Host plant influences pathogenicity of Beauveria bassiana to Bemisia tabaci and its sporulation on cadavers

Laboratory bioassays were conducted to determine host plant effect on pathogenicity of the entomopathogenic fungus Beauveria bassiana (Balsamo) Vuill. (Ascomycota: Hypocreales) to the sweetpotato whitefly, Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae). Fourth instar B. tabaci reared on cucumber, tomato, melon, green pepper, potato, eggplant, marrow, cabbage, bean or cotton, were treated with 1 × 10⁷ conidia/ml B. bassiana EABb 93/14-Tp isolate. Mortality caused by B. bassiana significantly increased with time and it was significantly affected by the host plant on which the nymphs were reared. Mean mortality of nymphs 8 days after inoculation ranged between 52.3±7.3 for nymphs reared on cotton and 91.8±5.8 for nymphs reared on cucumber. Average survival times of nymphs treated with the fungal suspensions were also significantly influenced by the host plant, with a mean of 4.7±0.1 days for nymphs reared on cucumber, 6.6±0.2 days for cotton and 6.9±0.1 days for green pepper. The production of newly formed conidia was also affected by host plant and varied from 111000±8600 conidia/cadaver for nymphs reared on cotton to 597000±28000 conidia/cadaver for those reared on melon.     

Insecticidal activity of entomopathogenic fungi (Hypocreales) for potato psyllid,Bactericera cockerelli (Hemiptera: Triozidae): Development of bioassay techniques,effect of fungal species and stage of the psyllid

The potato psyllid, Bactericera cockerelli (?ulc), is a pest of potato, tomato, and some other solanaceous vegetables and has also been incriminated in the transmission of a bacterial pathogen, Candidatus Liberibacter solanacearum, resulting in a serious disease known as ‘zebra chip’. Although there are several reports of fungal pathogens in psyllids, there are none from B. cockerelli, nor have any fungi been evaluated against it. Five isolates of fungi, one Beauveria bassiana, two Metarhizium anisopliae and two Isaria fumosorosea, were bioassayed against B. cockerelli on potato leaves under ideal conditions for the fungi. All applications were made with a Potter spray tower. With the exception of concentration-effect studies, all other applications were made using 10⁷ conidia/mL in a 2-mL aqueous suspension. All isolates except B. bassiana, produced 95–99% mortality, corrected for control mortality, in adults 2–3 days after application of conidia and 91–99% in nymphs 4 days after application. The corrected mortalities for adults and nymphs treated with B. bassiana were 53 and 78%, respectively, 4 days after application. I. fumosorosea Pfr 97 produced 95% corrected mortality in both first and late third instar nymphs. M. anisopliae (F 52) produced 96% corrected mortality in first and third instar nymphs. Pfr 97 and F 52 were evaluated for insecticidal activity against third instar B. cockerelli using 10⁵, 10⁶, and 10⁷ conidia per mL. Mortality produced by I. fumosorosea Pfr 97 ranged from 83 to 97% and that of M. anisopliae F 52 was 88 to 95% at these concentrations.     

Effect of culture medium on the production and virulence of submerged spores of Metarhizium anisopliae and Beauveria bassiana against larvae and adults of Aedes aegypti (Diptera: Culicidae)

Three Metarhizium anisopliae and three Beauveria bassiana isolates were cultivated in media containing casamino acids, soybean flour or sunflower seed flour and were shaken for three days. M. anisopliae presented similar yields of around 10⁶ submerged spores/ml without significant differences among them, whereas B. bassiana produced yields of around 10⁸ spores/ml, of which GHA strain produced more submerged spores in the casamino acids medium. The other two strains showed no significant difference in the production of submerged spores in the three media used. Differences in mortality on Aedes aegypti larvae were observed with the submerged spores of Metarhizium depending on isolate and medium used. M. anisopliae 2157 caused significantly higher mortality (40%) when cultivated in casamino acids medium. It presented an LC₅₀ of 8.93 × 10⁵ submerged spores/ml water against mosquito larvae five days after application, whereas it caused 27% mortality in Ae. aegypti adults 10 days after application. In conclusion, fungal nutrition affected virulence of some isolates of M. anisopliae against Ae. aegypti larvae while such an effect was not noted for B. bassiana isolates.     

