Weed In Canada Research Articles

Glyphosate-resistant and susceptible downy brome (Bromus tectorum) management with soil-applied residual herbicides

Abstract Downy brome is a cleistogamous facultative winter-annual grass weed that invades cropland, pastureland, and ruderal areas in western North America. Glyphosate-resistant downy brome, the first known glyphosate-resistant grass weed in Canada, was confirmed in a glyphosate-resistant canola field in southern Alberta in 2021. A controlled-environment study was conducted to determine the impact of preemergence soil-applied residual herbicides on glyphosate-resistant and susceptible downy brome in two field soils. Flumioxazin/pyroxasulfone (70/89 g ai ha−1), carfentrazone/pyroxasulfone (18/150 g ai ha−1), sulfentrazone/pyroxasulfone (100/100 or 150/150 g ai ha−1), and saflufenacil/pyroxasulfone (36/120 g ai ha−1) resulted in excellent (≥90%) visible control and downy brome biomass reduction 8 wk after treatment (WAT). The low rate of carfentrazone/pyroxasulfone (12/100 g ai ha−1) resulted in good (≥80%) visible control and biomass reduction 8 WAT, while the low and medium rates of saflufenacil/pyroxasulfone (18/60 or 25/84 g ai ha−1) resulted in ≥80% biomass reduction but suppression only (66% to 75%) based on visible control. Flumioxazin alone (105 g ai ha−1) resulted in good visible control (81%) 8 WAT in a sandy loam soil, but poor (13%) control in a clay loam soil. Soil type affected plant growth as evidenced by reduced growth in the untreated sandy loam soil compared to clay loam soil. The glyphosate-resistant population emerged and grew more vigorously than the glyphosate-susceptible population resulting in greater plant densities in the untreated control and some less-effective herbicide treatments. These results suggest that mixtures of a protoporphyrinogen oxidase-inhibiting herbicide with the very-long-chain fatty acid elongase inhibitor pyroxasulfone applied preemergence at ≥89 g ai ha−1 could be effective components of an herbicide layering strategy targeting glyphosate-resistant and glyphosate-susceptible downy brome.

Glyphosate-resistant downy brome (Bromus tectorum) control using alternative herbicides applied postemergence

AbstractDowny brome is a troublesome facultative winter-annual grass weed that invades agricultural and nonagricultural lands in western North America and can cause substantial crop yield losses particularly in no-till winter wheat. Glyphosate-resistant (GR) downy brome was identified in southern Alberta in 2021, representing the first confirmation of a GR grass weed in Canada. This study was designed to evaluate alternative herbicides and herbicide mixtures applied postemergence (POST) for control of GR and glyphosate-susceptible (GS) downy brome populations at the seedling stage under a controlled environment. The GR downy brome did not exhibit cross-resistance to other herbicides applied POST. Quizalofop alone or in combination with imazamox, imazamox + bentazon, or imazamox/imazethapyr, and glufosinate mixed with either clethodim or tiafenacil resulted in ≥80% visible control, plant mortality, and reduction in biomass of both GR and GS downy brome populations 21 d after treatment. Diligent stewardship of these remaining herbicide options is warranted since downy brome populations with resistance to herbicides that inhibit acetyl-CoA carboxylase or acetolactate synthase have been reported in neighboring states.

Potential spring canola yield losses due to weeds in Canada and the United States

