2440 Afr. J. Microbiol. Res.Figure 8. T. vaporariorum larvae with an abnormal shape surrounded with V. <strong>lecanii</strong>mycelium. G 40X0.75.Figure 9. An adult T. vaporariorum surrounded with V. <strong>lecanii</strong> mycelium G 40X0.75.larvae, adults and eggs showed that <strong>the</strong> observedmortality rates on <strong>the</strong> different stages treated byfungal spores are important and <strong>in</strong> particular <strong>in</strong> <strong>the</strong>case <strong>of</strong> larvae (LD 50 : 0.5x10 3 spores/ml).Drummond et al. (1987) found an LD 50 <strong>of</strong> 1.5x10 5spores/ml <strong>in</strong> <strong>the</strong> same conditions <strong>of</strong> temperature andhumidity which confirmed <strong>the</strong> efficiency <strong>of</strong> <strong>the</strong> usedstra<strong>in</strong> aga<strong>in</strong>st <strong>the</strong> development <strong>of</strong> T. vaporariorum. Incontrast to <strong>the</strong> results obta<strong>in</strong>ed with larvae, <strong>the</strong>efficiency <strong>of</strong> eggs treatment by fungal spores is lowerwith an LD 50 <strong>of</strong> 5.9x10 6 spores /ml. These results areconform<strong>in</strong>g to those obta<strong>in</strong>ed by Hall (1982), whichobserved that, for adult <strong>in</strong>sects, fungal sporetreatment was very efficient <strong>in</strong> small experimentalgreenhouses. In our experiments, <strong>the</strong> substitution <strong>of</strong>experimental greenhouses with desiccators confirmedthis efficiency. Therefore, 81 adults over 120 usedwere completely <strong>in</strong>vaded by <strong>the</strong> fungal mycelium.O<strong>the</strong>rs died by drown<strong>in</strong>g due to water condensationon desiccator’s walls.Eggs, larvae as well as adults treatment showedthat <strong>the</strong> used V. <strong>lecanii</strong> stra<strong>in</strong> is very pathogenicaga<strong>in</strong>st T. vaporariorum particularly <strong>in</strong> larval stage.The lower pathogenesis on eggs could be expla<strong>in</strong>edby <strong>the</strong> presence <strong>of</strong> <strong>the</strong> rigid cuticle which h<strong>in</strong>ders <strong>the</strong>penetration <strong>of</strong> <strong>the</strong> mycelium. Given that <strong>in</strong>sect
ProbitBouhous and Larous 2441Log time (hours)Development stages transfer <strong>of</strong> <strong>the</strong> <strong>in</strong>sectsma<strong>in</strong>ta<strong>in</strong>ed at an RH <strong>of</strong> 70% for 96 h did not<strong>in</strong>fluenced <strong>the</strong> mortality rates as compared with <strong>the</strong>control ma<strong>in</strong>ta<strong>in</strong>ed at 100% dur<strong>in</strong>g <strong>the</strong> completeexperimentation period.Drummond et al. (1987) observed that more <strong>the</strong><strong>fungus</strong> is pathogenic more it is <strong>in</strong>dependent on lowhumidity. However, Fargues et al. (2005) showed that<strong>the</strong> <strong>entomopathogenic</strong> Hyphomycetes have strongpotential for microbial control <strong>of</strong> whitefly larvae<strong>in</strong>fest<strong>in</strong>g tomato crops at moderate ambient humidity<strong>in</strong> Mediterranean greenhouses. The characteristic <strong>of</strong>V. <strong>lecanii</strong> regard<strong>in</strong>g <strong>the</strong> relative humidity could be <strong>of</strong>great importance for its application <strong>in</strong> greenhouseswhere important humidity fluctuations are noted.Figure 10. Effect <strong>of</strong> relative humidity (70%) on T.vaporariorum eggs mortality after treatment with V. <strong>lecanii</strong>.Used dose =1.5x10 7 spores /ml.development <strong>in</strong>evitably passes by larval stages, whichlet us suppose that eggs that have escaped <strong>the</strong>treatment will be attacked <strong>in</strong> <strong>the</strong> larval