Evaluation of Beauveria bassiana (Hyphomycetes) isolates as potential agents for control of Dendroctonus valens

Abstract The red turpentine beetle (RTB), Dendroctonus valens LeConte, as a destructive invasive pest, has become one of the most economically important forest pest in China. Effective control measures are desperately needed. Entomopathogenic fungi, such as Beauveria bassiana, have shown great potential for the management of some bark beetle species. In this study, 12 isolates of B. bassiana from bark beetle were examined for biological characteristics and virulence, to assess their potential as biocontrol agents for RTB. There were significant differences (at P= 0.05) in colony growth rate, conidial yield, conidial germination, tolerance to UV light and extracellular proteases activity among the tested B. bassiana isolates. Isolates, including Bb1801, Bb1906, Bb789 and Bb773, exhibited the best characteristics, because they have faster hyphal growth rate, higher spore production and faster spore germination, higher UV tolerance and protease (Pr1) production. The results of a pathogenicity test of B. bassiana on RTB larvae showed that most isolates of B. bassiana have demonstrated high efficacy and the highest virulent isolate was Bb1801, which killed 100% of the treated insects and had a median lethal time (LT₅₀) of 4.60 days at a concentration of 1×10⁷ conidia/mL. Therefore, isolate Bb1801 has a great potential for sustainable control of RTB in the forest. The correlation between biological characteristics and virulence of the fungal isolates is discussed and the possibility of combination of entomopathogenic fungi with semiochemicals, as one of the promising strategy for RTB control, is considered.     

Virulence,proteolytic and lipolytic activities of Brazilian Beauveria bassiana s.l. isolates (Hypocreales: Clavicipitaceae) to Rhipicephalus microplus ticks (Acari: Ixodidae)

Beauveria bassiana s.l. is a cosmopolitan fungus used in the control of different species of arthropods. The current study explored the virulence for ticks, proteolytic and lipolytic activities of 10 Brazilian B. bassiana s.l. isolates. For this purpose, Rhipicephalus microplus biological parameters was evaluated after immersion of the engorged females in fungal suspension (10⁸ conidia mL^?1) and the enzymatic activities were performed posteriorly the fungal growth in minimal medium. After the biological assays, five isolates changed all parameters analysed with highest efficacy of approximately 61% (CG 206) and 66% (CG 481). However, we observed that the most virulent isolates did not show the highest enzymatic activities. Interestingly, CG 500, considered an isolate of intermediate efficacy, demonstrated higher enzymatic activities than the other isolates in four of five analyses (total protease, Pr1, Pr2 and lipase; p?相似文献   

Effect of nutrition on growth and virulence of the entomopathogenic fungus Beauveria bassiana

Three isolates of the entomopathogen Beauveria bassiana along with one strain of Metarhizium anisopliae were cultured on seven media with different carbon/nitrogen (C/N) ratios. The effect of nutrition on virulence of the isolates was evaluated via measurement of colony growth, spore yield, germination speed, conidial C/N ratio and Pr1 (a serine protease) activity. 'Osmotic stress' medium produced the lowest colony growth with low numbers of conidia in all isolates. However, these conidia showed a high germination rate and virulence. However, conidial Pr1 activity was low in some isolates. In most but not in all cases conidia from 1% yeast extract, 2% peptone and low (10 : 1) C/N medium had higher Pr1 activity compared with conidia from other media. However, in some instances we could not conclude that there was a relationship among germination rate, conidial Pr1 activity and virulence. C/N ratio of conidia was statistically different among various media and fungal isolates. Conidia with lower C/N ratio generally produced lower LT(50) (lowest median lethal time) values (more virulent). Insect-passaged conidia from different media had lower C/N ratio compared with similar conidia from artificial cultures. Therefore, they should be more virulent than in vitro produced conidia. As germination rate, conidial Pr1 activity and C/N ratio are independent of host, it seems that host-related determinants such as insect cuticle and physiology and environmental conditions may influence host susceptibility and therefore fungal isolate virulence towards host insects.     