Sublethal effects of Beauveria bassiana (Balsamo) Vuillemin (Deuteromycotina: Hyphomycetes) on the whitefly Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) under laboratory conditions

Sublethal effects were evaluated in subsequent generations of whiteflies Bemisia tabaci (Gennadius) arising from parental generations exposed in the four nymphal stages to the fungus Beauveria bassiana (Balsamo) Vuillemin. Examples of such effects include impaired fertility, production of malformations or external variations, and reduced survival of later generations. Malformations of the head, legs, wings or abdomen were not observed in adults derived from treated nymphs, nor were negative effects observed in the fecundity and fertility of the descendants of the whiteflies treated with the fungus. However, moulting problems were observed in insects descended from B. bassiana-treated whiteflies. This is the first time that such effects have been reported, with almost 30% of imagos resulting from treated nymphs unable to detach completely from the exuvia. A gradual reduction in mortality rates between subsequent generations was observed. The importance of these results is discussed in the light of findings from other studies into the effects of entomopathogenic fungi on pest insects.     

Synergistic action of entomopathogenic hyphomycetes and the bacteria <Emphasis Type="Italic">Bacillus thuringiensis</Emphasis> ssp. <Emphasis Type="Italic">morrisoni</Emphasis> in the infection of Colorado potato beetle <Emphasis Type="Italic">Leptinotarsa decemlineata</Emphasis>

A synchronous coinfection of the Colorado potato beetle Leptinotarsa decemlineata (Say) with the entomopathogenic bacteria Bacillus thuringiensis ssp. morrisoni Bonnifoi & de Barjak var. tenebrionis Krieg et al. and hyphomycete Metarhizium anisopliae (Metsch.) Sorokin or Beauveria bassiana (Bals.) Vuill leads to the rapid death of 95–100% of larvae. The bacteria arrest the nutrition of insects, while the fungal spores kill the weakened larvae. The synergistic effect of two pathogens is recorded at a relatively low hyphomycete titer (1–5 × 10⁶ conidia/ml) and is evident in the mortality dynamics at all larval ages. These bacterial and fungal pathogens display no antagonism on artificial nutrient media. This microbial complex is highly efficient under natural conditions (80–90% larval mortality rate and no plant defoliation).     

Horizontal transmission of mycotic infection in adult tsetse,Glossina morsitans morsitans

AdultGlossina morsitans morsitans exposed to wet conidia ofBeauveria bassiana andMetarhizium anisopliae suffered high mortalities ranging from 90 to 100% by 2 weeks post-exposure. Infected ♂ ♂ maintained in the same cages with non-infected ♀♀ throughout the experimental period transmitted the fungal infection to the ♀♀ resulting in mortalities of 65% withB. bassiana and 55% withM. anisopliae. Likewise, infected ♀♀ maintained together with non-infected ♂♂ transmitted the infection to the ♂♂ resulting in mortalities of 75% withB. bassiana and 45% withM. anisopliae. Female tsetse flies infected withB. bassiana andM. anisopliae and maintained in the same cages with non-infected ♀♀ also transmitted infection to the non-infected tsetse resulting in mortalities of 62% and 48% withB. bassiana andM. anisopliae respectively. Infected tsetse exposed to non-infected tsetse of the opposite sex for only 30 min were also able to transmit the fungal infection. Pupae produced by female tsetse infected withB. bassiana andM. anisopliae exhibited higher pupal mortality than those produced by non-infected ♀♀. However, pupae exposed directly to dry spores ofB. bassiana andM. anisopliae had no increase in pupal mortality but adults emerging from theB. bassiana-exposed pupae had markedly reduced longevity.      

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.     

Efficacy of multiple biocontrol agents against the sweet potato whitefly Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) on tomato