AbstractWeeds represent one of the most important biotic threats to agricultural plant health, and the potential global impact of weeds on crop yields is similar to that of all other pests (animal pests and pathogens) combined. Canola is the most-grown crop in Canada based on seeded area and generates on average Can$29.9 billion in economic activity each year. The objective of this report, sponsored by the Weed Science Society of America Weed Loss Committee, was to provide an updated estimate of potential yield and monetary losses due to weed interference in spring canola grown in Canada and the United States. Quantitative yield data from field experiments were provided by researchers and weed science professionals in the northern Great Plains region; the major canola-producing area of North America. Overall, 89 yield loss estimates were compiled, covering the 18-yr period from 2003 to 2020. Average canola yield losses due to weed interference in Alberta, Saskatchewan, Manitoba, and North Dakota were 35%, 30%, 18%, and 28%, respectively. Potential yield losses weighted by canola harvested area averaged 30%, 28%, and 30% for Canada, the United States, and both countries combined, respectively. Therefore, unfettered weed interference in spring canola represents a potential monetary loss of Can$2.21 billion, $0.16 billion, and $2.37 billion for farmers in Canada, the United States, and both countries combined. The realization of such losses could manifest through continued selection for herbicide-resistant weeds, indicating the critical need for canola farmers to diversify resistance selection pressures by implementing proactive integrated weed management programs.

Hybridization rate and hybrid fitness for Camelina microcarpa Andrz. ex DC (♀) and Camelina sativa (L.) Crantz(Brassicaceae) (♂).

Hybridization between crops and their wild relatives has the potential to introduce novel variation into wild populations. Camelina (Camelina sativa) is a promising oilseed and cultivars with modified seed characteristics and herbicide resistance are in development, prompting a need to evaluate the potential for novel trait introgression into weedy relatives. Little‐podded false flax (littlepod; Camelina microcarpa) is a naturalized weed in Canada and the USA. Here we evaluated the hybridization rate between the three cytotypes of littlepod (♀) and camelina (♂), assessed characteristics of hybrids, and evaluated the fitness of hexaploid littlepod and camelina hybrids in the glasshouse and field. In total we conducted, 1,005 manual crosses with diploid littlepod, 1, 172 crosses with tetraploid littlepod, and 896 crosses with hexaploid littlepod. Hybrids were not produced by the diploids, but were produced by the tetraploids and hexaploids at rates of one hybrid for 2,000 ovules pollinated and 24 hybrids for 25 ovules pollinated, respectively. Hybrids between tetraploid littlepod and camelina showed low pollen fertility and produced a small number of seeds. In the glasshouse, hybrids between hexaploid littlepod and camelina also showed significantly lower pollen fertility and seed production than parental lines, but their seeds showed high viability. A similar pattern was observed in field trials, with hybrids showing earlier flowering, reduced biomass, seed production and seed weight. However, seed produced by the hybrids showed greater viability than that produced by hexaploid littlepod and is potentially the result of a shortened lifecycle. The introgression of lifecycle traits into littlepod populations may facilitate range expansion and contribute to crop gene persistence. Consequently, future work should evaluate the hybridization rate in the field, the fitness of advanced generation backcrosses, and the role of time to maturity in limiting hexaploid littlepod's distribution.

An assemblage of predatory mites (Phytoseiidae) associated with a potential biocontrol agent (Cecidophyes rouhollahi; Eriophyidae) for weed Galium spurium (Rubiaceae)

The weed Galium aparine L., commonly known as cleavers or goose grass, is an exotic plant in New Zealand (Webb et al. 1988; Reid 1998). Craemer et al. (1999) described a new gall mite species, Cecidophyes rouhollahi Craemer (Acari: Eriophyidae), from this weed in France and suggested that this new species might have potential as a biological control agent against G. spurium L. in Canada and western US where the weed was becoming more problematic (Malik & Vanden Born 1988; Whitson 1991). This mite species was later shown to result in severe stunting and complete prevention of seed production in a related weed G. spurium in greenhouse tests and was approved for field release against this weed in Canada (Sobhian et al. 2004). This mite species was recently reported on G. aparine from Auckland, New Zealand (Martin 2017). Mites prefer feeding on the young terminal leaves and can induce the leaf edges to roll inwards and curl (Fig. 1A, B, E). Martin (2017) noted: “No natural enemies of this species of mite have been recorded, but predatory mites may feed on these mites”. We report here on an assemblage of predatory mites (Phytoseiidae) associated with C. rouhollahi on G. aparine in Auckland.

