**1. Introduction**

The blood-sucking insect *Triatoma infestans*, Klug (Hemiptera: Reduviidae), the main Chagas disease vector in the Southern Cone of South America, has been the target of continuous control programs to reduce the disease transmission risk. The control strategy used has been the indoor application of pyrethroid insecticides [1]. However, it has been recognized that this strategy has limited efficacy, mainly confined to the Gran Chaco area shared by Argentina, Bolivia, and Paraguay [2]. Moreover, in the last years several foci of pyrethroid-resistant *T. infestans* have been documented in wide regions of Bolivia and Argentina [3,4]. These di fficulties make it necessary to search for alternative tools to insect control. The entomopathogenic fungus *Beauveria bassiana* (Ascomycota: Hypocreales) commercial strain GHA has shown to be useful in field interventions, showing promising results in houses infested with pyrethroid-resistant *T. infestans* [5,6].

Fungal pathogenesis depends on many factors that are related to germination capacity, growth rate, and spore yield and production, among other factors [7,8]. But the way in which fungus and host interact is key to knowing the final result of the infection process [9]. *B. bassiana* secretes toxic or immunosuppressive compounds (often referred to as secondary metabolites) during hemocoel invasion, such as the cyclooligomer nonribosomal peptides beauvericin and bassianolide, the diketomorpholine bassiatin, the cyclic peptides beauverolides, the dibenzoquinone oosporein, and the 2-pyridone tenellin [10–12]. Even though the genes involved in some of these secondary metabolites biosynthetic pathways have been studied, most of their biological roles remain to be uncovered [11]. In this regard, their expression pattern when the fungus grows within its insect host might help to better understand their role in pathogenesis. Our research group has developed a methodology based on absolute quantification by qPCR to follow it, reporting an induction of the genes encoding for the synthetase enzymes of the secondary metabolites beauvericin (*BbbeaS*), bassianolide (*BbbslS*), and tenellin (*BbtenS*) during the first days of infection, perhaps to be used as virulence factors, and then in moribund insects and/or cadavers to protect them from competitive microorganisms [13]. On the other hand, insects possess both innate cellular and humoral defense strategies to defend from microbe infections. Humoral response comprises the production of many di fferent antimicrobial peptides, including defensins [13]. *T. infestans* contains six genes encoding for defensins, and their expression is regulated at least by a pair of limpet transcription factors [14].

All these properties allow selecting strains with optimal characteristics to achieve e ffective control results; however, the fungus also need to be host-specific, virulent, and adapted to a regional environment [15]. In this regard, exploration for local isolates is crucial to establish long-term, effective, and sustainable biological control programs. There are only two reports on the isolation of entomopathogenic fungi naturally infecting *T. infestans* in Argentina; i.e., isolates of *B. bassiana* [16] and *Paecilomyces lilacinus* [17]. However, there is abundant information about *T.* infestans-*B. bassiana* interaction [13,18–22], even from a coevolutive perspective [23].

In the present study, we reported the isolation of a native *B. bassiana* strain from a *T. infestans* cadaver. We characterized its phylogeny, growth, virulence, and toxin expression during infection of both pyrethroid-susceptible and pyrethroid-resistant *T. infestans*, and also evaluated the host immune response within the context of a fungus–insect interaction.

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

## *2.1. Fungus Isolation*

As part of periodical entomological interventions performed by members of the Ministry of Public Health of Salta Province, Argentina, a cadaver of *T. infestans* adult with apparent signs of mycosis was isolated in August 2011 at the "Misión Aborigen el Cañaveral", Santa Victoria Este municipality, Rivadavia department, Salta province (22◦1652.78" S; 62◦425.60" W). This place belongs to the Gran Chaco region and is located at the edge of the Pilcomayo River. The cadaver of the infected insect was sterilized with 0.5% sodium hypochlorite and was sowed in a Petri dish with potato dextrose agar (PDA) with chloramphenicol and incubated at 27 ◦C for 7 d to isolate the fungus. The colony obtained was subcultured several times to obtain a pure culture, which was preliminary identified on the basis of macromorphological aspects [24] as *Beauveria* sp. and conserved at the Mycological Culture Collection of the School of Natural Sciences, National University of Salta, Argentina, under the code Bb-C001. The strain GHA *of B. bassiana* (obtained from Laverlam International, Butte, MT, USA) was used as reference for the entire characterization. In order to regain the optimal virulence parameters, both strains were inoculated on *T. infestans* nymphs and recovered in PDA prior to use [8].