Abstract: Biological control provides an environmentally harmonious and potentially stable management tactic to combat noxious pests such as Bemisia tabaci, notorious for its resistance to synthetic pesticides. Bioassays conducted under control chamber conditions integrating applications of the parasitoid Encarsia formosa, reared for 20 years on Trialeurodes vaporariorum, and the fungus Verticillium lecanii on the third‐fourth instar nymphs of B. tabaci on tomato, showed a comparable effect between the parasitoid‐fungus combined treatment and the fungus treatment alone (70.7% vs. 70.4%). Analysis of our results indicates antagonism between the two biocontrol agents related to the parasitoids’ ability to discriminate between infected and healthy B. tabaci nymphs. The parasitoid treatment alone produced 36.3% mortality, with no mortality in the distilled water controls. The behavioural performance of the parasitoid could have either genetic or environmental causes. Bioassays studying the feeding habit of the imported mirid predator Macrolophus caliginosus (adults) and the indigenous mirid Camptotylus reuteri (nymphs and adults) on eggs, or early second instar nymphs of B. tabaci, and choice preference tests indicated a significant difference in feeding between M. caliginosus and C. reuteri. There was no significant difference in percentage feeding of M. caliginosus on eggs (2.2%) or second instar nymphs (8.0%). There was a significant difference in feeding of M. caliginosus adults (18.6%) when offered eggs and second instars in the same arena compared with eggs or second instars offered separately. These results could be attributed to the biological behaviour of the predator having a type III functional response. Studies with the local C. reuteri species showed no significant difference in adult and nymphal consumption on second instars of B. tabaci compared with nymphs on eggs. However, C. reuteri adults fed less on eggs compared with nymphs. This local predatory species appears to be more efficient than M. caliginosus in feeding on particular stages of B. tabaci without depending on prey density. This is further supported by the low consumption of both adults and nymphs in the choice test (4% and 2.3%, respectively) compared with M. caliginosus adults (18.6%).     

Evaluating the Efficacy of Entomopathogenic Fungi against the Bark Beetle,Ips stebbingi Strohmeyer (Coleoptera,Curculionidae: Scolytinae) in India

The bark beetle, Ips stebbingi Strohmeyer (Coleoptera: Curculionidae: Scolytinae) is one of the most serious pests of Pinus wallichiana A. B. Jacks (Pinaceae) in Kashmir Himalaya. In order to find an effective biocontrol agent against this pest, we determined the effectiveness of entomopathogenic fungi, viz. Beauveria bassiana (Balsamo) Vuillemin, Metarhizium anisopliae sensu lato (Metchnikoff) Sorokin, and Lecanicillium lecanii (Zimmerman) Zare et Gams against I. stebbingi in the laboratory conditions. Each fungal suspension contained 1.0 × 10⁹ spores of fungi in 1 ml. The insecticide (cyclone 505 EC) was also used as positive control in the experiment. The mortality caused with these fungi was recorded in treated branches and Petri dish assay. In treated branches, B. bassiana and M. anisopliae caused higher mortality, i.e., 68% and 71.25%, respectively, 10 days after treatment, and 93.10% and 88%, respectively, 20 days after treatment. The results of Petri dish assay revealed that I. stebbingi adults were highly susceptible to both treated fungal species and insecticide. However, B. bassiana and M. anisopliae caused higher percentage mortality six days after treatment, i.e., 94.16% and 100% respectively. The percentage of mortality caused by treating with insecticide was 60%. Lecanicillium lecanii was found significantly less virulent (mortality 18.33%) in all fungal treatments. Results obtained in the present study are promising and may be used as alternative means of chemical control for management of this beetle pest; however, no recommendations concerning the potential use of these fungal pathogens for forest protection can be given, and further studies are needed in this respect, especially under field conditions.

     

Microbial control of cotton pests: use of the naturally occurring entomopathogenic fungus Aspergillus sp. (BC 639) in the management of Bemisia tabaci (Genn.) (Hemiptera: Aleyrodidae) and beneficial insects on transgenic cotton crops

The commercial adoption of transgenic Bacillus thuringiensis (Bt) cotton (Bollgard II®) reduced the use of insecticides to control Helicoverpa spp. However, the ineffectiveness of the Bt toxin against sucking pests such as silverleaf whiteflies (Bemisia tabaci) resulted in a marked increase in B. tabaci populations and in the use of insecticides to control this pest. The effect of the entomopathogenic fungus Aspergillus sp. BC 639 on B. tabaci and beneficial insects (predominantly predatory insects) was studied in commercial cotton field trials. The results showed that oil-based extracts of the entomopathogenic fungus BC 639 control the number of B. tabaci adults and nymphs in commercial transgenic cotton crops. The BC 639 fungus caused 60.0%, 67.2%, and 68.8% mortality in adults, and 54.6%, 62.3%, and 51.7% in nymphs at 7, 14, and 21 days after treatment, respectively, relative to the unsprayed controls. The effect of BC 639 at concentrations of 125, 250, and 500?ml/ha on low-density B. tabaci (～10 nymphs/leaf) did not differ significantly from that of the commercial insecticide (pyriproxifen). However, at higher densities (>50 nymphs per leaf), low concentrations of BC 639 (125 and 250?ml/ha) were not as effective as 500?ml/ha BC 639 in successfully controlling the pest. A simple graphic analysis suggested that the more B. tabaci nymphs per leaf, the fewer adults per leaf, and that once the number of nymphs increased to ～70 per leaf, a negative feedback regulatory effect reduced the survivorship of the nymphs and adults and/or caused the emigration of the adults from the contaminated leaves in search of new resources. Therefore, the ability of BC 639 to control B. tabaci adults and nymphs with minimal effects on predatory insects indicates its potential utility in supplementing integrated pest management programmes for cotton crops.     

