Efficacy Of Biological Control Agents Research Articles

Increasing survival and efficacy of a bacterial biocontrol agent of fire blight of rosaceous plants by means of osmoadaptation

The efficacy of Pseudomonas fluorescens EPS62e in the biocontrol of Erwinia amylovora, the causal agent of fire blight of apple and pear, depends on the colonization of plant surfaces after spray application. A procedure to increase cell survival in the phyllosphere was developed consisting of saline stress and osmolyte amendment to the growth medium during inoculum preparation. Hyperosmotic stress induced the synthesis of the osmolytes trehalose, N-acetylglutaminylglutamine amide and glucosyl-glycerol, but decreasing growth rate. Amendment of the growth medium with glycine betaine increased growth rate and cell yield and promoted its intracellular accumulation. Under controlled environment conditions, osmoadaptation increased by 10- to 100-fold cell survival to desiccation and to low relative humidity conditions on plant surfaces, in comparison with the nonosmoadapted controls. In the field, cell survival increased 100-1000 times in immature fruit upon osmoadaptation but was not significantly affected in flowers where active colonization occurs. The efficacy in the control of fire blight infections was increased 30-50% upon osmoadaptation on immature fruits but was not affected in blossoms. The method of osmoadaptation may be useful for improving the fitness and efficacy of biological control agents of phyllosphere pathogens under limiting humidity conditions.

Response of an invasive liana to simulated herbivory: implications for its biological control

Pre-release evaluation of the efficacy of biological control agents is often not possible in the case of many invasive species targeted for biocontrol. In such circumstances simulating herbivory could yield significant insights into plant response to damage, thereby improving the efficiency of agent prioritisation, increasing the chances of regulating the performance of invasive plants through herbivory and minimising potential risks posed by release of multiple herbivores. We adopted this approach to understand the weaknesses herbivores could exploit, to manage the invasive liana, Macfadyena unguis-cati. We simulated herbivory by damaging the leaves, stem, root and tuber of the plant, in isolation and in combination. We also applied these treatments at multiple frequencies. Plant response in terms of biomass allocation showed that at least two severe defoliation treatments were required to diminish this liana's climbing habit and reduce its allocation to belowground tuber reserves. Belowground damage appears to have negligible effect on the plant's biomass production and tuber damage appears to trigger a compensatory response. Plant response to combinations of different types of damage did not differ significantly to that from leaf damage. This suggests that specialist herbivores in the leaf-feeding guild capable of removing over 50% of the leaf tissue may be desirable in the biological control of this invasive species.

Herbivory alters competitive interactions between two invasive aquatic plants

We compared the effect of herbivory by two congeneric specialist weevils (Neochetina eichhorniae and Neochetina bruchi) on competition between their host Eichhornia crassipes and another floating aquatic plant (Pistia stratiotes) by experimentally manipulating plant densities, presence of either or both herbivore species, and nutrient levels. Measurements of E. crassipes yield per original plant included biomass, clonal expansion, and flower production. Without herbivory, intraspecific competition among E. crassipes individuals was 41 times more intense than interspecific competition in terms of biomass yield. Herbivory shifted competitive outcomes, bringing intraspecific competition in line with interspecific competition. Both weevil species reduced E. crassipes biomass and flowering, but N. bruchi also lessened clonal expansion (rosette density) whereas N. eichhorniae did not. Nutrient limitation reduced plant performance but did not alter the pattern of competitive interactions. However, low nutrients moderated differences between weevil species such that both seemed equally damaging. We conclude that while herbivory directly and indirectly affected plant performance by altering competition between these two invasive plant species, the competitive response varied depending upon the herbivore species and availability of nutrients. The influence of herbivores on competitive interactions may thus be an important consideration for evaluating the invasive potential of exotic plant species as well as for predicting the potential efficacy of biological control agents, but interactions between herbivory, competitors, and nutrient availability create complexities that must be considered.

Fermentation period, moisture content, and substrate effects on inoculum effectiveness of Dilophospora alopecuri: a biocontrol agent of annual ryegrass toxicity

Abstract The efficacy of biological control agents is affected by numerous cultural conditions. This study was undertaken to determine how the efficacy of Dilophospora alopecuri inoculum was affected by various conditions, including length of fermentation, moisture content, and growth substrate. Dilophospora alopecuri (Fr.) Fr. provides biological control of the organisms ( Rathayibacter toxicus and Anguina funesta ) responsible for annual ryegrass toxicity. The effectiveness of D. alopecuri inoculum was evaluated under different conditions for establishment in ryegrass. Prolongation of incubation of the inoculum for 3–10 weeks had no significant effect on the inoculum as a biological control agent. The effect of inoculum moisture content was mixed. With wheat as the substrate, air-drying the inoculum had no apparent effect. However, with ryegrass as the substrate, air-drying the inoculum decreased the effectiveness of the inoculum in the field experiment but not in the pot experiment. The substrate itself also had a significant effect. In the pots, application of colonized ryegrass gave a mean of 56.9 infected inflorescences per pot compared to 30.4 for colonized wheat. In the field experiment, 87.5% of plots inoculated with colonized ryegrass had 25 or more infected inflorescences, twice that of plots treated with colonized wheat.

