Reproductive variation in naturally occurring populations of the weed Parthenium hysterophorus (Asteraceae) in Australia

Parthenium weed, an annual herb native to tropical America, causes severe economic, human, and animal health and environmental impacts in Australia and in many countries in Asia, Africa, and the Pacific. There is little known about variation in reproductive output in naturally occurring populations of this weed. This information is vital to develop plant population models, devise management strategies to reduce seed output, and formulate parthenium weed pollen-induced human health (e.g., dermatitis and hay fever) risk assessment. Here, the variations in the number of capitula produced by the parthenium weed at two sites in Queensland, Australia, over a 4-yr period are reported. Under field conditions, parthenium weed produced up to 39,192 capitula per plant (> 156,768 seeds per plant), with majority of the plants (approximate to 75%) producing between 11 and 1,000 capitula, and less than 0.3% of the plants producing more than 10,000 capitula (> 40,000 seeds per plant). The number of capitula per plant in the field (297 +/- 22) was much lower than those reported from glasshouse and laboratory studies. Plant biomass contributed to 50 to 80% of the variation in capitulum production between plants within plots at each site, and weed density accounted for 62 to 73% of the variation in capitulum production between plots within each site. As plant size is directly correlated with reproductive output, plant size distributions in parthenium weed can be used to estimate effective population size. Information on variation in reproductive output will be used to implement management strategies to reduce parthenium weed seed output, resulting in reduced soil seed bank and weed seed spread.

Soil seed bank longevity information for weed eradication target species

The longevity of seed in the soil is a key determinant of the cost and length of weed eradication programs. Soil seed bank information and ongoing research have input into the planning and reporting of two nationally cost shared weed eradication programs based in tropical north Queensland. These eradication programs are targeting serious weeds such as Chromoleana odorata, Mikania micrantha, Miconia calvescens, Clidemia hirta and Limnocharis flava. Various methods are available for estimating soil seed persistence. Field methods to estimate the total and germinable soil seed densities include seed packet burial trials, extracting seed from field soil samples, germinating seed in field soil samples and observations from native range seed bank studies. Interrogating field control records can also indicate the length of the control and monitoring periods needed to exhaust the seed bank. Recently, laboratory tests which rapidly age seed have provided an additional indicator of relative seed persistence. Each method has its advantages, drawbacks and logistical constraints.

Evaluating the performance of weed containment programmes

Aim: To develop approaches to the evaluation of programmes whose strategic objectives are to halt or slow weed spread. Location: Australia. Methods: Key aspects in the evaluation of weed containment programmes are considered. These include the relevance of models that predict the effects of management intervention on spread, the detection of spread, evidence for containment failure and metrics for absolute or partial containment. Case studies documenting either near-absolute (Orobanche ramosa L., branched broomrape) or partial (Parthenium hysterophorus (L.) King and Robinson, parthenium) containment are presented. Results: While useful for informing containment strategies, predictive models cannot be employed in containment programme evaluation owing to the highly stochastic nature of realized weed spread. The quality of observations is critical to the timely detection of weed spread. Effectiveness of surveillance and monitoring activities will be improved by utilizing information on habitat suitability and identification of sites from which spread could most compromise containment. Proof of containment failure may be difficult to obtain. The default option of assuming that a new detection represents containment failure could lead to an underestimate of containment success, the magnitude of which will depend on how often this assumption is made. Main conclusions: Evaluation of weed containment programmes will be relatively straightforward if containment is either absolute or near-absolute and may be based on total containment area and direct measures of containment failure, for example, levels of dispersal, establishment and reproduction beyond (but proximal to) the containment line. Where containment is only partial, other measures of containment effectiveness will be required. These may include changes in the rates of detection of new infestations following the institution of interventions designed to reduce dispersal, the degree of compliance with such interventions, and the effectiveness of tactics intended to reduce fecundity or other demographic drivers of spread. © 2012 Blackwell Publishing Ltd.