Reaction of some tomato cultivars to tomato leaf curl virus and evaluation of the endophytic colonisation with Beauveria bassiana on the disease incidence and its vector,Bemisia tabaci

Tomato line LA1478 and Pusa Ruby were resistant to tomato leaf curl virus (TLCV) disease. They registered higher plant height, number of branches, total phenol content and yield per plant than the other cultivars. Variety Peto 86 was tolerant to the disease while the other popular tomato cultivars, i.e. Ace, Early Pack, Money Maker, Prichard and Strain B were highly susceptible to the disease. Plant height and number of branches per plant revealed significantly positive association with fruit yield per plant. The disease index of TLCV exhibited significant negative correlations with plant height, total phenol content and fruit yield per plant – 0–4 and 5–25 adult whiteflies were observed on resistant susceptible cultivars. In the case of epiphytically colonisation by Beauveria bassiana conidia, not all developing hyphae on the leaf surface penetrated the whitefly cuticle. Many of the germ tubes elongated to a short distance before terminating its growth. On the other hand, the rapid staining of tomato tissues injected with B. bassiana conidial suspension indicates that the entomopathogenic fungus was established inside tomato tissues until the end time of the trial. The direct injection with the spore suspension yielded high post-colonisation, where the fungus was recovered from sites distant from the point of inoculation. This indicates that the fungus has the potential to move throughout the plant tissues. Laboratory bioassay of tomato whitefly feeding on tomato tissues containing B. bassiana conidial spores indicates that plant endophytic colonisation with entomopathogenic fungi may reduce insect survival on these plants. LT₅₀ values of the test diet were between three and four days. The mortality of Bemisia tabaci was high in the case of endophytically colonisation compared to epiphytically one (90.0% compared to 10.0% during three days) for whiteflies fed tomato tissues containing 1.5 × 10⁷ B. bassiana spores/ml. Application of B. bassiana as an artificial endophyte inside tomato plants can be an important component in the integrated control of tomato whiteflies. The endophytic colonising can achieve biocontrol effect based on induced disease resistance in plant tissues. According the available references, this is the first report on B. tabaci controlling by plant endophytic treatment.     

Beauveria bassiana ARP14 a potential entomopathogenic fungus against Bemisia tabaci (Gennadius) and Trialeurodes vaporariorum (Westwood) (Hemiptera: Aleyrodidae)

The sweet potato whitefly, Bemisia tabaci (Hemiptera: Aleyrodidae), and the greenhouse whitefly, Trialeurodes vaporariorum (Hemiptera: Aleyrodidae), are important pests of protected crops grown in warm climates. We compared efficacy of a new strain of the entomopathogenic fungus Beauveria bassiana (ARP14) isolated from Riptortus pedestris (Hemiptera: Alydidae) with a commercial strain (GHA) against different life stages of both B. tabaci and T. vaporariorum. Eggs, nymphs, and adults were exposed to 1 × 10⁸ conidia/mL of each strain using the leaf-dipping method. The mycosis rate of B. tabaci eggs (as a proportion) was relatively low (0.13 for B. bassiana ARP14 and 0.10 for B. bassiana GHA), while, for T. vaporariorum eggs, mycosis rate was 0.44 for B. bassiana GHA and 0.27 for B. bassiana ARP14. However, mycosis rate of 1st instars of both whiteflies was much higher than for eggs, for both strains (ARP14 and GHA). The developmental period of B. tabaci eggs exposed to ARP14 was significantly shorter than for either eggs treated with GHA or the control. For 2nd and 4th instar nymphs and adults of both whiteflies there were no differences in mycosis rates between the two B. bassiana strains. These results suggest that, B. bassiana ARP14 could be commercialized as a native biological control agent for control of B. tabaci and T. vaporariorum.     