stage. Thepercentage <strong>of</strong> mortality <strong>of</strong> spore-treated eggs andlarvae <strong>in</strong>creased throughout <strong>the</strong> <strong>in</strong>cubation period <strong>in</strong>experimental conditions, thus, <strong>fungus</strong> efficiencyrequires some contact time.Treatment efficiency depends closely on a highrelative humidity rate (95 to 100%) necessary to <strong>the</strong>fungal spore germ<strong>in</strong>ation. A decrease <strong>in</strong> humidityattenuates <strong>the</strong> fungal <strong>in</strong>fectious capacity (Milner andLuton, 1986). It is <strong>the</strong>n recommended to ma<strong>in</strong>ta<strong>in</strong> avery high relative humidity after treat<strong>in</strong>g <strong>the</strong> aleurodeby <strong>the</strong> <strong>fungus</strong> (Drummond et al., 1987). It is clear thathumidity <strong>in</strong> greenhouses is not constant, its record<strong>in</strong>gperformed <strong>in</strong> Jijel dur<strong>in</strong>g May and June 2008, showeda decrease <strong>in</strong> <strong>the</strong> weekly mean from 57.34% <strong>in</strong> <strong>the</strong>morn<strong>in</strong>g to 96.22% <strong>in</strong> <strong>the</strong> night. The o<strong>the</strong>r importantfactor is <strong>the</strong> capacity <strong>of</strong> fungal spore germ<strong>in</strong>ationwhich should be higher than 90%, for <strong>the</strong> studiedstra<strong>in</strong>, spore germ<strong>in</strong>ation reached 92% after 12 h <strong>of</strong><strong>in</strong>cubation <strong>in</strong> <strong>the</strong> experimental conditions, which isconsidered as a good result.Germ<strong>in</strong>ation capacity could be <strong>in</strong>fluenced byseveral factors such as spore age, temperature aswell as relative humidity; consequently, evaluation <strong>of</strong>germ<strong>in</strong>ation capacity <strong>of</strong> <strong>the</strong> studied stra<strong>in</strong> should becarried out permanently before each biological controltreatment. It was reported that one <strong>of</strong> <strong>the</strong> properties <strong>of</strong>highly pathogenic and virulent stra<strong>in</strong>s is <strong>the</strong>ir rapidgerm<strong>in</strong>ation. Accord<strong>in</strong>g to Hall (1984), 50% <strong>of</strong> <strong>the</strong>spores <strong>of</strong> virulent stra<strong>in</strong>s germ<strong>in</strong>ate after 9 h <strong>of</strong><strong>in</strong>cubation at 100% RH.Spore germ<strong>in</strong>ation rate <strong>of</strong> <strong>the</strong> studied stra<strong>in</strong> reached65% after 9 h <strong>of</strong> <strong>in</strong>cubation at 100% RH. This fungalproperty is very important because a relative humidityhigher than 95% rarely persists <strong>in</strong> greenhouses dur<strong>in</strong>gmore than 12 h, as showed by <strong>the</strong> record<strong>in</strong>gs carriedout <strong>in</strong> a greenhouse <strong>of</strong> Jijel. Because <strong>of</strong> <strong>the</strong> rapidgerm<strong>in</strong>ation <strong>of</strong> <strong>the</strong> fungal stra<strong>in</strong>, it depends a little on<strong>the</strong> RH after an <strong>in</strong>cubation period <strong>of</strong> 16 h at 100% RH.ConclusionBased on <strong>the</strong> realised tests, <strong>the</strong> <strong>entomopathogenic</strong><strong>fungus</strong> V. <strong>lecanii</strong> is particularly <strong>in</strong>fectious for larvaland adult stages, and slightly <strong>in</strong>fectious for eggs. Inlarvae, LD 50 is relatively low (0.5x10 3 spores/ml) <strong>in</strong> <strong>the</strong>experimentation conditions. The action time <strong>of</strong> <strong>the</strong><strong>fungus</strong> reached its maximum <strong>in</strong> day 7. 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