**1. Introduction**

Synthetic insecticides are important pesticides for both agricultural and domestic pest control. Among them, neonicotinoids (e.g., acetamiprid, imidacloprid, thiacloprid) are the most extensively used worldwide because of their high effectiveness in controlling crop and domestic pests [1]. They account for more than 25% of the global insecticide market and are now being considered as a replacement for many existing conventional insecticide classes [2]. Neonicotinoids' popularity is largely due to their physicochemical properties, high effectiveness, low resistance, and the fact that they are less harmful to mammals compared to other insecticides. For example, to protect fruit plants against pests, up to 3 g per 100 m<sup>2</sup> of a popular pest control product (e.g., Mospilan 20 SP) is used with acetamiprid as the

active substance are used and, as a consequence, acetamiprid penetrates the soil and bioaccumulates. According to data from the European Food Safety Authority (EFSA), 14C-acetamiprid was identified as the major constituent of the radioactive residues in all plant parts (the study included eggplants, apples, carrots and cabbage) in an amount of 30–90% within 14–90 days after the last application [3]. The ability of neonicotinoids to accumulate in plants is known to increase the probability of environmental contamination and exposure to nontarget organisms [3–5]. In addition, despite the lack of recognition of acetamiprid as a compound persisting in soil, its degradation in environmental conditions has been found to last up to 43 days [3].

Among all methods of insect control, biological methods deserve special attention. *Metarhizium* sp., the common insect pathogens in wildlife, are very e fficient bioinsecticides usually applied in practice [6]. Their e ffectiveness against insects depends mainly on their infective potential, i.e., the ability to produce extracellular lytic enzymes and secondary metabolites [7,8]. While the extracellular enzymes are well studied, equally important secondary metabolites, which play critical roles in the ability of *Metarhizium* to successfully parasitize their hosts and ultimately contribute to the success or failure of these fungi as biological control agents, are quite often neglected [9]. Entomopathogenic fungi produce a variety of bioactive metabolites including >40 cyclic hexadepsipeptides destruxins (dtxs) [8]. It is suggested that in attacked insects, dtxs induce paralysis and muscle contraction via muscle depolarization by the direct opening of Ca2+ channels in the membrane [10,11]. Besides their insecticidal activity, dtxs have also potential as pharmaceuticals showing antiviral, antitumor, cytotoxic, immunosuppressant or antiproliferative e ffects [12,13]. However, due to the endophytic properties of *Metarhizium* sp., the presence of dtxs can be found in, e.g., potatoes [14], maize and strawberries [15], causing dtxs to enter the food chain and pose a threat to human health.

The aim of this work was to determine the influence of acetamiprid on the growth and the secondary metabolism of *Metarhizium,* which is considered as a significant factor during the pest infection process. Furthermore, we checked whether acetamiprid could be accumulated by *Metarhizium*, how it a ffected the production of dtxs, and whether it a ffected the ability of fungi to infect insects. This study could help to understand the potential risks of a harmful influence of acetamiprid on soil-inhabiting fungi and their infectious potential, which plays an important role in maintaining the ecological balance.