A Bioassay for Determining Pathogenicity of Entomogenous Fungi on Whiteflies

A simple and effective laboratory bioassay method for determining the pathogenicity of entomogenous fungi to whiteflies is presented. The bioassay is based on characterization of the growth rate and development of entomogenous fungi on fourth instar nymphs of the silverleaf whitefly (SLWF), Bemisia argentifolii (Bemisia tabaci, Strain B), and is useful in determining the effects of environmental factors (e.g., temperature and humidity) and additives (e.g., surfactants, adjuvants, and pesticides) on the development of entomogenous fungi. Such information can provide a better understanding of the influence of these factors on the performance of entomogenous fungi when tested under field conditions. The bioassay has been successfully implemented for the evaluation of the pathogenicity of Paecilomyces fumosoroseus, Verticillium lecanii, and Beauveria bassiana against fourth instar nymphs of SLWF. It has the potential to be useful as a screening tool for determining pathogenicity of new fungal isolates and for the standardization and quality control of commercial fungal preparations for viability and virulence against insect pests before they are used far field applications.     

Host plant effects on activity of the mitosporic fungi Beauveria bassiana and Paecilomyces fumosoroseus against two populations of Bemisia whiteflies (Homoptera: Aleyrodidae)

Laboratory bioassays were conducted to determine the effect of host plant on mycosis in two geographically distinct populations of early 2nd-instar nymphs of Bemisia argentifolii Bellows & Perring from the entomopathogenic fungi Beauveria bassiana (Balsamo) Vuillemin and Paecilomyces fumosoroseus (Wize) Brown & Smith. Mycosis in B. argentifolii nymphs varied according to the host plant on which the nymphs were reared but not according to the population. Both populations of whiteflies reared on cotton were consistently significantly less susceptible to infection by either fungus than when reared on melon. We hypothesized that the cotton plant produced a fungal inhibitor that may confer protection on whiteflies feeding (and possibly sequestering) upon it. Germination of conidia of both fungi was strongly inhibited (below 12% germination) on the cuticle of nymphs reared on cotton but was over 95% on the cuticle of nymphs reared on melon. We further hypothesized that the terpenoid gossypol, produced by many cultivars of cotton, might have been involved in antibiosis. Gossypol mixed with Noble agar at five concentrations was tested for its effects on germination of conidia of both fungi. P. fumosoroseus was highly tolerant of gossypol, even at the relatively high concentration of 1000 ppm, while B. bassiana tolerated gossypol at concentrations up to 500 ppm and strong inhibition only occurred in presence of gossypol at 1000 ppm. Our in vivo findings on cotton and on the insect's cuticle pointed at a potential host plant-mediated antibiosis. The in vitro tolerance of P. fumosoroseus and partial tolerance of B. bassiana to gossypol disagreed with our in vivo data. Gossypol concentrations higher than 1000 ppm might have increased the sensitivity of the fungi in our in vitro tests. Sequestered gossypol (and/or other cotton plant allelochemicals) by B. argentifolii nymphs would explain, at least partially, the insect's defense against the pathogens.This revised version was published online in October 2005 with corrections to the Cover Date.     

Combined effect of microbial and chemical control agents on subterranean termites

Termite mortality was measured when fungi were combined with bacteria or a chemical termiticide to determine whether a synergistic effect occurred. The fungus Beauveria bassiana was combined with the non-repellant chemical termiticide imidacloprid. Of the three B. bassiana strains tested one, B. bassiana ATCC 90519, was sufficiently pathogenic on its own that the advantage of a supplementary chemical treatment was marginal. The mortality caused by another fungal strain, B. bassiana ATCC 26037, was improved in combination with imidacloprid at both of the tested chemical concentrations over the first 14 days. The remaining fungal strain, B. bassiana ATCC 90518, demonstrated an overall mortality rate in combination with imidacloprid of 82.5%, versus a rate of 65.0% for the fungus alone. The fungus Isaria fumosorosea (Ifr) was combined with the bacterium Bacillus thuringiensis (Bt). On day 5, Ifr, Bt, and the combined treatment at a 10⁶ spores or cells/ml dosage caused 8.8%, 22.5%, and 15.0% mortality, respectively. The Bt and combined mortality rates are not significantly different. Control mortality on day 5 was 5.0%. On day 13 the combined 10⁶ treatment mortality rate was 91.3%, which was significantly higher than all other treatments: control at 17.5%, Ifr at 36.3% and Bt at 35.0%. When Ifr and Bt were applied at a 10⁹ spores or cells/ml dosage, Ifr alone caused a mortality rate of 97.5% as early as day 5. The combination with Bt could not significantly increase the effectiveness of this dosage. These data demonstrate the potential for synergistic effects of fungal and chemical treatment methods, thereby broadening the use of microbial control agents and reducing the quantity of chemical agents necessary to effect control.     