Control of glyphosate-resistant common waterhemp (Amaranthus tuberculatus var rudis) with dicamba and dimethenamid-P in Ontario

Glyphosate-resistant (GR) waterhemp is the fifth confirmed GR weed in Canada and was first confirmed in 2014 in the province of Ontario. In 2015 and 2016, two field experiments were conducted across four site–years each to determine the effectiveness of dicamba at two rates (300 and 600 g a.i. ha−1) in addition to dimethenamid-P (693 g a.i. ha−1) in a single and sequential management program. Due to traited soybean seed availability, all treatments were applied in corn. Waterhemp density, biomass, visual control estimates, and corn yield were collected. Waterhemp density and dryweight were found to be lower in treatments including a sequential application of dicamaba and those that included dimethenamid-P. The sequential management strategy provided >95% control of GR waterhemp at 12 weeks after application. Dicamba applied after emergence at 300 and 600 g a.i. ha−1 controlled GR waterhemp 80% and 91%, respectively; in contrast, dicamba applied before emergence at 300 and 600 g a.i. ha−1 provided only 9% ...

Genomic affinities between Brassica napus and Raphanus raphanistrum as revealed by meiotic GISH

AbstractAn intergeneric hybrid between Brassica napus (AC, n = 19) and Raphanus raphanistrum (Rr, n = 9) was developed using the sequential ovary–ovule culture. Morphological, molecular, cytological studies and fluorescent genomic in situ hybridization were conducted to establish the hybridity and genomic relatedness. F1 hybrid plants were male sterile and morphologically intermediate between the two parents. Cytological analysis of the hybrid plants revealed 10II + 8I as the predominant meiotic configuration with the mean bivalent frequency of 8.83. A maximum of 13II were observed in 8.24% of the pollen mother cells (PMCs). GISH studies facilitated the identification of two allopairs, confirming homoeologous pairing between A/C and Rr genomes. This may have significant implications in terms of transgene flow and possible introgression into R. raphanistrum, which is a common canola weed in Canada and Australia. The intergeneric hybrid B. napus × R. raphanistrum so produced also has the potential to be used as a bridging species for the transfer of desirable genes to amphiploid crop Brassica species carrying A/C genomes.

The Biology of Canadian Weeds. 154. Himalayan blackberry

Gaire, R., Astley, C., Upadhyaya, M. K., Clements, D. R. and Bargen, M. 2015. The Biology of Canadian Weeds. 154. Himalayan blackberry. Can. J. Plant Sci. 95: 557–570. Himalayan blackberry, an invasive weed in Canada, is particularly problematic in southwestern British Columbia. Isolated occurrences have also been recorded in Ontario. The nomenclature of this species has been under review recently; Rubus armeniacus Focke, R. discolor Weihe & Nees, and R. bifrons Vest have been frequently applied, as well as R. procerus auct. Non P. Muell. Ex Genev, Rubus praecox Bertol., R. vestitus Weihe and others. Himalayan blackberry microspecies in North America are within the species aggregate R. fruticosus L. Because the Canadian material has yet to be analysed genetically we refer to the species by the common name Himalayan blackberry in this account. Because of its vigorous growth in temperate, moist environments, it has become a problem weed in the Pacific Northwest, where it outcompetes native plant species by ...

