| Summary: | The tomato potato psyllid (TPP) <i>Bactericera cockerelli</i> is a serious pest of the Solanaceae family. The management of this pest using synthetic pesticides is problematic because of the development of pesticide resistance and environmental concerns including impacts on non-target organisms. The predatory bug <i>Engytatus nicotianae</i> has recently been identified as a useful biocontrol agent for TPP in greenhouses. The soil fungus <i>Trichoderma</i> Pers. is commonly used as a plant growth enhancer and biocontrol agent against phytopathogenic fungi. Therefore, there could be advantages associated with the combined use of these biocontrol agents. Some reports in other systems suggest that <i>Trichoderma</i> inoculation may alter the behaviour of pests and their natural enemies by modifying plant defence metabolites such as volatile organic compounds (VOCs). For this reason, this study aimed to investigate the individual and combined efficacy of these biocontrol agents (i.e., <i>Trichoderma atroviride</i> and <i>E. nicotianae</i>) against TPP in greenhouse grown tomatoes (<i>Solanum lycopersicum</i> cv. Merlice). To this end, we compared the effect of each biocontrol agent and their combination on TPP abundance across different developmental stages (egg, nymphs, adults) and the number of infested leaves. We also investigated plant VOC emissions under the different treatments. Across all measured TPP stages, the treatments tested (<i>E. nicotianae</i> alone, <i>T. atrovirdae</i> alone, and <i>T. atrovirdae</i> + <i>E. nicotianae</i>) significantly reduced mean TPP counts relative to the control, and no significant differences were observed in VOC emissions among treatments. Overall, <i>T. atrovirdae</i> alone was less effective than <i>E. nicotianae</i> alone and its combination with <i>T. atrovirdae</i> in suppressing TPP populations. However, the combined use of <i>Trichoderma</i> + <i>E. nicotianae</i> did not show significant advantages over the use of <i>E. nicotianae</i> alone in controlling TPP. Therefore, their combined use needs to be further assessed in light of other advantages of <i>Trichoderma</i> to the crop (e.g., growth promotion or pathogen defence).
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