Premature Death in Parasitized Listronotus bonariensis Adults Can be Caused by Bacteria Transmitted by the Parasitoid Microctonus hyperodae

Abstract

During experiments to determine the impact of the introduced parasitoid Microctonus hyperodae Loan (Hymenoptera: Braconidae) on the Argentine stem weevil Listronotus bonariensis (Kuschel) (Col.: Curculionidae), it was discovered that weevils frequently died prior to development of the parasitoid. It was postulated that this premature mortality could be caused by an ingress of pathogens in the puncture wounds created during parasitoid oviposition. This hypothesis was tested using the pathogenic bacterium Serratia marcescens Bizio as a model indicator of bacterial transmission by the parasitoid. When the bacteria were injected with a pin, simulating parasitoid attack, 92% of weevils died. Weevils exposed to parasitoids contaminated with bacteria suffered significantly higher mortality than those exposed to parasitoids untreated with bacteria. W hen parasitoids and weevils were confined in Petri dishes on filter paper contaminated with S. marcescens , there was significantly higher weevil mortality than in dishes without bacteria. The treatment of weevils, using the pin injection method, with bacteria isolated from weevils and soil ( Serratia spp., Bacillus spp. and Pseudomonas fluorescens (Trevisan)), produced similar levels of weevil mortality to the test strain. These results strongly suggest that parasitoid-mediated transmission of bacteria causing rapid death of the host can act as a significant mortality factor in biological control programmes, and should be considered in their interpretation.