Environmental Impact of Glyphosate-Resistant Weeds in Canada

Glyphosate-resistant (GR) giant ragweed, horseweed, and common ragweed were confirmed in southwestern Ontario, Canada in 2008, 2010, and 2011, respectively. In the western prairie provinces of Alberta and Saskatchewan, GR (plus acetolactate synthase inhibitor-resistant) kochia was discovered in 2011. This symposium paper estimates the environmental impact (EI) of the top herbicide treatments or programs used to manage these GR weed species in the major field crops grown in each region. For each herbicide treatment, EI (per ha basis) was calculated as the environmental impact quotient (EIQ), which quantifies the relative potential risk of pesticide active ingredients on human and ecological health based on risk components to farm workers, consumers, and the environment, multiplied by the application rate (kg ai ha−1). Total EI is defined as EI (per ha basis) multiplied by the application area (i.e., land area affected by a GR weed). It was assumed that all herbicide treatments would supplement the continued usage of glyphosate because of its broad spectrum weed control. For the control of these GR weeds, most treatments contain auxinic or protoporphyrinogen oxidase (PPO)-inhibiting herbicides. The majority of auxinic herbicide treatments result in low (EI ≤ 10) to moderate (11 to 20) EI, whereas all treatments of PPO inhibitors have low EI. Total EI of GR horseweed and kochia will generally be greater than that of giant or common ragweed because of rapid seed dispersal. For recommended herbicide treatments to control GR weeds (and herbicide-resistant weeds in general), EI data should be routinely included with cost and site of action in weed control extension publications and software, so that growers have the information needed to assess the EI of their actions.

Biological and genetic characterisation of Phoma macrostoma isolates with bioherbicidal activity

The fungus Phoma macrostoma Mont. isolate 94-44B was registered as a bioherbicide for control of broadleaved weeds in Canada and the USA in 2011 and 2012, respectively. To obtain the registrations, the fungus had to be characterised both biologically and genetically. The objectives of this study were to demonstrate that bioherbicidal activity was associated with specific genetic markers and to determine whether bioherbicidal activity was a general trait of the species or only selected isolates. A collection of 64 isolates of P. macrostoma was established. A greenhouse bioassay and bioherbicidal-specific primers were used to determine bioherbicidal activity of all isolates. Only isolates originating from Canada thistle demonstrated the ability to reduce dandelion seedlings and display the 853 bp amplicon for the bioherbicidal-specific primer. Bioherbicidal isolates were consistently differentiated from all other isolates with two main genotypic groupings (I and II) arising from internal transcribed spacer (ITS) and amplified fragment length polymorphisms (AFLP) sequence analyses. Using AFLP, two biotypes of bioherbicidal isolates were also differentiated by the presence or absence of an AFLP marker at a single polymorphic locus. The genetic divergence among the bioherbicidal and nonbioherbicidal isolates of P. macrostoma was only 2.21% which was lower than that reported for other related Phoma sp. Other than the bioherbicidal trait, there was no apparent affiliation of the genetics with known varietal types, host or geographic origin. ITS sequence analysis and AFLP fingerprinting may be used as tools to detect bioherbicidal isolates of P. macrostoma.

Occurrence and distribution of glyphosate-resistant giant ragweed (Ambrosia trifida L.) in southwestern Ontario

Vink, J. P., Soltani, N., Robinson, D. E., Tardif, F. J., Lawton, M. B. and Sikkema, P. H. 2012. Occurrence and distribution of glyphosate-resistant giant ragweed ( Ambrosia trifida L.) in southwestern Ontario. Can. J. Plant Sci. 92: 533–539. Giant ragweed is the first confirmed glyphosate-resistant (GR) weed in Canada. A survey was conducted to document the distribution of GR giant ragweed in southwestern Ontario. Giant ragweed seed was collected from 102 sites in Essex (70), Chatham-Kent (21), Lambton (10) and Waterloo counties (1) during the autumn of 2009 and 2010 prior to soybean harvest. Plants were grown in a growth room, sprayed with glyphosate at 1800 g a.e. ha−1 and classified as resistant or susceptible. GR giant ragweed has been confirmed at 47 new locations in three counties (Essex, Chatham-Kent and Lambton). The results from this survey indicate that GR giant ragweed biotypes occur across a greater area in southwestern Ontario than originally thought. This survey provides an important baseline for future surveys in the province.

First Report of a Leaf Spot on Conyza sumatrensis Caused by Phoma macrostoma in China.

