Above- and below-ground applications of fungal biocontrol agents have variable effects on two-spotted spider mites in three strawberry cultivars

Entomopathogenic fungi (EPF) can be applied directly to strawberry canopies as biopesticides, while many EPF strains also display a plant-associated lifestyle as root colonisers and endophytes. Fungal root colonisation can affect aboveground herbivores indirectly, potentially providing an additional beneficial effect of biocontrol fungi applied to plant growth substrates. Here, we applied two strains of the EPF Metarhizium brunneum (KVL16-36 and KIS1868) and a biocontrol fungus used against plant pathogens, Clonostachys rosea (KIS1881) to strawberry plants in two different ways; either by immersion of leaf discs in conidial suspension (aboveground application) or as drench inoculation to the growth substrate (belowground application). Three strawberry cultivars ('Clery', 'Faith', 'Rumba') were tested. We evaluated the effects of fungal strains and cultivars on individual development of the two-spotted spider mite Tetranychus urticae eggs on leaf discs and on T. urticae population growth on leaves of whole plants, respectively. In experiments with immersed leaf discs, both M. brunneum strains significantly decreased egg survival and prolonged the development time of hatched T. urticae compared to the control treatment, while C. rosea only had a significant effect on developmental time on the cultivars 'Clery' and 'Rumba'. However, using drench inoculation in whole plant experiments, M. brunneum KIS1868 increased mite population size on strawberry leaves compared to M. brunneum KVL16-36 and C. rosea, although this increase was not significantly higher compared to the control. Individual fungal biocontrol agents may therefore differently affect the same pest organisms depending on the application method, which must be considered in biocontrol programmes.