Impact of culture age on conidial germination,desiccation and UV tolerance of entomopathogenic fungi

The impact of culture age on conidial yields, germination and tolerance to UV exposure of freshly harvested and dry conidia produced by five entomopathogenic fungal (EPF) isolates was studied. Beauveria bassiana, Metarhizium anisopliae, Lecanicillium lecanii and Lecanicillium muscarium were grown on potato dextrose agar (PDA) medium for 7 or 14 days at 25°C. While the age of cultures had a significant impact on the germination rate of conidia produced by isolates L. lecanii CBS 122.175 and B. bassiana LMSA 1.01.093, other EPF isolates germinated at the same rate regardless of the culture age. When exposed to UV radiation, conidia produced by all isolates germinated at a lower rate compared to the non-irradiated conidia, although this decrease in germination (20–80% decrease) was unaffected by the culture age. Air-drying had only a slight impact on conidial germination (0–60% decrease). Under the conditions of this study, the stability of irradiated conidia produced by M. anisopliae LMSA 1.01.197 and B. bassiana CBS 110.25 was significantly increased when conidia were dried prior to UV exposure. This increase in tolerance to stress of dried conidia might be caused, at least partially, by the low metabolic activity associated with dehydration.     

Investigation of native isolates of entomopathogenic fungi for the biological control of three citrus pests

The pathogenicity of 15 isolates of Beauveria bassiana (Balsamo) Vuillemin, five isolates of Metarhizium anisopliae (Metschnikoff) Sorokin and one isolate of M. flavoviride (Gams and Rozsypal) were tested under laboratory conditions against the subterranean life stages of the citrus pests, Ceratitis rosa Karsch, C. capitata Wiedemann (Diptera: Tephritidae) and Thaumatotibia leucotreta Meyrick. (Lepidoptera: Tortricidae). When these citrus pests were treated with a concentration of 1×10⁷ conidia mL^?1, fungal isolates had a significantly greater effect on the adults of C. rosa and C. capitata than they did on the puparia of these two fruit fly species. Further, C. rosa and C. capitata did not differ significantly in their response to entomopathogenic fungi when adult and pupal mycosis were considered. Depending on fungal isolate, the percentage of T. leucotreta adults which emerged from fungal treated sand ranged from 5 to 60% and the percentage of pupae with visible signs of mycosis ranged from 21 to 93%. The relative virulence of the four most promising fungal isolates, as well as the commercially available B. bassiana product, BroadBand® (Biological Control Products, South Africa), were compared against one another as log-probit regressions of mortality against T. leucotreta which exhibited a dose-dependent response. The estimated LC₅₀ values of the three most virulent B. bassiana isolates ranged from 6.8×10⁵ to 2.1×10⁶ conidia mL^?1, while those of the least pathogenic ranged from 1.6×10⁷ to 3.7×10⁷ conidia mL^?1.     

Selection of Beauveria bassiana isolates for control of the whiteflies Bemisia tabaci and Trialeurodes vaporariorum on the basis of their virulence, thermal requirements, and toxicogenic activity

As part of a 3-fold approach to select potential mycoinsecticides for whitefly control, we evaluated infectivity, thermal requirements, and toxicogenic activity of the entomopathogenic fungus Beauveria bassiana (Ascomycota: Clavicipitaceae) under laboratory conditions. Twenty-five native B. bassiana isolates and a commercially available mycoinsecticide (based on B. bassiana) were evaluated for virulence to fourth instar nymphs of sweetpotato whitefly, Bemisia tabaci, and greenhouse whitefly, Trialeurodes vaporariorum, at a concentration of 1 × 10⁷ conidia/ml. All isolates were pathogenic for both whitefly species, whereas mortality rates varied from 3 to 85%. A second series of bioassays was conducted on 10 selected isolates using four 10-fold concentrations ranging from 1 × 10⁵ to 1 × 10⁸ conidia/ml. Median lethal concentrations (LC₅₀) of the four most virulent isolates varied from 1.1 × 10⁵ to 6.2 × 10⁶ conidia/ml and average survival time (AST) of treated nymphs from 5.9 to 7.4 days. T. vaporariorum were significantly more susceptible to all B. bassiana isolates than B. tabaci. The thermal biology of the eight most virulent isolates to both whitefly species was investigated at six temperatures (10–35 °C). The colony radial growth rate was estimated from the slope of the linear regression of colony radius on time and data were then fitted to a modified generalized β function that accounted for 90.5–99.3% of the data variance. Optimum temperatures for extension rate ranged from 23.1 to 27.1 °C, whereas maximum temperatures for fungal growth varied from 31.8 to 36.6 °C. On the basis of their virulence and thermal requirements, three isolates showed promise as candidates for whitefly management in Mediterranean greenhouses. Whilst in vitro production of macromolecular compounds toxic to Galleria mellonella larvae was not a requisite for virulence, ASTs of larvae injected with Sephadex G-25 fractions from candidate isolates ranged from 1.4 to 3.7 days compared with 5–6 days for non-toxic G-25 fractions. In addition, proteinase K treatment significantly reduced their toxic activity suggesting that they were proteins and revealing the potential of these isolates to be further improved through biotechnology to kill the pest more quickly.     

