The overall objective of this PhD thesis was to investigate the interactions that may occur when combining natural enemies of an herbivore. This was done by assessing the non-target effects of the generalist entomopathogenic fungus Metarhizium brunneum on four different predatory arthropods (Publication I) as well as the effects on the resulting biocontrol in a multitrophic context (Publication II) and predator behavioral changes resulting of the entomopathogen presence (Publication III). Exposure to soil-applied M. brunneum under laboratory conditions resulted in a range of fitness responses to the high concentration of M. brunneum in soil, from no response, to intermediate and high response. A greenhouse study was conducted to assess the possible effects of combining the predatory midge Aphidoletes aphidimyza and M. brunneum applied in soil to suppress the aphid population infesting maize plants. A. aphidimyza applied alone suppressed the aphid population more effectively than M. brunneum applied alone. However, the aphid population was most suppressed when both agents were combined, even though the suppression was less than additive. The study of the behavioral changes of A. aphidimyza females in the presence of M. brunneum revealed that gravid A. aphidimyza are able to perceive the risk posed by M. brunneum and react to that by choosing a pathogen-free site for offspring. In conclusion, non-target effects of M. brunneum on predatory arthropods may be expected. However, knowledge of the life cycles of the predatory arthropods and the optimal timing for releasing the natural enemies can reduce the risk of antagonistic interactions. Findings confirm that A. aphidimyza females are able to change their oviposition behavior in the presence of the entomopathogen. It furthermore confirms that the side-effects of soil application of M. brunneum on the performance of a predatory arthropod can be considered minor since it did not disrupt the suppression of aphids.