Economic aspects of post-border weed risk management

Aim: Decision-making in weed management involves consideration of limited budgets, long time horizons, conflicting priorities, and as a result, trade-offs. Economics provides tools that allow these issues to be addressed and is thus integral to management of the risks posed by weeds. One of the critical issues in weed risk management during the early stages of an invasion concerns feasibility of eradication. We briefly review how economics may be used in weed risk management, concentrating on this management strategy. Location: Australia. Methods: A range of innovative studies that investigate aspects of weed risk management are reviewed. We show how these could be applied to newly invading weeds, focussing on methods for investigating eradication feasibility. In particular, eradication feasibility is analysed in terms of cost and duration of an eradication programme, using a simulation model based on field-derived parameter values for chromolaena, Chromolaena odorata. Results: The duration of an eradication programme can be reduced by investing in progressively higher amounts of search effort per hectare, but increasing search area will become relatively more expensive as search effort increases. When variation in survey and control success is taken into account, increasing search effort also reduces uncertainty around the required duration of the eradication programme. Main conclusions: Economics is integral to the management of the risks posed by weeds. Decision analysis, based on economic principles, is now commonly used to tackle key issues that confront weed managers. For eradication feasibility, duration and cost of a weed eradication programme are critical components; the dimensions of both factors can usefully be estimated through simulation. © 2013 John Wiley & Sons Ltd.

Weed age affects chemical control of Conyza bonariensis in fallows

In the last decade, Conyza bonariensis has become a widespread and difficult-to-control weed in Australian broad-acre cropping, particularly in glyphosate-based zero-tilled fallows of the subtropical grain region. The first Australian populations of C. bonariensis, where it is known as flaxleaf fleabane, were confirmed resistant to glyphosate in 2010. Control with alternative herbicides in fallows has been inconsistent, with earlier research indicating that weed age could be a potential contributing factor. In two field experiments, the impact of weed age (one, two and three months) was measured on the efficacy of six non-selective herbicide mixtures and sequential applications for control in fallows. In another two experiments we evaluated 11 non-selective herbicides, mixtures and sequential applications applied to one and three month old weeds using higher rates on older weeds. When herbicide rates were consistent for different weed ages, efficacy was reduced only by an average of 1% when two month old weeds were treated compared to one month old weeds. However when applied to three month old weeds, efficacy of treatments was significantly (P < 0.001) reduced by 3-30%. When herbicide rates were increased, weed age had no adverse effect on efficacy, which ranged from 90 to 100%, for amitrole, glyphosate mixed with 2,4-D amine plus picloram, and three sequential application treatments of glyphosate mixtures followed with bipyridyl products. Thus, this problem weed can be controlled effectively and consistently at the rosette stage of one to two months old, or three month old weeds with several different treatments at robust rates. These effective glyphosate alternatives and sequential-application tactics will minimise replenishment of the soil seed-bank and further reduce the risk for further evolution of glyphosate resistance. (C) 2012 Elsevier Ltd. All rights reserved.

Effect of temperature on the survival of Aconophora compressa Walker (Hemiptera: Membracidae): implications for weed biocontrol

Abstract In weed biocontrol, similarity of abiotic factors between the native and introduced range of a biocontrol agent is critical to its establishment and effectiveness. This is particularly the case for weeds that have a wide geographical distribution in the native range. For such weeds, the choice of a specialist insect that has narrow tolerance limits to important abiotic factors can diminish its ability to be an effective biocontrol agent. The membracid Aconophora compressa was introduced in Australia from Mexico for biocontrol of Lantana camara, a plant with a wide climatic tolerance. In this study we investigated the effect of constant and alternating temperatures on A. compressa survival. Longevity of adults and nymphs declined with increasing temperatures, and at 39°C individuals survived for less than a day. At lower temperatures, nymphs survived longer than adults. Survival at alternating temperatures was longer than at constant temperatures, but the general trend of lower survival at higher temperatures remained. Spatially and temporally, the climatic tolerance of A. compressa appears to be a subset of that of lantana, thereby limiting its potential impact.

