Jacques Fargues

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.

Histological and ultrastructural studies of Beauveria bassiana infection in Leptinotarsa decemlineta larvae during ecdysis

Abstract Studies of Leptinotarsa decemlineata larvae infected by Beauveria bassiana during ecdysis have enabled us to define the modes of fungal penetration employed to enter the ecdysial cuticle. We have observed the mechanically active passage of the penetrant hyphae and have followed the growth of the filaments and blastospore formation in the molting fluid. The attack of the new integument and its consequent alteration and the entry into the body cavity have also been studied. The infection develops rapidly in some of the larvae which die in premolt, while others are able to molt. Conditions rendered abnormal due to the presence of the fungus cause integumentary injuries which serve as an important factor in pathogenesis since they enhance the entry of fungal elements and bacteria thereby inducing septicemia. Contaminated larvae are able to molt, showing no signs of injury or disease, and survive for a long time, until the fungus finally invades the organism and causes death. This postponement of mortality shows that molting and hemocytic reactions are, to a certain extent, an effective defense mechanism. These last observations can be useful in the understanding of pathological processes associated with a hidden phase of fungal infection.

Effect of temperature on vegetative growth among isolates of Metarhizium anisopliae and M. flavoviride

Effects of temperature on vegetative growth on a semi-synthetic medium of 22 isolates of Metarhizium anisopliae and 14 isolates of M. flavoviride were determined. The majority of isolates of both species grew between 11 and 32°C; several isolates grew at 8 and 37 °C. None of the isolates grew at 40 °C. Relative growth rate, calculated from the maximum growth rate for each isolate, was significantly affected by temperature and isolate, with significant isolate * temperature interactions. The maximum absolute growth rates among the isolates ranged from 2.5 mm to 5.9 mm/day. Optimal temperatures were generally between 25 and 32 °C with several isolates exhibiting optimal growth at temperatures as high as 32 °C. Overall, relative growth rates were greater in isolates of M. anisopliae than M. flavoviride at temperatures of 25 °C or lower; conversely mean relative growth rates were greater in M. flavoviride than M. anisopliae at temperatures higher than 25 °C. However, the two most cold tolerant isolates at 8 °C were M. flavoviride and the three most heat tolerant at 35 °C were M. anisopliae. Since temperature growth responses varied considerably between isolates, strain selection according to thermal tolerance may be warranted when choosing a strain for development as a microbial control agent.

Effects of Temperature, Humidity and Inoculation Method on Susceptibility of Schistocerca gregaria to Metarhizium flavoviride

The effects of temperature on conidial germination and susceptibility of adults of the desert locust , Schistocerca gregaria, to four isolates of Metarhizium flavoviride were determined . In addition , the effects of inoculation method (topical or spray) , spore carrier (oil or water) and ambient relative humidity (RH) on susceptibility of the locust to the most thermotolerant isolate (Mf324) were investigated . There were differences among the isolates in the effects of temperature on germination of conidia after a 24 - h incubation period . Over 90% of conidia of all isolates germinated after 24 h at 30 o C . In contrast , at 40 o C , none of the isolates germinated for up to 72 h . However , there were differences in germination between the isolates at 35 o C . Locust mortality and disease progression were significantly affected by temperature . At both 25 and 30 o C , all isolates induced 98 - 100% mortality within 8 days; however , there were differences between isolates at 35 o C . None of the isola...

Modelling temperature-dependent bionomics of Bemisia tabaci (Q-biotype)

Abstract The influence of temperature (17, 21, 25, 30 and 35 °C) on life‐history traits of a Q‐biotype Bemisia tabaci population on tomato is studied. Temperature‐dependent relationships are characterized for immature developmental rate, immature survival, fecundity, longevity and intrinsic rate of increase. Development time vary from 20 days at 30 °C to 56 days at 17 °C and the lowest thermal threshold is estimated at 10.2 °C. The optimal temperature for immature development is 32.5 °C. Total fecundity (eggs per female) ranges from 105.3 (at 21 °C) to 41 (at 35 °C). The longevity decreases with temperature increase. The intrinsic rate of increase ranges from 0.0450 (at 17 °C) to 0.123 (at 30 °C). The functional relationships between temperature and life‐history parameters are used to evaluate the effect of temperature on the population dynamics. Such mathematical relationships could provide a basis for future development of population models.

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.

Dependence of the entomopathogenic fungus, Beauveria bassiana, on high humidity for infection of Rhodnius prolixus.

The impact of relative humidity (RH) on the infective potential of the isolate Bb INRA 297 of Beauveriabassiana (Bals.) Vuillemin (Deuteromycotina: Hyphomycetes) against first in star nymphs of Rhodnius prolixus Stål. (Hemiptera: Reduviidae) was determined. Fungus-treated insects were exposed to RHs ranged from 75 to 100% at 25 °C. Results clearly showed a threshold of humidity at ca. 96% for high and rapid mortality. After initial exposure to increasing periods of 97% (4, 8, 16, 24, 36 and 48 h) and subsequent transfer to constant lower RHs (43, 53,75 and 86%) at a constant 25 °C, an incubation of at least 48 h at 97% RH was necessary to kill all insects. On changing RHs of 97/75% and different regimes of temperature (15/28 °C,20/25 °C, 25/28 °C,and 25/35 °C), at least 72 h of initial exposure at 97% RH for the 15/28 °C regime, 48 h for the 20°/25°C and25/28 °C regimes and 36 h for25/35 °C were needed to kill all insects over a 6-day incubation time. Delayed exposure to favorable moisture condition (97% RH), significantly affected infection for up to a 3-day delay within the various temperature-humidity regimes tested.

Effect of various liquid culture media on morphology, growth, propagule production, and pathogenic activity to Bemisia argentifolii of the entomopathogenic Hyphomycete, Paecilomyces fumosoroseus

The entomopathogenic Hyphomycete Paecilomyces fumosoroseus was grown in five different liquid media, which have been developed for mass production of Beauveria spp. or P. fumosoroseus. Production was followed for 96 h by measuring both biomass and concentration of propagules. Maximal biomass was obtained with two media, Jackson and Catroux media (40–60 mg ml-1 suspension produced after 42 h incubation), where the exponential phase of growth began earlier than in the other media. While high concentrations of propagules (1.4–5.5 × 108 propagules ml-1) were produced in three media (Jackson, Paris, and Catroux media) after 48–72 h incubation, production of propagules was lower in the two other media, containing maltose as carbon source (Goral and Kondryatiev media) with 0.4–3.7 × 107 propagules ml-1 after 96 h incubation. P. fumosoroseus produced oblong blastospores in the three most productive media and conidiospore-like (ovoid to subspherical) propagules in the two other media. Infection potential of produced propagules was tested on the silverleaf whitefly ( Bemisia argentifolii). Whiteflies were sprayed as 2nd instars with P. fumosoroseus propagules produced in the five liquid media (1.9 × 103 and 1.9 × 104 propagules cm-2). All the media produced propagules that were infectious for larvae. With the lower dosage, mortality rates were significantly lower with propagules produced in one of the two least productive media (57%) (in the Kondryatiev medium) compared to those obtained with the three most productive media (>90%). However, when whiteflies were treated with the higher dosage, mortality rates (91–99%) between media were not significantly different.