Efficiency of the entomopathogenic fungus Verticillium lecanii in the ...

ProbitBouhous and Larous 2441Log time (hours)Development stages transfer of the insectsmaintained at an RH of 70% for 96 h did notinfluenced the mortality rates as compared with thecontrol maintained at 100% during the completeexperimentation period.Drummond et al. (1987) observed that more thefungus is pathogenic more it is independent on lowhumidity. However, Fargues et al. (2005) showed thatthe entomopathogenic Hyphomycetes have strongpotential for microbial control of whitefly larvaeinfesting tomato crops at moderate ambient humidityin Mediterranean greenhouses. The characteristic ofV. lecanii regarding the relative humidity could be ofgreat importance for its application in greenhouseswhere important humidity fluctuations are noted.Figure 10. Effect of relative humidity (70%) on T.vaporariorum eggs mortality after treatment with V. lecanii.Used dose =1.5x10 7 spores /ml.development inevitably passes by larval stages, whichlet us suppose that eggs that have escaped thetreatment will be attacked in the larval stage. Thepercentage of mortality of spore-treated eggs andlarvae increased throughout the incubation period inexperimental conditions, thus, fungus efficiencyrequires some contact time.Treatment efficiency depends closely on a highrelative humidity rate (95 to 100%) necessary to thefungal spore germination. A decrease in humidityattenuates the fungal infectious capacity (Milner andLuton, 1986). It is then recommended to maintain avery high relative humidity after treating the aleurodeby the fungus (Drummond et al., 1987). It is clear thathumidity in greenhouses is not constant, its recordingperformed in Jijel during May and June 2008, showeda decrease in the weekly mean from 57.34% in themorning to 96.22% in the night. The other importantfactor is the capacity of fungal spore germinationwhich should be higher than 90%, for the studiedstrain, spore germination reached 92% after 12 h ofincubation in the experimental conditions, which isconsidered as a good result.Germination capacity could be influenced byseveral factors such as spore age, temperature aswell as relative humidity; consequently, evaluation ofgermination capacity of the studied strain should becarried out permanently before each biological controltreatment. It was reported that one of the properties ofhighly pathogenic and virulent strains is their rapidgermination. According to Hall (1984), 50% of thespores of virulent strains germinate after 9 h ofincubation at 100% RH.Spore germination rate of the studied strain reached65% after 9 h of incubation at 100% RH. This fungalproperty is very important because a relative humidityhigher than 95% rarely persists in greenhouses duringmore than 12 h, as showed by the recordings carriedout in a greenhouse of Jijel. Because of the rapidgermination of the fungal strain, it depends a little onthe RH after an incubation period of 16 h at 100% RH.ConclusionBased on the realised tests, the entomopathogenicfungus V. lecanii is particularly infectious for larvaland adult stages, and slightly infectious for eggs. Inlarvae, LD 50 is relatively low (0.5x10 3 spores/ml) in theexperimentation conditions. The action time of thefungus reached its maximum in day 7. Concerning theeffect of humidity on the development of the fungusand on its virulence, results presented here allowedus to consider a future practical application of V.lecanii in the biological control of T. vaporariorum.Further applied research is required to determine thegreenhouse application conditions.REFERENCESBliss CI (1935). 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