Assessing the potential of the rust fungus Puccinia spegazzinii as a classical biological control agent for the invasive weed Mikania micrantha in Papua New Guinea

The rust fungus Puccinia spegazzinii was introduced into Papua New Guinea (PNG) in 2008 as a classical biological control agent of the invasive weed Mikania micrantha (Asteraceae), following its earlier release in India, mainland China and Taiwan. Prior to implementing field releases in PNG, assessments were conducted to determine the most suitable rust pathotype for the country, potential for damage to non-target species, most efficient culturing method and potential impact to M. micrantha. The pathotype from eastern Ecuador was selected from the seven pathotypes tested, since all the plant populations evaluated from PNG were highly susceptible to it. None of the 11 plant species (representing eight families) tested to confirm host specificity showed symptoms of infection, supporting previous host range determination. A method of mass-producing inoculum of the rust fungus, using a simple technology which can be readily replicated in other countries, was developed. Comparative growth trials over one rust generation showed that M. micrantha plants infected with the rust generally had both lower growth rates and lower final dry weights, and produced fewer nodes than uninfected plants. There were significant correlations between the number of pustules and (a) the growth rate, (b) number of new nodes and (c) final total dry weight of single-stemmed plants placed in open sunlight and between the number of pustules and number of new nodes of multi-stemmed plants placed under cocoa trees. The trials suggest that field densities of M. micrantha could be reduced if the rust populations are sufficiently high. Crown Copyright (C) 2013 Published by Elsevier Inc. All rights reserved.

How weed control and fertilisation influence tree physiological processes and growth at early establishment in an exotic F1 hybrid pine plantation of subtropical Australia

Purpose This study investigated how nitrogen (N) nutrition and key physiological processes varied under changed water and nitrogen competition resulting from different weed control and fertilisation treatments in a 2-year-old F1 hybrid (Pinus elliottii Engelm var. elliottii × P. caribaea var. hondurensis Barr. ex Golf.) plantation on a grey podzolic soil type, in Southeast Queensland. Materials and methods The study integrated a range of measures including growth variables (diameter at ground level (DGL), diameter at breast height (DBH) and height (H)), foliar variables (including foliar N concentration, foliar δ13C and δ15N) and physiological variables (including photosynthesis (An), stomatal conductance (gs), transpiration (E), intrinsic water use efficiency (WUEi) (A/gs) and xylem pressure potential (ΨXPP)) to better understand the mechanisms influencing growth under different weed control and fertilisation treatments. Five levels of weed control were applied: standard (routine), luxury, intermediate, mechanical and nil weed control, all with routine fertilisation plus an additional treatment, routine weed control and luxury fertilisation. Relative weed cover was assessed at 0.8, 1.1 and 1.6 years after plantation establishment to monitor the effectiveness of weed control treatments. Soil investigation included soil ammonium (NH4 +-N), nitrate (NO3 −-N), potentially mineralizable N (PMN), gravimetric soil moisture content (MC), hot water extractable organic carbon (HWETC), hot water extractable total N (HWETN), total C, total N, stable C isotope composition (δ13C), stable N isotope composition (δ15N), total P and extractable K. Results and discussion There were significant relationships between foliar N concentrations and relative weed cover and between tree growth and foliar N concentration or foliar δ15N, but initial site preparation practices also increased soil N transformations in the planting rows reducing the observable effects of weed control on foliar δ15N. A positive relationship between foliar N concentration and foliar δ13C or photosynthesis indicated that increased N availability to trees positively influenced non-stomatal limitations to photosynthesis. However, trees with increased foliar N concentrations and photosynthesis were negatively related to xylem pressure potential in the afternoons which enhanced stomatal limitations to photosynthesis and WUEi. Conclusions Luxury and intermediate weed control and luxury fertilisation positively influenced growth at early establishment by reducing the competition for water and N resources. This influenced fundamental key physiological processes such as the relationships between foliar N concentration, A n, E, gs and ΨXPP. Results also confirmed that time from cultivation is an important factor influencing the effectiveness of using foliar δ15N as an indicator of soil N transformations.

