Nathalie Smits

EFFECT OF TEMPERATURE ON VEGETATIVE GROWTH OF BEAUVERIA BASSIANA ISOLATES FROM DIFFERENT ORIGINS

AbstractEffects of temperature on growth on a semisynthetic medium of 65 isolates of Beauveria bassiana from different geoclimatic and host origins were determined. The isolates were grouped accord...

Variability in susceptibility to simulated sunlight of conidia among isolates of entomopathogenic Hyphomycetes

The influence of simulated sunlight on survival of conidia of 4 species of entomopathogenic Hyphomycetes was investigated. Conidia from 65 isolates ofBeauveria bassiana, 23 ofMetarhizium anisopliae, 14 ofMetarhizium flavoviride and 33 isolates ofPaecilomyces fumosoroseus were irradiated by artificial sunlight (295 to 1,100 nm at an ultraviolet-B irradiance of 0.3 W m−2) for 0, 1, 2, 4 and 8 h. Survival was estimated by comparing the number of colony forming units (CFU) produced by conidia exposed to irradiation to the number of CFUs produced by an unexposed control. Survival decreased with increased exposure to simulated sunlight; exposure for 2 h or more was detrimental to all isolates tested. Overall, isolates ofM. flavoviride were the most resistant to irradiation followed byB. bassiana andM. anisopliae. Conidia ofP. fumosoroseus were most susceptible. In addition to the large interspecies differences in susceptibility to irradiation, there was also an intraspecies variation indicating that strain selection to irradiation tolerance may be important in the development of microbial control agents where increased persistence in an insolated environment is desirable.

Tomato leaf boundary layer climate: implications for microbiological whitefly control in greenhouses

The efficiency of mycoinsecticides as a means of whitefly control was examined under greenhouse conditions, focusing in particular on the high humidity levels favouring infection of the whitefly larvae infesting the under leaf surface. The theory of the moisture transfers occurring within the leaf boundary layer under laminar conditions is outlined and a model for the air temperature and humidity distribution is developed. Measurements were made of the humidity of the air in the boundary layer on the underside of leaves of a tomato crop grown in a full scale greenhouse. A considerable increase in relative humidity was detected 5 mm from the underside of the leaves, particularly during day-time when crop transpiration reached its maximum. These measurements were compared with the results of the model based on boundary layer theory and it was established that the data recorded were accurately predicted by the model during day-time from both the qualitative and quantitative points of view, but they were underestimated at night. The implications of these findings were examined and discussed in the context of the microbiological control of whiteflies under the conditions of confinement pertaining in various greenhouses and similar cultural systems. It is suggested that the concept of the microclimate pertaining in the leaf boundary layer developed here could be applied to the biological control of insects and microbial pathogens, in both protected and open crop growth systems.

Effect of liquid culture media on morphology, growth, propagule production, and pathogenic activity of the Hyphomycete, Metarhizium flavoviride

Two isolates of Metarhizium spp. were studied for propagule production, because of their pathogenic activity towards locusts and grasshoppers (Mf189 = M. flavoviride (or M. anisopliae var. acridum) strain IMI 330189, and Mf324 = M. flavoviride strain ARSEF324). Both isolates were grown in seven different liquid media, which have been developed for mass production of various Hyphomycetes, considered as candidates for microbial control of noxious insects. Shake-flask experiments were carried out at 28 °C in the dark. Production was quantified for 72 h and the effects of the tested media were evaluated on propagule concentration, morphology and pathogenicity. Based on preliminary experiments, all tested media were supplemented with 0.4% Tween 80 to avoid the formation of pellets and to produce unicellular propagules. Submerged propagule yields were higher withMf189 than with Mf324 in all seven media. While high concentrations of propagules (1.4 to 2.4 × 108 propagules ml-1 for MF189 and1.4 to 8.3 × 107 propagules ml-1 for Mf324) were produced in four media (Adamek, Catroux, Jackson, and Jenkins–Prior media), production of propagules was lower in the three other media (Goral, Kondryatiev, and Paris media). Both isolates produced oblong blastospore-like propagules, except in Kondryatiev medium in which they provided ovoid propagules. In this case, Mf189 submerged propagules looked like aerial conidia, but scanning observations did not demonstrate a typical conidiogenesis via phialides. In Kondryatiev medium, Mf324 submerged propagules were significantly smaller than aerial conidia. Infection potential of submerged propagules was assayed on Schistocerca gregaria. Second-instar larvae fed for 48 h on fresh wheat previously contaminated by a spraying suspension of each inoculum titrated at 107 propagules ml-1. All seven media produced submerged propagules that were highly infectious for S. gregaria larvae. Shake flask culture assays permitted us to select three low-costmedia, Adamek, Jenkins–Prior, and Catroux for improving scale-up of liquid fermentation focused on mass-production of Metarhizium propagules for mycoinsecticides devoted to locust control.

