INSECT-MEDIATED DISPERSAL OF THE RHIZOBACTERIUM PSEUDOMONAS-CHLORORAPHIS

The southern corn rootworm, Diabrotica undecimpunctata subsp. howardi, a common and mobile insect pest, was shown to transmit the rhizobacte rium Pseudomonas chlororaphis strain L11 between corn plants. Strain L 11 has been genetically modified to contain the lacZY genes from Esche richia coli. It can reach high densities on roots and invade the roots and move into the foliage. D. undecimpunctata subsp. howardi became i nfested with L11 as larvae while feeding on roots of seed-inoculated c orn and retained the bacteria through pupation, molting to the adult s tage, and emergence from the soil. Bacterial densities on or in the in sects increased 100-fold after they fed again as adults on L11-infeste d foliage. Adults retained the bacteria for at least 2 weeks after las t exposure and could transmit L11 to new plants. The likelihood of tra nsmission decreased with time since last exposure to L11, but increase d with time spent on the new plants. This research demonstrates that r hizobacteria can escape the rhizosphere by moving in or onto foliage, where they can then be acquired and transmitted by insects. This trans mission route may be common among naturally occurring rhizobacteria an d facilitate the dispersal of both beneficial and harmful soilborne mi croorganisms.