Oral insecticidal activity of plant‐associated pseudomonads

Summary Biocontrol pseudomonads are most known to protect plants from fungal diseases and to increase plant yield, while intriguing aspects on insecticidal activity have been discovered only recently. Here, we demonstrate that Fit toxin producing pseudomonads, in contrast to a naturally Fit ‐deficient strain, exhibit potent oral activity against larvae of S podoptera littoralis , H eliothis virescens and P lutella xylostella , all major insect pests of agricultural crops. Spraying plant leaves with suspensions containing only 1000 P seudomonas cells per ml was sufficient to kill 70–80% of S podoptera and H eliothis larvae. Monitoring survival kinetics and bacterial titres in parallel, we demonstrate that P seudomonas fluorescens   CHA0 and P seudomonas chlororaphis   PCL1391 , two bacteria harbouring the Fit gene cluster colonize and kill insects via oral infection. Using Fit mutants of CHA0 and PCL1391 , we show that production of the Fit toxin contributes substantially to oral insecticidal activity. Furthermore, the global regulator GacA is required for full insecticidal activity. Our findings demonstrate the lethal oral activity of two root‐colonizing pseudomonads so far known as potent antagonists of fungal plant pathogens. This adds insecticidal activity to the existing biocontrol repertoire of these bacteria and opens new perspectives for applications in crop pest control and in research on their ecological behaviour.