**1. Introduction**

Box trees, *Buxus* sp., are important evergreen shrubs occurring in natural *Buxus* forests [1] or grown as ornamental trees in city parks in Europe. They are now endangered by the box tree moth (BTM), *Cydalima perspectalis* (Walker) (Lepidoptera: Crambidae), which is a serious invasive pest native to Asia that was first detected in Germany in 2007 and has since invaded a large area causing significant damage [2,3]. This pest overwinters at the larval stage [4,5] and can have two to four generations per year in Europe depending on abiotic conditions [6]. Natural enemies do not suppress the *C. perspectalis* population, which is probably because this exotic species does not seem to be a good host for native parasitoids [4,7,8]. Thus, the pest is able to destroy *Buxus* tree completely in one season [9]. Some synthetic chemical insecticides are effective in *C. perspectalis* control [10]. Still, their application in natural habitats is problematic because of their adverse side e ffects on non-target species. Their frequent application possibly leads to the risk of resistance development in the pest.

The use of microbial biopesticides against BTM offers a unique alternate solution to broad-spectrum chemical insecticides. The best results have been obtained by using products based on*Bacillus thuringiensis* Berliner (Bacillales: Bacillaceae) while entomopathogenic nematode *Steinernema carpocapsae* (Weiser) (Rhabditida: Steinernematidae) was less successful [11]. Entomopathogenic fungi (EPFs) represent other promising biocontrol agents. Their advantages are that they do need to be ingested as they are able to penetrate the host cuticle and can be relatively easily produced [12]. A number of mycoinsecticides, most commonly based on *Beauveria bassiana* (Balsamo) Vuillemin (Hypocreales: Cordycipitaceae), *Metarhizium anisopliae* (Metsch.) Sorokin, (Hypocreales: Clavicipitaceae), *Isaria fumosorosea* (WIZE) Brown & Smith (Hypocreales: Cordycipitaceae), and *B. brongniartii* (Saccardo) (Hypocreales: Cordycipitaceae) have been developed in the world [13]. To our knowledge, only *B. bassiana* strain GY1-17 was tested against BTM in Korea, but larvae were not a ffected significantly [14].

The present study aimed to assess the possibility of fungal biocontrol of *C. perspectalis* by *I. fumosorosea*, which is known to be virulent to many insect species including a wide variety of butterflies and moths [15–17] and has received significant attention as a possible biological control agen<sup>t</sup> for several economically important pests [18]. The obtained results showed low e fficacy of the fungus against this pest. Therefore, additional experiments were conducted to test the hypothesis that the low e ffectiveness might be due to the antifungal activity of some host plant phytochemicals consumed by the moth larvae. Low-temperature scanning electron microscopy revealed that the number of *I. fumosorosea* conidia did not germinate. In vitro experiments confirmed that the hydroalcoholic extract of *Buxus* leaves suppressed spore germination and fungus growth.

#### **2. Materials and Methods**
