Biological control of weeds by means of plant pathogens: Significance for integrated weed management in modern agro-ecology

Biological control of weeds by using plant pathogens has gainedacceptance as a practical, safe, environmentally beneficial, weedmanagement method applicable to agro-ecosystems. The interest in thisweed control approach from public and private groups, and support forresearch and developmental effort, are on the upswing. This increasinginterest is stimulated largely due to major economic, social, andenvironmental forces that are directing our choices in crop productionpractices. Some of these changes are market-driven while others aresocial and ecological in nature. These changes are in turn influencingthe choices in weed control methods. In this regard, biocontrol withplant pathogens has been proven a feasible, albeit minor, component ofmodern integrated weed-management systems. This environmentallybeneficial method should be promoted and exploited further to meet thecurrent and future challenges in weed management in agro-ecosystems.     

Interactions between density-dependent processes, population dynamics and control of an invasive plant species, Tripleurospermum perforatum (scentless chamomile)

Tripleurospermum perforatum is an invasive weedy species which exhibits strong over-compensating density dependence. Interactions between density-dependent survival, probability of flowering and fecundity were modelled and their impact on the population dynamics were examined. When only fecundity was density-dependent, the dynamics were similar to those observed in the model containing all three density-dependent terms. Density-dependent survival was a stabilizing process when acting in combination with density-dependent fecundity and probability of flowering; removing density-dependent survival from the model produced two-point cycles. The addition of a seed bank was also stabilizing. Simulations of control strategies at different life-history stages indicated that full control would be difficult due to the strong over-compensating density dependence, with severe reductions in fecundity and late season survival necessary in order to reduce equilibrium seed density and biomass.     

Insect herbivory stimulates allelopathic exudation by an invasive plant and the suppression of natives

Exotic invasive plants are often subjected to attack from imported insects as a method of biological control. A fundamental, but rarely explicitly tested, assumption of biological control is that damaged plants are less fit and compete poorly. In contrast, we find that one of the most destructive invasive plants in North America, Centaurea maculosa, exudes far higher amounts of (±)‐catechin, an allelopathic chemical known to have deleterious effects on native plants, when attacked by larvae of two different root boring biocontrol insects and a parasitic fungus. We also demonstrate that C. maculosa plants experimentally attacked by one of these biocontrols exhibit more intense negative effects on natives.     

Comparative population dynamics of Eucalyptus cladocalyx in its native habitat and as an invasive species in an urban bushland in south-western Australia

Abstract.  Eucalyptus cladocalyx F. Muell., is a tree with a restricted distribution in the Southern Flinders Ranges, South Australia. It was originally introduced into the urban bushland of Kings Park, Perth, Western Australia in 1932 as an ornamental. Since its planting, E. cladocalyx has become invasive, spreading into the bushland up to 70 m away from planting sites. Anecdotal evidence suggests that the E. cladocalyx population is increasing at a greater rate than the two principal native tree species, E. gomphocephala DC. and E. marginata Donn ex Smith, but little is known about the factors influencing its invasion, or its biology. This study describes the population structure of E. cladocalyx , E. gomphocephala and E. marginata in Kings Park and the role of fire in the recruitment process. The study indicated that the three species have characteristics common to temperate Eucalyptus species that mass recruit seedlings following fire, with high numbers of seedlings found in recently burnt areas and low numbers in unburnt areas. Seedling survival in E. cladocalyx was higher than either of the native species. Furthermore, E. cladocalyx adults showed higher rates of canopy recovery following fires. It is argued that although fire in Kings Park is providing opportunities for E. cladocalyx , E. gomphocephala and E. marginata recruitment, the E. cladocalyx population is more resilient in an environment frequently disturbed by fire compared with the native populations.     