Invasion Dynamics of Cytisus scoparius: A Matrix Model Approach

It is at the level of population dynamics that an invasion either fails or succeeds. By elucidating patterns of variation in population growth rates or demographic rates, it is possible to forge a connection between quantitative field data and theoretical ideas about invasiveness, invasibility, and rates of spread. Demographic models also provide a tool to guide control strategies for invasive pests. Here I report the results of a demographic study of Cytisus scoparius, an exotic shrub on the west coast of North America. I used matrix population models to describe demographic patterns in six populations (three in prairies and three in urban fields) and across advancing stages of invasion. At the edge of the invading front, all populations showed finite rates of increase (λ) >1; however, prairie populations were increasing much more rapidly than urban ones. While many individual vital rates differed between prairie and urban populations, Life Table Response Analysis revealed that seedling establishment made by far the largest contribution to the difference in growth rate between the habitats. Establishment is much higher in the prairies, which are also less anthropogenically disturbed and show higher plant species diversity. From the edge of the invading population to the center, λ generally decreased, and the elasticity pattern changed from one evenly distributed across life history stages to one dominated by the survivorship of large adults. Comparing the matrix model predictions to direct estimates of invasion (change over time of various measures of density and biomass), λ was most closely correlated with the increase of total biomass. From a control perspective, elasticities did not suggest one particularly sensitive life history stage (“Achilles heel”) for this pest plant. A simulation was used to evaluate the potential efficacy of biological control agents that attack seeds. Based on model predictions, under current conditions a control agent would have to destroy over 99.9% of seeds in prairies, and 70% of seeds in urban populations, to suppress the invasion of C. scoparius populations.

Effect of carbohydrate type and concentration on polyhydroxy alcohol and trehalose content of conidia of three entomopathogenic fungi

The entomopathogenic fungi Beauveria bassiana, Metarhizium anisopliae and Paecilomyces farinosus were cultured on solid agar media containing different carbohydrate components (glycerol, glucose, trehalose or starch) at concentrations of ≤ 142.7 g added carbon I-1 for 30 d at 25 °C. The water activity (aw) of the media ranged from 0.925 to 0.998. Growth of M. anisopliae and P. farinosus was stimulated between 0.975 and 0.995 aw on glucose media and that of P. farinosus at 0.975 aw on glycerol media. At 50.0 g added carbon I-1 than that in conidia produced at lower glucose concentrations. Conidia contained between 10.8 and 20.8 mg glycerol plus erythritol g-1 on glucose media with 142.7 g added carbon I-1, depending on species. Conversely, conidia of B. bassiana and P. farinosus contained maximum amounts of trehalose (≤ 23.5 mg g-1) when produced on glucose media with 25.0 g added carbon I-1, conidia of B. bassiana contained up to 154.0 mg glycerol plus erythritol g-1. When B. bassiana and P. farinosus were grown on trehalose media, conidia contained up to 222.1 mg trehalose g-1. By contrast, conidia of M. anisopliae contained < 17.0 mg trehalose g-1 under all conditions tested. The water availability of solutions of different polyols is discussed in relation to their potential to act in osmotic adjustment during germination. The ability to manipulate polyol and trehalose content of fungal propagules may be critical in enhancing the storage life and efficacy of biological control agents.

Effect of prey stage on life-table attributes of a genetically manipulated strain ofMetaseiulus occidentalis (Acari: Phytoseiidae)

Metaseiulus occidentalis females from the carbaryl-organophosphate-sulfur resistant strain (COS) lived longer (25.3 days versus 19.7 days), had a higher total fecundity (43.8 eggs female−1 versus 33.6 eggs female−1) and a higher daily fecundity rate (2.4 eggs female−1 day−1 versus 2.0 eggs female−1 day−1), and exhibited a higher intrinsic rate of increase (0.243 individuals female−1 day−1 versus 0.182 individuals female−1 day−1) and shorter generation time (13.9 days versus 17.0 days), at 24–28°C, 47–56%rh under continuous fluorescent light, when reared on a diet of 0–48-h-old eggs rather than a diet of mixed actives ofTetranychus pacificus McGregor on bean leaf disks. The sexratio of the progeny was female-biased for both diets, 2.1 females to 1 male forM. occidentalis reared on eggs and 2.0:1 ♀:♂ forM. occidentalis reared on mixed actives, suggesting that diet influences sex-ratio in some unknown way. There was no significant difference in oviposition rates for repeatedly-mated and once-matedM. occidentalis females reared on a diet of younger (0–24-h old) eggs compared to a diet of older (72–96-h old) eggs ofT. pacificus. The COS strain ofM. occidentalis exhibited life-table parameters comparable to the other strains reported in the literature, suggesting that the reproductive attributes of this acarine predator were not reduced as a result of artificial laboratory selection. Diet, a biotic factor, produced substantial differences in life-table parameters, suggesting that this factor can influence conclusions regarding the potential efficacy of biological control agents.

The importance of understanding host plant phenology in the biological control of Acacia longifolia*

SUMMARYIn order to assess the effect of an introduced gall wasp on the weed Acacia longifolia, it was necessary to study the vegetative growth and reproductive potential of uninfested trees. The growth of branches on uninfested trees was studied over one year. The results yielded a growth phenology which is notably different from the growth phenology of A. longifolia described by Milton (1980). Milton (1980) observed no flushing, but a gradual increase and decline in growth during spring and late summer, respectively. The present study showed distinct flushing, and a marked lack of growth during spring. Examination of the effect of reproduction on the vegetative growth of A. longifolia explains the disparity. A. longifolia reproduces during spring. Comparison of phyllode addition during spring with pod production for three successive years at one site, and for two years at another, shows that phyllode addition is low during years of high pod production, and vice versa. Between trees, phyllode addition is also inversely related to pod production within a reproductive season. Because the present growth phenology was done in a year of high pod production, growth during spring was negligible, in contrast with Milton's (1980) phenology which was done in a year of low pod production and high phyllode addition. Depending on the year in which the plant was studied, its reproductive potential and vegetative growth could have been under‐ or overestimated by as much as 62% and 90%, respectively. This study shows that the phenology of weeds with an annual cycle needs to be examined for at least two years in order not to under‐ or overestimate their reproductive or growth potential. This is especially important when assessing the efficacy of biological control agents.