Effects of weed control and fertilization at early establishment on tree nitrogen and water use in an exotic F1 hybrid pine of subtropical Australia

Purpose We investigated the effects of weed control and fertilization at early establishment on foliar stable carbon (δ13C) and nitrogen (N) isotope (δ15N) compositions, foliar N concentration, tree growth and biomass, relative weed cover and other physiological traits in a 2-year old F1 hybrid (Pinus elliottii var. elliottii (Engelm) × Pinus caribaea var. hondurensis (Barr. ex Golf.)) plantation grown on a yellow earth in southeast Queensland of subtropical Australia. Materials and methods Treatments included routine weed control, luxury weed control, intermediate weed control, mechanical weed control, nil weed control, and routine and luxury fertilization in a randomised complete block design. Initial soil nutrition and soil fertility parameters included (hot water extractable organic carbon (C) and total nitrogen (N), total C and N, C/N ratio, labile N pools (nitrate (NO3 −) and ammonium (NH4 +)), extractable potassium (K+)), soil δ15N and δ13C. Relative weed cover, foliar N concentrations, tree growth rate and physiological parameters including photosynthesis, stomatal conductance, photosynthetic nitrogen use efficiency, foliar δ15N and foliar δ13C were also measured at early establishment. Results and discussion Foliar N concentration at 1.25 years was significantly different amongst the weed control treatments and was negatively correlated to the relative weed cover at 1.1 years. Foliar N concentration was also positively correlated to foliar δ15N and foliar δ13C, tree height, height growth rates and tree biomass. Foliar δ15N was negatively correlated to the relative weed cover at 0.8 and 1.1 years. The physiological measurements indicated that luxury fertilization and increasing weed competition on these soils decreased leaf xylem pressure potential (Ψxpp) when compared to the other treatments. Conclusions These results indicate how increasing N resources and weed competition have implications for tree N and water use at establishment in F1 hybrid plantations of southeast Queensland, Australia. These results suggest the desirability of weed control, in the inter-planting row, in the first year to maximise site N and water resources available for seedling growth. It also showed the need to avoid over-fertilisation, which interfered with the balance between available N and water on these soils.

Changes in weed species since the introduction of glyphosate-resistant cotton

Weed management practices in cotton systems that were based on frequent cultivation, residual herbicides, and some post-emergent herbicides have changed. The ability to use glyphosate as a knockdown before planting, in shielded sprayers, and now over-the-top in glyphosate-tolerant cotton has seen a significant reduction in the use of residual herbicides and cultivation. Glyphosate is now the dominant herbicide in both crop and fallow. This reliance increases the risk of shifts to glyphosate-tolerant species and the evolution of glyphosate-resistant weeds. Four surveys were undertaken in the 2008-09 and 2010-11 seasons. Surveys were conducted at the start of the summer cropping season (November-December) and at the end of the same season (March-April). Fifty fields previously surveyed in irrigated and non-irrigated cotton systems were re-surveyed. A major species shift towards Conyza bonariensis was observed. There was also a minor increase in the prevalence of Sonchus oleraceus. Several species were still present at the end of the season, indicating either poor control and/or late-season germinations. These included C. bonariensis, S. oleraceus, Hibiscus verdcourtii and Hibiscus tridactylites, Echinochloa colona, Convolvulus sp., Ipomea lonchophylla, Chamaesyce drummondii, Cullen sp., Amaranthus macrocarpus, and Chloris virgata. These species, with the exception of E. colona, H. verdcourtii, and H. tridactylites, have tolerance to glyphosate and therefore are likely candidates to either remain or increase in dominance in a glyphosate-based system.