| Summary: | The galling insect <i>Leptocybe invasa</i> Fisher & La Salle (Hymenoptera: Eulophidae) is a major limiting factor in the cultivation of drought-tolerant eucalyptus. The insect <i>L. invasa</i> is a major pest of eucalyptus plantations, and <i>Beauveria bassiana</i> (Bals.) Vuill (Ascomycota: Hypocreales) is being investigated as a potential biocontrol agent against this pest. The fungus <i>B. bassiana</i> can produce metabolites that affect insect biology and survival. Here, we investigated the ability of the entomopathogenic <i>B. bassiana</i> to endophytically develop and induce resistance to <i>L. invasa</i> in a drought-tolerant eucalyptus hybrid. In a greenhouse under semi-controlled conditions, a group of seedlings were sprayed with a solution containing the fungal spores of <i>B. bassiana</i>. The uninoculated seedlings and seedlings inoculated were infested with <i>L. invasa</i>, and their morphometric responses, gas exchange, and chlorophyll indexes were assessed. The number of leaves and height of the inoculated plants was higher than those of the uninoculated plants. The mean CO<sub>2</sub> assimilation rate (A) and transpiration rate (E) were higher for inoculated plants. The inoculated plants showed higher chl a and chl b contents. Compared to the uninoculated plants, the inoculated plants developed much fewer galls, while some showed only scar formations where <i>L. invasa</i> deposited its eggs. These results indicated that inoculating <i>Eucalyptus</i> with <i>B. bassiana</i> promoted resistance to <i>L. invasa</i>. To the best of our knowledge, this is the first study showing that an entomopathogenic fungus can develop endophytically to promote resistance against a galling insect pest.
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