Response of the Wasp Cephalonomia tarsalis (Hymenoptera: Bethylidae) to Beauveria bassiana (Hyphomycetes: Moniliales) as Free Conidia or Infection in Its Host, the Sawtoothed Grain Beetle, Oryzaephilus surinamensis (Coleoptera: Silvanidae)

Cephalonomia tarsalis, an ectoparasitoid, and Beauveria bassiana, an entomopathogenic fungus, are potential biological control agents for the sawtoothed grain beetle, Oryzaephilus surinamensis. Several experiments were conducted to determine whether the two beneficial organisms are compatible. Wasps exhibited little avoidance behavior toward the fungus. Adult wasps oviposited on B. bassiana-infected larvae up to within 1 day of the host's death and the appearance of red fungal pigment. Wasp larvae are susceptible to the fungus and die within 1 day of oviposition on host larvae with mycosis. A 3-h exposure of adult wasps to 100 mg of B. bassiana/kg of wheat resulted in 52.7% mortality. Nevertheless, the wasps entered into grain containing B. bassiana conidia as freely as they entered into conidia-free grain. The mean prevalence of B. bassiana in 46 samples of pooled wheat representing 276 locations was 7.5 colony-forming units/g of wheat. Natural C. tarsalis exposure to B. bassiana in untreated stored wheat is likely to be below lethal quantities, and the introduction of the fungus in insecticidal quantities would have a negative impact on C. tarsalis populations.     

Virulence potential of some fungal isolates and their control-promise against the Egyptian cotton leaf worm,Spodoptera littoralis

A preliminary virulence test of four fungal isolates, Beauveria bassiana IMI 382302, Beauveria bassiana IMI 386701, Trichoderma harzianum T24 and Aspergillus flavus Link against larvae of Spodoptera littoralis was performed. The most effective isolates against larvae of S. littoralis were B. bassiana 302 and T. harzianum T24, which also showed the lower percentage of pupation compared with the other two isolates under the same conditions of treatments. Three concentrations (1 × 10⁶, 1 × 10⁷ and 1 × 10⁸ ml^?1) of the aqueous conidial suspension of the four tested isolates were carried out against both larval and pupal stages of S. littoralis within five days post-treatment. T. harzianum T24 showed 80% larval mortality only when applied at the highest conidial concentration, while A. flavus showed 100% pupal mortality only, at all of its conidial concentrations. However, B. bassiana IMI 382302 showed relatively high dose-dependant larval and pupal mortalities, while strain IMI 386701 of B. bassiana showed a very weak mortality against pupae at higher concentrations, and no virulence against larvae was recorded. Enzymatic and antibiosis bioassays of the four fungal isolates showed relatively high activities against Fusarium spp. for most of the tested isolates. Clear zone of enzyme activity on agar plates proportionally increased with increasing the concentration of enzyme substrate and prolongation of the incubation period. Mtabolites produced in the agar culture inhibited the growth of Fusarium spp. and the productivity differed greatly among isolates or strains of the same isolate. Volatile and non-volatile compounds produced by A. flavus Link showed a higher inhibition activity against Fusarium spp. compared with the other fungal isolates. The humoral antifungal response of insect host is relatively high compared to the anti-bacterial one. Injection of larvae with the immune sensitive bacteria Micrococcus luteus (5 × 10³ bacteria/larva) showed a detectable humoral response by 2 h, peaked around 12 h and became hardly detectable by 24 h post-injection. Injection of larvae with conidial suspension (5 × 10³ conidia/larva) from each of the fungal isolates showed humoral antifungal activity against B. bassiana IMI 386701 and A. flavus only. This activity was detectable by 12 h, peaked around 36 h and became hardly detectable by 48 h post-injection. Although the humoral antifungal response was started slowly compared to the antibacterial one, it lasted for longer and enabled larvae to withstand the infection with these immune-sensitive fungal strains. No humoral activity was detected against B. bassiana IMI 382302, although however, weak activity was detected against T. harzianum T24 only at the low conidial concentration but not at the higher one (1 × 10⁸ ml^?1). Thus, this study concludes that larvae of S. littoralis showed immune-dependant sensitivity to T. harzianum T24 and B. bassiana IMI 382302. Therefore, this study may recommend these two fungal isolates as mycoinsecticides in the battle against cotton leaf worm in Egypt. Hence, they have been selected for future comprehensive bioassays in the laboratory under conditions similar to that in the field. This, in fact, may help for developing effective mycoinsecticides against this pest. Penetration mechanims of insect cuticle by entomopathogenic fungi will be discussed.     

