*Article* **E** ff**ect of Juvenile Hormone on Resistance against Entomopathogenic Fungus** *Metarhizium robertsii* **Di** ff**ers between Sexes**

#### **Markus J. Rantala 1, Ivan M. Dubovskiy 2,3, Mari Pölkki 1, Tatjana Krama 4,5, Jorge Contreras-Garduño 6 and Indrikis A. Krams 4,7,8,\***


Received: 17 September 2020; Accepted: 16 November 2020; Published: 19 November 2020

**Abstract:** Juvenile hormone has been suggested to be a potential mediator in the trade-o ff between mating and insects' immunity. Studies on various insect taxons have found that juvenile hormone interferes with humoral and cellular immunity. Although this was shown experimentally, studies using highly virulent parasites or pathogens are lacking so far. In this study, we tested if juvenile hormone administration a ffected resistance against entomopathogenic fungi, *Metarhizium robertsii*, in the mealworm beetle, *Tenebrio molitor*. In previous studies with *T. molitor,* juvenile hormone has been found to reduce a major humoral immune e ffector-system (phenoloxidase) in both sexes and decrease the encapsulation response in males. Here, we found that juvenile hormone administration prolonged survival time after infection with *M. robertsii* in males but reduced survival time in females. This study indicates that the e ffects of juvenile hormone on insect immunity might be more complicated than previously considered. We also sugges<sup>t</sup> that there might be a trade-o ff between specific and non-specific immunity since, in males, juvenile hormone enhances specific immunity but corrupts non-specific immunity. Our study highlights the importance of using real parasites and pathogens in immuno-ecological studies.

**Keywords:** immune defense; immunocompetence; *Metarhizium robertsii*; pathogens; sex; *Tenebrio molitor*
