Low Efficacy of Isaria fumosorosea against Box Tree Moth Cydalima perspectalis: Are Host Plant Phytochemicals Involved in Herbivore Defence against Fungal Pathogens?

Buxus sp. is an important native and ornamental tree in Europe threatened by a serious invasive pest Cydalima perspectalis. 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 Buxus trees grown wildly or in city parks. Laboratory experiments were conducted to assess the efficacy of entomopathogenic fungus Isaria fumosorosea strain CCM 8367 against C. perspectalis. The last-instar larvae of the box tree moth were treated by the suspension of fungus conidia at concentrations ranging from 1 × 104 to 1 × 108 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 C. perspectalis to I. fumosorosea. 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 B. sempervirens leaves significantly inhibits both the germination of I. fumosorosea conidia and fungus growth. It can be speculated that the strain CCM 8367 of I. fumosorosea is not a potent biocontrol agent against C. perspectalis 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.