Phenotypic variability of environmental isolates of the entomopathogenic fungus <Emphasis Type="Italic">Beauveria bassiana</Emphasis>

A total of 35 isolates of Beauveria bassiana (Bals.) Vuill. isolates obtained from various insects of Novosibirsk oblast were investigated. The fungal morphotypes were found to include cultures of high, medium, and low virulence. Low correlation (r < 0.48) was observed between virulence and the morphophysiological characteristics of the isolates (lipase and protease activity, biomass, radial growth rate, conidia productivity, and relief). Isolates exhibiting high virulence to insects of a certain order proved to be virulent to the insects of other orders. A high correlation (r > 0. 74) was revealed between the virulence of the isolates to the potato beetle Leptinotarsa decemlineata Say and the locusts Calliptamus barbarus Costa and Locusta migratoria L. Isolates obtained from insects of the same species in the same site may differ significantly in virulence.     

The development and multiple uses of a standardised bioassay method to select hypocrealean fungi for biological control of aphids

A technically standardised bioassay method was designed, evaluated and used to assess virulence and host range of hypocrealean fungi against aphids. A track mounted sprayer was used to apply conidia because hand held versions of the same sprayer can be used for field applications, thereby allowing the outcome from laboratory experiments to predict activity in the field accurately. Eighteen fungal isolates were assessed in single concentration bioassays against the black bean aphid Aphis fabae Scopoli. Isolates comprised commercially available mycoinsecticides (based on Beauveria bassiana and Lecanicillium longisporum) and isolates of B. bassiana, Lecanicillium spp., Paecilomyces fumosoroseus and Metarhizium anisopliae. Aphid mortality was in excess of 80% for 15 isolates, and HRI 1.72 (L. longipsorum), Z11 (P. fumosoroseus), Mycotech strain GHA (B. bassiana) and ARSEF 2879 (B. bassiana) were studied further. Multiple concentration bioassays identified HRI 1.72 as the most virulent isolate against A. fabae with significantly smaller LC₅₀ and LT₅₀ values compared to other isolates. A precise LC₅₀ value (2.95 × 10² conidia ml⁻¹) was calculated for HRI 1.72 using a second multiple concentration assay with smaller concentrations of conidia. The four isolates were applied at a single concentration (1 × 10⁸ conidia ml⁻¹) against Myzus persicae, A. fabae, Acyrthosiphon pisum, Metopolophium dirhodum, Sitobion avenae and Rhopalosiphum padi. A ranking of aphid susceptibility was obtained, such that S. avenae > M. persicae, A. pisum, A. fabae > R. padi. Results indicate the importance of standardising bioassay methods to reduce bioassay variability without compromising the ability to use the bioassay to investigate fungus–host interactions under varying abiotic and biotic conditions.     

Role of cuticle-degrading enzymes of Beauveria bassiana and Metarhizium anisopliae in virulence on Plodia interpunctella (Lepidoptera,Pyralidae) larvae

Entomopathogenic fungi (EPF) are important biological control agents in pest management programs in agroecosystems against insect pests. EPF such as Beauveria bassiana (Bals.) Vuillemin and Metarhizium anisopliae (Metchn.) Sorokin produce a wide range of extracellular enzymes involved in disturbance of the first barrier in the insect cuticle comprising proteins, chitin, and lipids. Realizing relationships between the expression of these enzymes and fungal virulence might aid in development of effective mycoinsecticides. The virulence of B. bassiana (isolates TV and OZ1) and M. anisopliae (isolate CS1) were investigated on Plodia interpunctella (Hübner) larvae in this study. The third instar larvae were immersed in a suspension containing 1 × 10⁸ conidia mL^?1 of fungal conidial inoculum. The results revealed that all three fungal isolates caused mortality in larvae, but there was a considerable variation in their virulence. Total proteinase, exochitinase and lipase assays were done for these isolates. The TV isolate with the highest mortality with 41.7%, had the highest level of specific activity of exochitinase, protease, and lipase with 0.148, 0.654, and 0.190 U. mg^?1 protein, respectively. In the current study, a positive correlation was determined between the virulence of fungal isolates and the activities of protease and lipase, but this link was not significant for exochitinase. Our results demonstrated that extracellular enzymes, particularly protease and lipase, may play a crucial role in the virulence of these fungal isolates against P. interpunctella larvae.     