Comparison of non-linear temperature-dependent development rate models applied to in vitro growth of entomopathogenic fungi

Five non-linear models with three to five parameters, built to quantify the effect of temperature on insect development and microbial growth, were tested to describe the influence of temperature on in vitro-measured growth rates of entomopathogenic hyphomycetes. Data from two isolates of each of the four fungal species, Paecilomyces fumosoroseus, Beauveria bassiana, Metarhizium anisopliae, Metarhizium flavoviride, were used to assess the features of each model. Criteria for model evaluation included the statistical quality of parameters estimates, the goodness of fit to data, as well as the ability to provide estimates of several key parameters: the upper and lower development thresholds, the thermal optimum and the maximal growth rate at thermal optimum. The second model proposed by Brière et al. (1999) was found to be the best, and Ratkowskys model (1983) also exhibited good features.

Effects of temperature and solar radiation interactions on the survival of quiescent conidia of the entomopathogenic hyphomycetePaecilomyces fumosoroseus (Wize) Brown and Smith.

The detrimental effect of solar radiation on the survival of conidia of the entomopathogenic fungusPaecilomyces fumoroseus was studied by monitoring germinability and ability to form colonies (CFU) of conidia irradiated at two temperatures, 25 and 35 °C, harmless to shaded conidia. There was no apparent effect when spores were exposed to a high level of artificial radiation (0.66 W m−2 UVB). However, at a lower level of irradiance (0.33 W m−2), effects of radiation occurred more quickly at 35 °C than at 25 °C. Under natural solar radiation, the rate of decrease in germinability or viability was doubled at 35 °C as compared to 25 °C, indicating an interaction between temperature and radiation effects under natural conditions. This interaction was not detected in indoor experiments, indicating that the spectral distribution of UV radiation has to be taken in account as well as its irradiance when studying its effects.

Modelling the Persistence of Quiescent Conidia of the Entomopathogenic Hyphomycete Paecilomyces fumosoroseus Exposed to Solar Radiation

Persistence of conidia of the entomopathogenic fungus Paecilomyces fumosoroseus exposed to artificial and solar radiation at a constant temperature was studied by monitoring the ability to germinate and to form colonies (colony - forming units , CFUs) . The photic effect of radiation on each of these variables was modelled by a decreasing function of UVB irradiation ( in J m 2) . Germination ability was represented by a logistic function and viability (log CFU) by an infinitely decreasing function . Experiments carried out under artificial conditions , at three different UVB irradiances ( from 0 . 3 to 1 . 6 W m 2) , similar to those observed in nature , confirmed the adequacy of the predictor variable and of the functions chosen for describing these data . The proposed models appeared to be irradiance independent . Under solar radiation , the models were able to describe data collected on three different summer days in France (48 o 51 N , 2 o 06 E) . However , it took a greater amount of solar UVB radiat...

Unexpected lack of influence of tree rows on the dynamics of wheat aphids and their natural enemies in a temperate agroforestry system

Trees at low density and cereals may be combined in silvoarable agroforestry systems: tree rows and their companion vegetation introduce an heterogeneity in the cropped field that may modify the dynamics of insects crop pests and their predators. This hypothesis was tested in tree-wheat systems by monitoring aphids and their predators for two years in southern France, in two fields in 2006 and one in 2007. Tree row vegetation diversity was either suppressed by weeding or stimulated by seeding flowering species that could attract adult predators of aphids. The dynamics of aphids and predators were recorded by tiller sampling and yellow trap monitoring. Between 3500 and 5000 aphids were collected from each studied field. Surprisingly, no differences could be evidenced on the dynamics of both aphids and their predators between agroforestry and monocropping plots, nor between weeded and flowered plots. The heterogeneity induced by the tree row in the cropped field was probably not strong enough to alter the populations dynamics. A high landscape diversity in the area due to nearby forest patches and fallows may have blurred the impact of the tree lines. Different results may be expected in areas with lower biodiversity like cereal monocropping zones.