Conyza sumatrensis (Asteraceae), an annual or biennial plant, is native to North and South America. It is an invasive, noxious weed that is widespread in southern and southeastern China. It invades farm land and causes great losses to dry land crops, including wheat, corn, and beans. It also reduces biological diversity by crowding out native plants in the infested areas (3,4). During a search for fungal pathogens that could serve as potential biological control agents of C. sumatrensis, a leaf spot disease was observed in 2010 in Chongqing, China. An isolate (SMBC22) of a highly virulent fungus was obtained from diseased leaves. Pathogenicity tests were performed by placing 6-mm-diameter mycelial disks of 7-day-old potato dextrose agar (PDA) cultures of SMBC22 on leaves of 15 healthy greenhouse-grown plants of C. sumatrensis; the same number of control plants was treated with sterile PDA disks. Treated plants were covered with plastic bags for 24 h and maintained in a growth chamber with daily average temperatures of 24 to 26°C, continuous light (3,100 lux), and high relative humidity (>90%). Lesions similar to those observed in the field were first obvious on the SMBC22-inoculated leaves 3 days after inoculation. Symptoms became severe 7 to 9 days after inoculation. Control plants remained healthy. The fungus was reisolated from inoculated and diseased leaves and it was morphologically the same as SMBC22. The pathogenicity test was conducted three times. A survey of 10 southern and southeastern Chinese provinces revealed that the disease was widespread and it attacked leaves and stems of seedlings and mature plants of C. sumatrensis. Lesions on leaves were initially small, circular, and water soaked. The typical lesion was ovoid or fusiform, dark brown, and surrounded by a yellow halo. The spots coalesced to form large lesions and plants were often completely blighted. Fungal colonies of SMBC22 on PDA plates were initially white and turned dark gray. Colonies were circular with smooth edges with obvious rings of pycnidia on the surface. Aerial hyphae were short and dense. Pycnidia, black and immersed or semi-immersed in the medium, were visible after 12 days of incubation. Pycnidia were 72 to 140 μm in diameter. Conidia were produced in the pycnidia and were hyaline, unicellular, ellipsoidal, and 4.4 to 6.1 × 1.6 to 2.2 μm. To confirm identification of the fungus, genomic DNA was extracted from mycelia of a 7-day-old culture on PDA at 25°C (2). The internal transcribed spacer (ITS) gene of rDNA was amplified using primers ITS4/ITS5. The gene sequence was 524 bp long and registered in NCBI GenBank (No. HQ645974). BLAST analysis showed that the current sequence had 99% homology to an isolate of Phoma macrostoma (DQ 404792) from Cirsium arvense (Canada thistle) in Canada and reported to cause chlorotic symptoms on that host plant (1). To our knowledge, this is the first report of P. macrostoma causing disease on C. sumatrensis in China. P. macrostoma, thought of as a biocontrol agent of broadleaf weeds in Canada, has been patented in the United States. The current isolate of P. macrostoma is considered as a potential biocontrol agent of C. sumatrensis.

A review of the recruitment biology of winter annual weeds in Canada

Typically, summer annual weeds have been the primary management target for field crop farmers in Canada, but with changes in cropping systems and with acknowledged global climate change there will be an increasing need to consider the management of species that are present very early in the spring, including winter annual weeds. Knowledge of weed recruitment biology and emergence patterns can be used to guide weed management practices. A review was conducted of the recruitment biology of winter annual weeds in Canada. The key finding was that all of the significant winter annual weeds in Canadian agriculture are facultative, with the majority of species emerging at two peak periods, April-May and September-October. For the weed species included in this review, the information available on recruitment and seed biology was far from comprehensive, and for some species there was very little published information, in particular on specific base temperatures for germination, documented field-based emergence periods and data that could be used to create simple predictive population dynamics models, including fecundity (× environment), seed longevity and overwintering probability. This is particularly true with respect to information in Canadian agricultural contexts. A number of questions result from this review, a key one being whether spring versus fall emergence creates significant differences in fundamental biological characteristics of winter annual weeds including seed dormancy status, microsite requirements, phenology, plant vigour and competitive ability. Given that none of the winter annuals in Canada are constitutive and given ongoing climate change, farmers in Canada should be careful not to encourage summer annuals to become winter annuals. Movement away from fall weed management, including tillage and repeated sowing of winter annual crops will encourage facultative winter annual behaviour. In addition, farmers should be wary of invasion by populations of persistent winter annuals including stinkweed, chickweed and American dragonhead.Key words: Facultative winter annual, dormancy, microsites, emergence pattern, seed rain, seed longevity, weed management