Integrating species distribution models and interacting particle systems to predict the spread of an invasive alien plant

Aim We demonstrate how to integrate two widely used tools for modelling the spread of invasive plants, and compare the performance of the combined model with that of its individual components using the recent range dynamics of the invasive annual weed Ambrosia artemisiifolia L. Location Austria. Methods Species distribution models, which deliver habitat‐based information on potential distributions, and interacting particle systems, which simulate spatio‐temporal range dynamics as dependent on neighbourhood configurations, were combined into a common framework. We then used the combined model to simulate the invasion of A. artemisiifolia in Austria between 1990 and 2005. For comparison, simulations were also performed with models that accounted only for habitat suitability or neighbourhood configurations. The fit of the three models to the data was assessed by likelihood ratio tests, and simulated invasion patterns were evaluated against observed ones in terms of predictive discrimination ability (area under the receiver operating characteristic curve, AUC) and spatial autocorrelation (Moran’s I). Results The combined model fitted the data significantly better than the single‐component alternatives. Simulations relying solely on parameterized spread kernels performed worst in terms of both AUC and spatial pattern formation. Simulations based only on habitat information correctly predicted infestation of susceptible areas but reproduced the autocorrelated patterns of A. artemisiifolia expansion less adequately than did the integrated model. Main conclusions Our integrated modelling approach offers a flexible tool for forecasts of spatio‐temporal invasion patterns from landscape to regional scales. As a further advantage, scenarios of environmental change can be incorporated consistently by appropriately updating habitat suitability layers. Given the susceptibility of many alien plants, including A. artemisiifolia, to both land use and climate changes, taking such scenarios into account will increasingly become relevant for the design of proactive management strategies.     

The role of long‐distance seed dispersal in the local population dynamics of an invasive plant species

Aim Long‐distance dispersal is important for plant population dynamics at larger spatial scales, but our understanding of this phenomenon is mostly based on computer modelling rather than field data. This paper, by combining field data and a simulation model, quantifies the fraction of the seed of the alien species Heracleum mantegazzianum that needs to disperse over a long distance for successful invasion. Location Central Europe, Czech Republic. Methods To assess the role of random dispersal in long‐term population dynamics of the studied species, we combined longitudinal data covering 50 years of the invasion of this plant from its very start, inferred from a series of aerial photographs of 60‐ha plots, with data on population dynamics at a fine scale of 10‐m² plots. Results A simulation model based on field data indicates that the fraction of seed that is dispersed from source plants not described by the short‐distance dispersal kernel ranges from 0.1 to 7.5% of the total seed set. The fraction of long‐distance dispersed seed that provides the best prediction of the observed spread was significantly negatively correlated with the percentage of habitats suitable for invasion. Main conclusions Our results indicate that the fraction of seeds that needed to be dispersed over long distances to account for the observed invasion dynamics decreased with increasing proportion of invasible habitats, indicating that the spatial pattern of propagule pressure differs in landscapes prone to invasion. Long‐distance dispersal is an important component of the population dynamics of an invasive species even at relatively small scales.     

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     

A Simulation model for the arrowhead scale (Hemiptera: Diaspididae) population dynamics on citrus trees in relation to pest-management programs

1. A mathematical model was constructed describing population dynamics of the arrowhead scale, Unaspis yanonensis Kuwana. The population occurrence patterns simulated by the model from 1971 to 1976 were relatively consistent with the actual findings in the field.
2. The effects of control tactics for this pest were examined on the basis of the model.
3. Effective timing of insecticide (petroleum oil and organophosphorous insecticide) application was indicated to be 40 days after the initial appearance of 1st-instar nymphs in both the 1st and 2nd generations of the host.
4. The number of overwintering adult females in May of the next year (i.e., the starting point of the next year's population) decreased linearly with the increment in the degree of insecticide coverage. The pest populations were expected to be kept to low densities by petroleum oil sprays alone for the 1st generation when overall, complete coverage was accomplished.
5. Examination of the effects of two introduced parasitoids, Aphytis yanonensis and Coccobius fulvus, showed that they were able to control the scale population by themselves when more than 70% parasitism was achieved. When petroleum oil was applied to overwintering adults (termed winter petroleum oil) at the degree of coverage of 0.8, host populations were reduced by ca. 60% parasitism. When the winter petroleum oil was combined either with additional petroleum oil 40 days after the initial appearance of 1st-instar nymphs of the 1st generation or with an organophosphorous insecticide 40 days after the initial appearance of 1st-instar nymphs of the 2nd generation (degree of coverage=0.8 in each case), 40% parasitism led the reduction of the pest population.
6. The most desirable control program was considered to be one in which winter petroleum oil was used every year and additional petroleum oil or an organophosphorous insecticide was also used in those years when host density was high.

