Transmission of a genetically engineered rhizobacterium by grasshoppers inthe laboratory and field

Root-colonizing bacteria have little inherent dispersal ability. However, w e have shown previously that they routinely move into plant stems, sometime s at high densities, and might therefore be acquired and transmitted by fol iage-feeding insects. Here we describe laboratory and field experiments on the transmission of a genetically engineered strain of the root-colonizing bacterium Pseudomonas chlororaphis (formerly aureofaciens) by a common herb ivorous insect, the red-legged grasshopper Melanoplus femurrubrum, In laboratory and field microcosms, the engineered P. chlororaphis were app lied to corn seeds at planting and allowed to colonize the developing roots and to invade the aerial tissues naturally. Adult M. femurrubrum, that lat er fed on the foliage of these plants became infested with the bacteria. Th e bacteria were harbored throughout the digestive tracts and elsewhere in t he exposed grasshoppers and could be recovered from their frass. Grasshoppe rs carried the bacteria for greater than or equal to 1 wk after removal fro m the source of inoculum and could transmit the bacteria to new plants. In the laboratory, the likelihood of transmission declined as the proportion o f grasshoppers remaining infested decreased through time. Transmission was less predictable in the field. Transport by insects may make naturally occurring soil bacteria more mobile than previously thought and could make the containment of engineered strai ns unlikely. However, at least in this system, microbial movement through p lants and insects in the field was accurately predicted by laboratory studi es, which therefore remain useful in basic research in microbial ecology, a nd in risk assessment.