The biology of Canadian weeds. 125. Symphyotrichum ericoides (L.) Nesom (Aster ericoides L.) and S. novae-angliae (L.) Nesom (A. novae-angliae L.)

Symphyotrichum ericoides, the heath aster, is a herbaceous perennial, arising from woody, corm-like rootstocks or herbaceous rhizomes. This facultative upland species occurs in open, dry ground in plains, railway sidings, prairies, ranges, glades, grassy hillsides, dunes, sand bars, river banks, shore salt flats, and thickets. A weed in Canada, and sometimes declared so in the United States and other times not, S. ericoides has an extensive North American distribution, occurring from Nova Scotia and Maine in the east, westward to southern British Columbia, and southward to northern Mexico. No infraspecific taxa are recognized in this treatment of the species. Symphyotrichum novae-angliae, the New England aster, is a herbaceous perennial, arising from thick, short-branched, woody rootstocks. This facultative wetland species occurs in open places such as along roadside ditches and fence rows, on dry ground in plains, prairies and glades, in clearings, meadows and abandoned fields, and along stream banks and swamps, or in moist open or sometimes wooded places. A weed in both Canada and the United States, S. novae-angliae occurs commonly throughout the northern two-thirds of the eastern deciduous forest region of North America and onto the Great Plains. In the east, Nova Scotia populations are likely escaped cultivars, but in the west, isolated natural populations occur from the Black Hills region of South Dakota south to New Mexico. No infraspecific taxa are reco gnized in this treatment of the species. In those habitats where the two species co-occur, the morphologically intermediate F1hybrid S. amethystinum is often found. Key words: Symphyotrichum ericoides, Aster ericoides, Aster multiflorus, heath aster, Symphyotrichum novae-angliae, Aster novae-angliae, New England aster

The Biology of Canadian Weeds. Revised Instructions and Format, Updated Lists of Accounts Published and in Preparation

Three years ago, we published a revised format for the series on the Biology of Canadian Weeds, as well as instructions for the preparation of Updates of Previous Accounts (Cavers and Warwick 2000). This led to renewed interest in the series with 15 new accounts and two updates being published from April 2000 to October 2003. In addition, four new accounts and two updates have been accepted for publication, pending revision. There have also been many changes in the list of manuscripts in preparation. Since there are also a number of changes that need to be made in the Format and the Instructions to Authors, we are reprinting all of this information here. The term “weed”, as used in this series, refers to any vascular plant that perpetuates itself in habitats where it is not wanted. Individual contributions should contain accounts of a single weed species or a group of related species. Prospective authors should note that a new series, the Biology of Invasive Alien Plants in Canada, is being launched in this journal issue (Warwick et al. 2003). Before submitting an offer of contribution, authors should consider carefully the status and history of their species in Canada and then select the appropriate series. The Biology of Canadian Weeds series addresses undeniable problematic weeds in Canada, whereas the new series is designed to cover aliens in Canada that have demonstratable potential risk. In cases where the assignment to a particular series is not clear, the editors of both series may be called upon to make a judgment call. Please consult the new series (Warwick et al. 2003) for further information. If you wish to write an account on a weed species, you must first check to ensure that the species has not been assigned to someone else. This involves sending an “offer of contribution” to Dr P. Cavers. After approval of your offer, you may proceed with writing the paper. The completed paper is to be submitted directly to the Canadian Journal of Plant Science (see below). Offers of contributions by individuals or groups should be made to Dr. Paul B. Cavers, Department of Biology, University of Western Ontario, London, Ontario, N6A 5B7, e-mail pcavers@uwo.ca. These offers of contributions will be subject to approval by a committee of the Canadian Weed Science Society (CWSS). Approved offers will be subject to re-approval after three years if the article has not been completed during that period. Any inquiries about the format to be used or advice on preliminary drafts should be directed to Dr. Cavers. He can also supply blank maps of Canada and will suggest specialists or institutions that will provide various types of services for authors. The Canadian Weed Science Society has a web site [http://cwss-scm.ca/] with on-line references to common English and French names, links to provincial and federal legislation and current lists of species published and in preparation for this series. The finished manuscript should be submitted to: Canadian Journal of Plant Science; Mr. T. Fenton, Head, Journals Section, Agricultural Institute of Canada, 141 Laurier Ave. W., Suite 1112, Ottawa, Ontario, Canada K1P 5J3. All contributions must comply with the format of this journal. Accounts on a single species, including all maps, drawings, diagrams and photographs, should not exceed 50 pages [12 point font, double-spaced with numbered lines]. Of course, accounts on more than one species can be longer. The authors should also provide the names and addresses [including phone numbers and e-mails if available] of three potential referees.