     

Use of an individual-based model to forecast the effect of climate change on the dynamics, abundance and geographical range of the pest slug Deroceras reticulatum in the UK

Slugs are serious agricultural pests and their activity is strongly driven by ambient temperature and soil moisture. The strength of this relationship has been shown through the development of a deterministic model, based upon temperature and soil moisture conditions alone, which accurately describes the population dynamics and abundance of Deroceras reticulatum . Because of this strong climatic dependence, slug abundance and dynamics are likely to be affected by climate change. We used a validated individual-based model (IbM) of D. reticulatum , to assess the effects of climate change on the abundance of this species in the UK. Climatic scenarios were based on the UKCIP02 predictions and constructed using the LARS-WG stochastic weather generator. The IbM of slugs predicted population dynamics at three time slices (2020s, 2050s and 2080s), and two scenarios of greenhouse gas emissions. The maximum generation number, the number of population peaks, the number of slug-days in each season, the percentage of years when the population passes over a threshold for damage and the percentage of years in which populations go extinct were investigated. Currently, the south-west of the UK has the best conditions for D. reticulatum to thrive, with the north-east of Scotland having the most adverse. By 2080 under both low- and high-emissions scenarios, the north and west of Scotland will have the most favourable conditions for the survival of this species and the east of the UK and Scotland will have the harshest. By 2080 the climate in the north-west of Scotland will become more like the current climate in south-east England, which explains the shift in the pattern of abundance. The north-west of Scotland will have increased slug damage and south-west England and west-Wales will have decreased slug damage with some changes becoming evident by 2020.     

Using delimiting surveys to characterize the spatiotemporal dynamics facilitates the management of an invasive non-native insect

The ability to ascertain abundance and spatial extent of a nascent population of a non-native species can inform management decisions. Following initial detection, delimiting surveys, which involve the use of a finer network of samples around the focal point of a newly detected colony, are often used to quantify colony size, spatial extent, and the location of the population core. Despite the widespread use of pheromone-baited traps in delimitation surveys to manage invading populations of Lymantria dispar (L.) in North America, there has been no prior comprehensive attempt to analytically determine the adequacy of these surveys. We used data from 2,190 delimiting grids collected from 2000 to 2010 in the United States to quantify the information gained from delimiting surveys. The use of delimiting surveys revealed that ≈53 % of populations of low initial abundance persisted as detectable populations in the following year; however, when trap data from delimiting surveys were excluded, only ≈16 % of these low density populations were detected in the following year. Measurements of abundance and spatial extent of a detected population were affected by the increased use of delimiting traps after accounting for initial abundance, the distance from an infested area, and colony area. The use of delimiting traps had a lesser effect on the estimation of the spatial location of the population core, indicating that initial detection of a population often reflects the population core. The need to prioritize resources in efforts to manage non-native species is paramount, and early detection is a key in invasive species management.     

Combined effects of drought stress and psyllid herbivory on the invasive weed Scotch broom,Cytisus scoparius

Increases in the frequency and duration of droughts under global climate change could have implications for plant–insect interactions, and could either increase or decrease the effects of weed biological control agents. In this study, we used greenhouse and field experiments to examine the impacts of drought stress on the abundance and impacts of the adventive psyllid Arytainilla spartiophila Forster (Hemiptera: Psyllidae, Arytaininae) on its target weed Scotch broom, Cytisus scoparius (L.) Link (Fabaceae), in California, USA. The psyllid impacted plant growth in both field and greenhouse experiments, whereas drought stress affected plant growth in the greenhouse only, suggesting that other factors besides water availability may be more limiting for plants in the field. Effects on psyllid survival were consistent with the plant vigor hypothesis, which predicts that herbivores will perform better on vigorously growing plants; psyllid numbers were lower on drought-stressed plants in the greenhouse and were correlated with plant growth in both the greenhouse and the field. In the greenhouse, the combined effects of the psyllid and drought stress were additive, indicating that the effects of the psyllid were consistent across unstressed and drought-stressed plants. Although the psyllid is unlikely to control Scotch broom on its own, results suggest that it may work in conjunction with drought stress to suppress Scotch broom.     