Isolation and virulence of entomopathogenic fungi against the great spruce bark beetle,Dendroctonus micans (Kugelann) (Coleoptera: Scolytidae)

Twelve fungal strains including Lecanicillium muscarium (Petch.) Zare and Gams, Isaria farinosa (Holmsk.) Fr., Fusarium sp., Beauveria bassiana Sensu Lato and Beauveria sp. were isolated from larvae and adults of D. micans. In addition, virulence of these isolates against this pest was determined. Conidia suspensions of 1×10⁶ conidia mL^–1 were applied to larvae and adults. The highest mortality and mycosis for larvae were obtained from isolate ARSEF 9271 (Beauveria bassiana) with 90% mortality and mycosis within 10 days. ARSEF 9271 also produced 93% mortality and mycosis in adults. On the other hand, the highest mortality and mycosis for adults were obtained with isolate ARSEF 9272 (Beauveria sp.), with 100% mortality and 80% mycosis within 10 days. These results indicate that isolates ARSEF 9271 and ARSEF 9272 seem to be the most promising potential fungal biocontrol agents against D. micans.     

Pathogenicity of <Emphasis Type="Italic">Beauveria bassiana</Emphasis> and <Emphasis Type="Italic">Metarhizium anisopliae</Emphasis> to the tobacco spider mite <Emphasis Type="Italic">Tetranychus evansi</Emphasis>

Seventeen isolates of Metarhizium anisopliae (Metschnikoff) Sorokin and two isolates of Beauveria bassiana (Balsamo) Vuillemin were evaluated for their pathogenicity against the tobacco spider mite, Tetranychus evansi Baker & Pritchard. In the laboratory all the fungal isolates were pathogenic to the adult female mites, causing mortality between 22.1 and 82.6%. Isolates causing more than 70% mortality were subjected to dose–response mortality bioassays. The lethal concentration causing 50% mortality (LC₅₀) values ranged between 0.7×10⁷ and 2.5×10⁷ conidia ml⁻¹. The lethal time to 50% mortality (LT₅₀) values of the most active isolates of B. bassiana and M. anisopliae strains varied between 4.6 and 5.8 days. Potted tomato plants were artificially infested with T. evansi and treated with B. bassiana isolate GPK and M. anisopliae isolate ICIPE78. Both fungal isolates reduced the population density of mites as compared to untreated controls. However, conidia formulated in oil outperformed the ones formulated in water. This study demonstrates the prospects of pathogenic fungi for the management of T. evansi.     

Different host plant species modifies the susceptibility of Bactericera cockerelli to the entomopathogenic fungus Beauveria bassiana

Bactericera cockerelli (Sulc.) is a serious pest of solanaceous crops and a vector of the plant pathogen Candidatus Liberibacter psyllaurous. Entomopathogenic fungi are the most important biological control alternatives for this pest. Host plant species, however, can modify the outcomes of insect–pathogen interactions. We conducted laboratory experiments to quantify the virulence of two isolates of the entomopathogenic fungus Beauveria bassiana (Bals. [Vuill.]), BB40 and BB42, against third instar B. cockerelli nymphs maintained on chilli pepper plants. Owing to the lack of difference in virulence against B. cockerelli nymphs on chilli pepper between the two B. bassiana isolates, only BB42 was used to: compare virulence against nymphs maintained on either chilli pepper, potato or tomato; and in vivo conidia production from nymphs maintained on different host plants. Virulence of the two B. bassiana isolates against B. cockerelli nymphs was similar. Bactericera cockerelli nymphs maintained on tomato were more susceptible to B. bassiana than nymphs maintained on potato or chilli peppers. Infected nymphs maintained on chilli peppers produced the greatest number of conidia followed by infected nymphs maintained on tomato and potato. Host plant affected the susceptibility of B. cockerelli to B. bassiana isolate BB42 and subsequent conidia production. The implications of our results for microbial control of B. cockerelli by B. bassiana are discussed.