A cuticle-degrading protease (CDEP-1) of Beauveria bassiana enhances virulence

In order to investigate virulence enhancement of entomopathogenic fungi, a Beauveria bassiana-sourced Pr1 protease (CDEP-1) was expressed by a methylotrophic yeast Pichia pastoris and then used as an additive to three gradient sprays of B. bassiana strain (Bb0062) onto apterous green peach aphid Myzus persicae adults in six bioassays. The resultant data fit well to a time–concentration–mortality model. Generally, the LC₅₀ estimates of the fungal pathogen against the aphid species decreased with increasing CDEP-1 concentrations from 0 to 100 µg mL^?1. The LC₅₀s on days 5–7 after spray were reduced by 1.5–2.5-fold at the concentrations of 20–100 µg mL^?1. However, sprays of 20–100 µg CDEP-1 mL^?1 aqueous solution alone had no significant effect on aphid mortality compared to water spray only. Neither did inclusion of inactivated CDEP-1 at a concentration of 50 µg mL^?1 affect significantly the fungal virulence to aphids. Our results confirm for the first time that the cuticle-degrading protease CDEP-1 enhanced fungal virulence due to acceleration of conidial germination and cuticle penetration. This suggests a new approach to utilising the protease in microbial control.     

Pathogenicity, formulation and storage of insect pathogenic hyphomycetous fungi tested against Diabrotica speciosa

Studies were conducted tosearch for fungal strains with potentialpathogenicity against Diabrotica speciosa(Germar) (Coleoptera: Chrysomelidae).Among sixteen fungal isolates screenedthe most virulent was a Beauveria bassiana(Balsamo) Vuillemin isolate (FHD13) thatcaused 70% mortality of D. speciosathird instar larvae. The LC₅₀ value ofB. bassiana isolate FHD13 was3.48 × 10¹⁰ conidia/ml.Different temperatures (4, 17 and 26 °C)and vegetable oils (corn, sunflower and canola)used for storage did not significantly affectviability of conidia. A pathogenicity trialagainst D. speciosa larvae performed withthe corn oil formulation (1 × 10⁸ conidia/mlof oil) caused 65% of mortality.     

Attenuation of the entomopathogenic fungus Beauveria bassiana following serial in vitro transfers

Stability of pathogenicity in continuous in vitro cultivation is desirable for the purpose of large-scale production of a mycoinsecticide. Fungal biocontrol agents may lose virulence when maintain on artificial media, resulting in products of commercially inferior quality. In this research, two isolates (DEBI007 and DEBI008) of entomopathogenic fungus Beauveria bassiana were investigated for their stability following fifteen serial in vitro transfers assaying virulence to mealworm larvae, conidiation, and hyphal development on artificial culture as some fungal virulence determinants. Moreover, role of insect cuticle on fungal virulence restoration and protease 1 (Pr1) activity was considered as the most important factor. Although radial hyphal development and colony colour on in vitro culture was not affected following serial transfers, conidiation and Pr1 activity of both fungal isolates were reduced remarkably after fifteen transfers compared with control. Similarly, mean lethal concentration (LC₅₀) values were increased as the number of serial transfers on artificial diet increased, although these increases were not statistically significant in both isolates as the confidential limits of LC₅₀ values were overlapping. Our results revealed that attenuation of entomopathogenic fungi following serial in vitro transfers is a combination of interconnected factors. Other probable components such as pathogenicity determinants in this interaction should be explored in next researches.     

腺苷酸核糖基化因子BbarfA介导白僵菌孢子萌发和毒力

【目的】探究腺苷酸糖基化因子ARF在球孢白僵菌(Beauveria bassiana)中存在种类及生物学功能。【方法】利用BLASTp搜索球孢白僵菌非冗余蛋白数据库,鉴定ARF并进行聚类分析,结合表达分析、反义抑制、超量表达野生型基因和GTP解离位点与结合位点突变的基因,解析其中1个ARF与白僵菌发育分化、逆境胁迫反应和毒力的关系。【结果】球孢白僵菌中存在至少6个ARF或类似蛋白,分别聚类于酵母、人类ARF及其类似蛋白的不同类群。其中BBA_01574与人类的ARF3、ARF4和ARF5聚为一类,命名为BbarfA。BbarfA在成熟的分生孢子和球形膨大时期表达明显高于芽管伸长期。反义抑制BbarfA加速了孢子萌发,提高了菌株毒力,而超量表达BbarfA和点突变GTP解离区域的BbarfA则延迟了孢子萌发速度,降低了菌株毒力。尽管BbarfA转录受高盐、髙渗、氧化和高温胁迫的诱导,但遗传修饰的转化子与野生菌株对上述胁迫反应的敏感性无明显差异。【结论】BbarfA介导分生孢子萌发和毒力。     