Development of genetic markers for risk assessment of biological control agents

Prior to the release of any organism for use as a biological control agent, it is necessary to assess the possible effects of deploying a single isolate over a wide geographic area. There is a distinct possibility that a rare virulence gene or allele could be introduced into a local population of the pathogen following release. Factors such as the isolation of populations by distance or natural barriers to dispersal can allow evolutionary forces such as selection, mutation, and genetic drift to affect gene frequencies in local populations. Analysis of the genetic structure of a species can reveal information about its population structure and permit risk analysis. We have developed a series of molecular markers to assess the impact of releasing selected isolates of the biological control fungus Chondrostereum purpureum. These markers were used to analyze gene flow between populations and to assess the risk of the deployment of C. purpureum as an agent for the biological control of forest weeds in Canada. Avant d'autoriser un nouvel organisme comme agent de lutte biologique, il est nécessaire d'évaluer les conséquences éventuelles du relâchement d'un seul isolat sur une grande région géographique. La possibilité existe qu'un gène ou un allèle de virulence peu fréquent puisse être introduit dans une population locale de l'agent pathogène à la suite d'un relâchement. Des facteurs tels que l'isolement des populations par la distance ou par des barrières naturelles qui gênent la dispersion peuvent permettre aux forces de l'évolution telles que la sélection, la mutation et la dérive génétique d'affecter la fréquence des gènes dans les populations locales. L'analyse de la structure génétique d'une espèce peut informer sur la structure de sa population et permettre l'analyse du risque. Nous avons développé une série de marqueurs moléculaires afin d'évaluer l'impact de la dissémination d'isolats sélectionnés du champignon de lutte biologique Chondrostereum purpureum. Ces marqueurs furent utilisés pour analyser le flux de gènes entre populations et pour évaluer le risque du relâchement du C. purpureum comme agent de lutte biologique contre les mauvaises herbes forestières au Canada.

Einfluß des Verhaltens adulter Insekten auf Ressourcen-Nutzung und Populationsdynamik: Ein Drei-Komponenten-Modell der Populationsdichte-Steuerung

