. With appropriate funding, a full field release of at least one species of South African bracken-feeding moth should be achieved during the mid-1990s.     

Further observations on the use of fungicides to control Peronosclerospora sacchari (T. Miyake) Shirai and K. Hara in sugarcane in Papua New Guinea

Abstract

Many biologists perceive organisms as constantly evolving and therefore consider the host plant ranges of biological control agents as labile. Host plant ranges are thus likely to undergo adaptive change should environmental conditions change, for example following successful biological control. As a consequence, the introduction of biological control agents against weeds is considered by many to be an inherently unsafe practice with non‐target plants at risk of attack. However, despite the introduction of over 600 insect species from one geographic region to another for biological weed control during this century, there are relatively few documented cases of changes in host plant range. Purported instances are discussed in relation to behavioural and genetic concepts. It is concluded that apparent additions to the host range can, in all of the cases examined, be explained in terms of established behavioural concepts of pre‐adaptation, threshold change resulting from host deprivation, and effects of experience (learning). The inappropriateness of the often‐used term host shift’ to describe these cases is demonstrated, and it is concluded that evidence from biological weed control contradicts some aspects of ecological and evolutionary theory.     

Strategic tillage in Australian conservation agricultural systems to address soil constraints: How does it impact weed management?

In the conservation agricultural systems practised in Australia, cultivation is not commonly utilised for the purpose of weed control. However, occasional use of tillage (strategic tillage) is implemented every few years for soil amelioration, to address constraints such as acidity, water repellence or soil compaction. Depending on the tillage method, the soil amelioration process buries or disturbs the topsoil. The act of amelioration also changes the soil physical and chemical properties and affects crop growth. While these strategic tillage practices are not usually applied for weed control, they are likely to have an impact on weed seedbank burial, which will in turn affect seed dormancy and seedbank depletion. Strategic tillage impacts on seed burial and soil characteristics will also affect weed emergence, plant survival, competitive ability of weeds against the crop and efficiency of soil applied pre-emergent herbicides. If growers understand the impacts of soil amelioration on weed demography, they can more effectively plan management strategies to apply following the strategic tillage practice. Weed seed burial resulting from a full soil inversion is understood, but for many soil tillage implements, more data is needed on the extent of soil mixing, burial of topsoil and the weed seedbank, physical control of existing weeds and stimulation of emergence following the tillage event. Within the agronomic system, there is no research on optimal timing for a tillage event within the year. There are multiple studies to indicate that strategic tillage can reduce weed density, but in most studies, the weed density increases in subsequent years. This indicates that more research is required on the interaction of amelioration and weed ecology, and optimal weed management strategies following a strategic tillage event to maintain weeds at low densities. However, this review also highlights that, where the impacts of soil amelioration are understood, existing data on weed ecology can be applied to potentially determine impacts of amelioration on weed growth.