The use of fluorescent powders to track autocontamination of emerald ash borer (Coleoptera: Buprestidae) by the entomopathogen Beauveria bassiana (Ascomycota: Hypocreales)

We evaluated the use of fluorescent powders for tracking dispersal by the emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), of Beauveria bassiana isolates from an autocontamination device. Neither of the two DayGlow powders tested (Arc Yellow and Aurora Pink) interfered with fungal germination or growth, nor did they affect survival of beetles in the laboratory, or affect virulence of the fungus. The powders persisted at least 10 days out-of-doors on dead beetles in sticky band traps, and at least 14 days on pouches inside autocontamination traps. During field trials of autocontamination traps with powder-dusted fungal pouches in southwestern Ontario, 8.0% of the 4010 beetles captured in green prism and sticky-band traps were positive for fluorescent powders. Only half (46.2–57.8%) of the powder-positive beetles actually carried viable fungal conidia, as determined by plating of beetle rinses, possibly as a result of patchy growth of fungal isolates and reduced conidia production on pouch surfaces during the 16-day trapping experiment. The presence of viable conidia (either one or both isolates) on about 10% of beetles that did not carry any visible powder particles may be an indication of horizontal transmission of the fungus by beetles that had visited the autocontamination traps.     

Repeated in vitro subculturing alters spore surface properties and virulence of Metarhizium anisopliae

Repeated subculturing caused rapid changes in the spore surface properties and virulence of Metarhizium anisopliae. Of the two strains evaluated, M. anisopliae V245 attenuated more rapidly than V275. Electrophoretic mobility and Radial Flow Chamber assays were used for the first time to generate qualitative and quantitative information on the adhesive forces of M. anisopliae conidia. Independent of strain, adhesion, hydrophobicity and spore-bound Pr1 declined after the first subculture; however, spore surface charge decline was erratic. Adhesion and hydrophobicity stabilized after the third subculture, whereas spore-bound Pr1 continues to decline following repeated subculturing. Decline in spore bound Pr1 was directly correlated with decline in virulence, however, such correlation with adhesion, hydrophobicity or surface charge could not be established. Because spore-bound Pr1 activities were directly correlated with M. anisopliae virulence; it could be used as a quality-control marker to monitor changes in virulence.     

Addition of exogenous carbon and nitrogen sources to aphid exuviae modulates synthesis of proteases and chitinase by germinating conidia of Beauveria bassiana

Secretion of catabolic extracellular enzymes (ECE) is the hallmark of the infection of insects through the cuticle by entomopathogenic fungi (EPF). In this paper, we show that germinating conidia of Beauveria bassiana (Bb) regulate the synthesis of ECE through a multiple control mode during the initial stages of germination. We tested Bb conidial growth on aphid exuviae with or without supplementation of additional carbon and/or nitrogen (C/N) compounds. To understand the interrelation between conidial germination during growth, the synthesis of ECE activity, free amino nitrogen (FAN), glucose and fungal dry weight biomass were measured. Immediately (0.25 h) upon incubation of conidia, activity of subtilisin-like Pr1 and trypsin-like Pr2 enzymes and chitinase (NAGase) was observed in the culture filtrates. At 0.25 h, addition of exogenous C-source resulted in higher activities of Pr1 and Pr2, respectively. Conversely at 0.25 h, addition of N-sources repressed the synthesis of Pr2, but that of Pr1. C/N repression was observed only for exponentially growing mycelia. NAGase activity remained at basal level and unaffected by added C/N. We conclude that C/N repression occurs only when it is necessary for the Bb infective structures to establish a nutritional relationship with the host structures.     

