Serratia marcescens as a rapid indicator of Microctonus hyperodae oviposition activity in Listronotus maculicollis and potential application of the technique to host-specificity testing

Listronotus maculicollis (Dietz) (Coleoptera: Curculionidae) is a potential novel host of the braconid parasitoid Microctonus hyperodae Loan, but init ial studies have shown that levels of parasitism are lower than in the natu ral host L. bonariensis (Kuschel). A novel bacterial indicator test was use d to determine whether the lower level of parasitism was due to behavioural factors, lack of oviposition, or host resistance. The incidence of oviposi tor penetration by the parasitoid M. hyperodae into adult L. maculicollis w as measured by immersing the ovipositor of the parasitoid in the facultativ e pathogen, Serratia marcescens Bizio. Adult weevils were then exposed to p arasitoids for up to 72 h and rapid mortality used as an indicator of ovipo sition penetration. Survival was assessed after six days and surviving weev ils were dissected and examined for parasitoid larvae. Mortality among L. m aculicolis exposed to parasitoids treated with S. marcescens was significan tly higher (P < 0.001) than the controls but significantly lower (P < 0.001 ) than in the natural host, L. bonariensis. Dissection of weevils exposed t o uncontaminated parasitoids revealed that parasitism in L. maculicolis was significantly (P < 0.001) less than parasitism in L. bonariensis. Serratia marcescens-induced mortality plus parasitism of surviving weevils in the p arasitoid plus bacteria treatments produced a similar overall effect. Appli cation of bacteria to the parasitoid ovipositor provided a rapid, simple te st for ovipositor penetration, which shows potential for separation of beha vioural and physiological defence mechanisms in parasitoid/host range studi es.