| Summary: | <i>Buxus</i> sp. is an important native and ornamental tree in Europe threatened by a serious invasive pest <i>Cydalima perspectalis</i>. The larvae of this moth are able to defoliate box trees and cause their death. The development of novel biopesticides targeting this pest might help protect <i>Buxus</i> trees grown wildly or in city parks. Laboratory experiments were conducted to assess the efficacy of entomopathogenic fungus <i>Isaria fumosorosea</i> strain CCM 8367 against <i>C. perspectalis</i>. The last-instar larvae of the box tree moth were treated by the suspension of fungus conidia at concentrations ranging from 1 × 10<sup>4</sup> to 1 × 10<sup>8</sup> spores per 1 mL. Fungus infection was observed mostly in pupae, but the maximum mortality did not exceed 60%, indicating a very low susceptibility of <i>C. perspectalis</i> to <i>I. fumosorosea</i>. Furthermore, a number of ungerminated fungal conidia were found on larval cuticles using a low-temperature scanning electron microscopy. Our data also reveal that the hydroalcoholic extract from <i>B. sempervirens</i> leaves significantly inhibits both the germination of <i>I. fumosorosea</i> conidia and fungus growth. It can be speculated that the strain CCM 8367 of <i>I. fumosorosea</i> is not a potent biocontrol agent against <i>C. perspectalis</i> and low virulence of the fungus might be due to the accumulation of host plant phytochemicals having antimicrobial activity in larval cuticle of the pest.
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