Bottom-up processes (limitation by resources) and top-down processes (limitation by antagonists) are usually considered to be the driving forces in the population dynamics of insects. This view implies that a permanent underutilization of resources by phytophagous insects or parasitoids is due to population control by natural enemies and/or disease organisms. Our contribution presents examples of phytophagous insects and parasitoids with a level of resource utilization which cannot be satisfactorily explained by the dichotomy of bottom-up and top-down processes. We start by defining the term available resources. We then bring an evaluation of the success rate of biocontrol agents introduced against allochthonous noxious weeds in Canada. A recent status report [Harris 1995] shows that a considerable number of phytophagous insect species established in Canada without their specialized natural enemies exploit only a small proportion of the available biomass of host plants. For one of these species, the tephritid gallformer Urophora cardui, we show that its host selection behaviour leads to a spatial dispersion pattern of galls resulting in a strongly inverse density dependence of resource utilization. Further examples of an underutilization of hosts are given for parasitoids attacking Urophora species in flower heads of Centaurea and for parasitoids of aphids. Neither a bottom-up nor a direct top-down process, but rather specific behaviour patterns are involved in these cases of resource underutilization. These patterns may be adaptive in the sense of spreading the risk, of escaping enemies operating in a density-dependent manner or as a trade-off between host quality and host quantity. They may also be historically constrained. To account for the possible impact of behaviour, we propose to replace the dichotomy of bottom-up and top-down processes by a triangle scheme where changing combinations of resource limitation, limitation by antagonists and specific behaviour patterns control population densities.

Sustainability, conservation tillage and weeds in Canada

The sustainability of conservation tillage is dependent on the extent of changes in weed community composition, the usage of herbicides, and the development of integrated weed management (IWM) strategies, including biological weed control. The objective of this paper is to review research on conservation tillage and weed management in light of these factors. Recent Canadian research has found that changes in weed communities due to the adoption of conservation tillage are not necessarily those expected and were not consistant by species, location, or year. Changes reflected the use of different selection pressures, such as different crop rotations and herbicides, within the studies to a greater extent than weed life cycle groupings. Therefore, research that determines the reasons for change or the lack of change in weed communities is required to provide the scientific basis for the development of IWM strategies. Documented herbicide usage in conservation tillage varies from less than to more than conventional-tillage systems. Potential to reduce herbicide usage in conservation-tillage systems exists. Furthermore, the herbicides used in western Canada are different from those causing ground water contamination in the United States, are less volatile, and are used at lower rates. The presence of surface crop residues in conservation tillage may provide a unique environment for classical and inundative biological control agents. Some insects, fungi, and bacteria have the potential to survive to a greater extent in undisturbed plant residues. Residue management and conservation tillage systems are evolving in Canada. Research must keep pace by providing weed management strategies that enchance the sustainability of these systems. Key words: Biological control, zero tillage, integrated weed management, residues, herbicides, rhizobacteria.

Crop Losses Due to Weeds in Canada

Concerns about the effect of pesticides on our environment has stimulated questions about the direction of future research in weed science. Knowledge of crop losses due to weeds would be helpful in guiding research objectives by identifying areas of greatest need. A survey was conducted in 1991 to estimate losses due to weeds in 58 commodities in Canada. Data generated included: percent loss in production, quantity loss in production, and loss of production value (monetary value) for each commodity in each province. The estimated average annual losses caused by weeds in the 58 commodities were $984 million, with losses of $372 million in Eastern Canada and $612 million in Western Canada. In Eastern Canada, approximately 50% of the total loss occurred in hay crops and 33% in field crops, whereas in Western Canada, 84% of the total loss occurred in field crops. Loss of potential production due to weeds for each commodity is shown by province.

The development and potential role of mycoherbicides for forestry

With increasing intensification of forest management and limited options for control of competing vegetation, there is need for research on alternative vegetation management methods, including biological control. Most forest weeds in Canada are native species with useful as well as detrimental roles, and therefore classical biological control with introduced natural enemies generally cannot be considered. At present, use of native fungal pathogens, or mycoherbicides, is one of the most promising approaches, and recent advancements in agriculture indicate that effective, site-specific controls using mycoherbicides are possible. Mycoherbicide use in forestry appears attractive because of the likelihood of fewer off-target effects than present vegetation management methods and because it could provide either selective controls for specific weeds or broad spectrum controls.Vegetation management in forestry has some unique aspects which will make the development of biological controls different from that in agriculture. There are many indigenous plant pathogens that are potential mycoherbicides, but their efficacy will need to be enhanced by adjuvants, stress treatments, and integration with other vegetation control practices. Currently, at three Canadian institutions and several other locations worldwide, there are research programs on biological control of forest vegetation.