**Low <sup>E</sup>**ffi**cacy of** *Isaria fumosorosea* **against Box Tree Moth** *Cydalima perspectalis***: Are Host Plant Phytochemicals Involved in Herbivore Defence against Fungal Pathogens?**

#### **Rostislav Zemek 1,2,3,\*, Jana Konopická 3,4 and Zain Ul Abdin 1,2,5**


Received: 16 November 2020; Accepted: 3 December 2020; Published: 6 December 2020

**Abstract:** *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 × 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 *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 agen<sup>t</sup> 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.

**Keywords:** *Buxus*; entomopathogenic fungi; invasive pests; virulence; alkaloids; antimicrobial activity