Quantifying patch‐specific seed dispersal and local population dynamics to estimate population spread of an endangered plant species

AimHabitat loss and fragmentation impose high extinction risk upon endangered plant species globally. For many endangered plant species, as the remnant habitats become smaller and more fragmented, it is vital to estimate the population spread rate of small patches in order to effectively manage and preserve them for potential future range expansion. However, population spread rate has rarely been quantified at the patch level to inform conservation strategies and management decisions. To close this gap, we quantify the patch‐specific seed dispersal and local population dynamics of Minuartia smejkalii, which is a critically endangered plant species endemic in the Czech Republic and is of urgent conservation concern.LocationŽelivka and Hrnčíře, Czechia.MethodsWe conducted demographic analyses using population projection matrices with long‐term demographic data and used an analytic mechanistic dispersal model to simulate seed dispersal. We then used information on local population dynamics and seed dispersal to estimate the population spread rate and compared the relative contributions of seed dispersal and population growth rate to the population spread rate.ResultsWe found that although both seed dispersal and population growth rate in M. smejkalii were critically limited, the population spread rate depended more strongly on the maximal dispersal distance than on the population growth rate.Main conclusionsWe recommend conservationists to largely increase the dispersal distance of M. smejkalii. Generally, efforts made to increase seed dispersal ability could largely raise efficiency and effectiveness of conservation actions for critically endangered plant species.     

Wildfire effects and post-fire responses of an invasive mesquite population: the interactive importance of grazing and non-native herbaceous species invasion

Aim To determine how responses of an established velvet mesquite (Prosopis velutina Woot.) population to a 2002 wildfire were shaped by grazing and non‐native herbaceous species invasions, both of which influenced fire behaviour. Location The study was conducted on contiguous ranches (one actively grazed by cattle, one that had not been grazed since 1968) in the Sonoita Valley of southern Arizona. Plant communities on both ranches were comprised of Chihuahuan semi‐desert grassland, savanna, and Madrean evergreen woodland ecosystems, but large areas were dominated by Lehmann and Boer lovegrass, African grass species that were introduced more than 50 years ago. Methods We selected 243 individuals that had been defoliated and bark scorched during the fire using a stratified random design based on pre‐fire grazing status and dominant grass cover. After the start of the 2003 growing season, we recorded individual tree characteristics, fire damage, and measures of post‐fire response, and tested for relationships among classes of: grazing status, bark damage, dominant grass cover type, abundance of live and dead aboveground branches, flowering status, and sprout number and size. Analyses of fire damage and post‐fire response were interpreted with respect to values of fireline intensity, scorch height and energy release that were projected by a fire behaviour model, nexus . Results Nearly all of the trees on grazed areas suffered low levels of fire damage, while a majority on ungrazed areas suffered moderate to severe damage. Trees on grazed areas consequently had significantly more leaf‐bearing twigs and branches in 2003 but a very low number of root sprouts, while individuals on ungrazed areas had a greater density of root sprouts but little post‐fire dead branching and almost no living branches. Among the ungrazed grassland types, more than 75% of the trees on Boer lovegrass plots suffered moderate to severe damage, while a similar percentage of trees in native grass areas suffered low damage. These differences were: (1) attributed to variations in fire characteristics that were caused by differences in litter production and removal, and (2) ecologically significant because trees in the severe damage class showed almost no aboveground post‐fire branching, either live or dead in 2003, while trees in the low damage class exhibited a greater amount of both. Main conclusions Our results affirm the notion that effective management of western grasslands where mesquite encroachment has or will become a problem requires a better understanding of how interactions among key ecosystem influences (e.g. fire, grazing, non‐native species) affect not only mesquite seedlings and saplings but also larger, established individuals and thereby the long‐term structure and functioning of semi‐desert grassland ecosystems. As managers shift their focus from eradication to management of mesquite in western grasslands and savannas, our results provide insights into how prescribed fires (and their effects on mesquite populations) differ from wildfires and how such effects may be mediated by the altered land uses and ecosystem characteristics that now exist in many western ecosystems.