Entomopathogenic fungal endophyte-mediated tritrophic interactions between Spodoptera littoralis and its parasitoid Hyposoter didymator

Abstract

The use of entomopathogenic fungi for pest control is gaining increasing attention. These fungi act as contact biological insecticides but also via endophytic colonization of targeted crops. In addition, the joint use of entomopathogenic fungi and natural enemies hold potential in many pest control programs. Here, we evaluated in vitro and in planta multitrophic interactions among the endophytic fungus Metarhizium brunneum colonizing melon (Cucumis melo) plant, the parasitoid Hyposoter didymator and the pest Spodoptera littoralis. In all experiments, total mortality of S. littoralis larvae was significantly affected by the presence of the parasitoid; the treatments including the parasitoid achieved the highest mortality rates both in vitro and in planta. Simultaneous exposure to the fungus and the parasitoid did not significantly increase the total mortality of S. littoralis larvae than the parasitoid alone. The time between exposure to fungus and parasitoid attack did not affect S. littoralis mortality, nor parasitism (parasitoid) or infection rates (fungus). However, the parasitoid showed a significant preference for larvae fed on control plants (24.4% parasitism) compared with larvae fed on fungus-colonized plants (4.4%). A histological study of S. littoralis larvae simultaneously attacked by H. didymator and M. brunneum showed, for the first time, the coexistence of both agents within the same host; parasitoid larvae developed inside the host despite fungal colonization. This provides key information about intra-host interactions between two important biological control agents when used together for S. littoralis control.