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Effect of Air Humidity on the Infection Potential of Hyphomycetous Fungi as Mycoinsecticides for Trialeurodes vaporariorum

Two commercial mycoinsecticides, 'Mycotal', based on Verticillium lecanii and 'Naturalis-L', on Beauveria bassiana, were used at dosages recommended by manufacturers to examine effects of a range of medium and high humidities on infection towards Trialeurodes vaporariorum. Two series of bioassays were carried out using special devices. In one series, a non-excised bean leaf was sandwiched in order to create a chamber above the whitefly-infected area. In another series, three potted whole bean plants were maintained in an airtight plastic box. In both series, conducted at 25°C, relative humidity (RH) was controlled by injecting a circulating constant flow of humidity-regulated air which had passed over a salt solution used to maintain targeted air humidity levels (ranging from 53 to 98.5 and 75 to 98.5% RH, respectively). Microclimatic measurements demonstrated that non-excised leaves significantly increased the air humidity in the air circulating in the immediate environment of the leaf surface as well as that of the whole plant. In the first series of assays under all RH conditions tested (from 70 to 98% RH after passage above the leaves), both mycoinsecticides induced high mortality rates (92-100%). The fungal effect occurred early, since at least 70% of whiteflies died as second instars, before the next molt occurred. Fungus-induced mortality appeared to be independent of the expected humidity level. In the second series of assays, mortality rates were high (98-100%) and did not depend on the ambient humidity conditions tested (from 80 to 100% RH). Mortality rates occurring during the second instar ranged from 46 to 88%. Implication of these results for microbial control of whiteflies in greenhouses and in the field is discussed in relation to plant transpiration-induced increase in air humidity prevailing in the habitat of the targeted whitefly larvae.