Host-plant mediated interactions between two aphid species

Three isolates of Isaria fumosoroseus Wize (Hyphomycetes) were cultured on six media composed of differing amounts of chitin, carbon, and nitrogen. The effect of nutrition on growth and virulence was studied by measuring colony growth, spore yield, germination rate, spore-bound protease (Pr₁) and lipase activity, and virulence of inoculum produced by different media against second instars of diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae). Chitin peptone nutrition media resulted in the highest colony growth but spore yields were lowest for this medium, whereas the osmotic stress medium resulted in the lowest colony growth with fewer spores. Highest lipase activity (4.14 µmol/ml/min) was observed for spores produced on high C/N medium for isolate IF28.2, whereas highest Pr1 activity (2.64 µmol/ml/min) was also observed on high C/N medium for isolate IF28.2. A higher rate of Pr1 and lipase activity for all three isolates was observed on low C/N, 2% peptone, and the osmotic stress media. Conidia from nutrient-poor media (2% peptone) proved to be the least virulent for all three isolates with median survival time values of 2.23, 2.05, and 1.79 days for IF32, IF28.2, and IF49, respectively. Median survival time values for the various nutrient media proved to be positively correlated with spore-bound Pr1 and lipase activity, having correlation coefficient values of 0.115 and 0.538, respectively.     

Characterisation of Isaria fumosorosea isolates and their virulence toward the Diamondback Moth,Plutella xylostella

Some morphological and physiological characteristics of an Isaria fumosorosea isolate with diminished virulence, IFCF01-D, and its parent isolate, IFCF01, were evaluated and laboratory bioassays were performed to assess their virulence against Plutella xylostella. The relationship among these traits and virulence against P. xylostella is discussed. There were no significant differences in conidial viability, spore production and the time required for 50% germination (GT₅₀). Spore viability after incubation for 24 h at 25°C was greater than 98% for both isolates tested. Spore production on potato dextrose agar after 14 days incubation at 25°C was 4.68 × 10⁸ and 4.59 × 10⁸ conidia/mL for IFCF01 and IFCF01-D, respectively. When exposed to high temperatures (40, 45, 50 or 55°C) through a water bath for 10 min, conidial germination ranged from 0.83% to 84.0% for IFCF01 and 0% to 86% for IFCF01-D. Germination rate showed a negative relationship with the exposure temperature for both isolates. The per cent germination of isolate IFCF01 24 h after ultraviolet (UV) radiation (18 W, 240–260 nm) varied from 0% to 92% and 0% to 81% for IFCF01-D. Germination rate and the exposure time exhibited a negative correlation for both isolates tested. Conidial surface hydrophobicity of IFCF01 (60%) was significantly higher than that of isolate IFCF01-D (53%). Subsequently, using the cicada exuviae as the substrate for enzymatic analysis, Pr1 and chitinase activity demonstrated the contrasting virulence traits: higher specific activities for the more virulent IFCF01 and lower enzymatic levels for isolate IFCF01-D.     

Temperature-dependent germination,growth and co-infection of Beauveria spp. isolates from different climatic regions

Biological control agents based on entomopathogenic fungi traditionally contain a single strain that is efficient under certain biotic and abiotic conditions. Since particularly abiotic conditions vary, biological control efficiency may become more resilient at extreme temperatures if two or more fungal strains are combined based on their adaptations to their original environment. Here we evaluated the in vitro temperature-dependent germination and growth rate for six Beauveria spp. isolates originating from either arctic or tropical regions. Isolates of arctic origin showed higher germination and growth rate at 8°C and 12°C than isolates from the tropics while the latter group showed highest germination and in vitro growth at 32°C. Three of the isolates belonging to Beauveria bassiana were further tested in vivo for temperature-dependent infection in the mealworm beetle Tenebrio molitor both individually and combined. The same amounts of conidia were used in all bioassays. Virulence was isolate dependent at all temperatures with no additional effect at the low (12°C) and high (32°C) temperatures of combinations of arctic and tropical isolates. The results therefore indicate that adaptations to abiotic conditions in the natural environment do not directly reflect the effect of biotic environment (such as host infection) under similar conditions. Selection of isolates for biocontrol agents should not be based solely on in vitro experiments, while isolate selection based on virulence should also include considerations of the abiotic conditions the isolates are expected to function.     

Germination polarity of Beauveria bassiana conidia and its possible correlation with virulence

Three different germination types of conidia; unidirectional, bidirectional and multidirectional, were revealed through microscopic observations for eight Beauveria bassiana isolates germinated on Sabouraud dextrose agar. Canonical correlation analysis indicated that there is a positive correlation between unidirectional germination and virulence against diamondback moth, Plutella xylostella and the Colorado potato beetle, Leptinotarsa decemlineata. Scanning electron microscopy revealed different in vivo behaviors for unipolar- and bipolar-germinated conidia. Unipolar-germinated conidia produced a strong germ tube with mostly appressorium-like structures while bipolar-germinated conidia continued to invasive hyphal growth without any penetration, indicating that germination polarity in one way or another may be an indicator of pathogenic ability.