A microcosm method was developed to investigate survival and fate of genetically engineered bacteria associated with plant surfaces and a plant-feeding insect, the variegated cutworm, Peridroma saucia. Larvae on radish plants in microcosms were sprayed with nonrecombinant Pseudomonas cepacia and a recombinant strain of P. cepacia carrying the transmissible plasmid R388::Tn1721. Immediately after spraying, P. cepacia comprised about 20%-30% of the total population on leaves. After 2 days, the total epiphytic population had increased approximately fourfold, while the P. cepacia strains had decreased to 2%-30% of their initial numbers. Plasmid transfer from P. cepacia R388::Tn 1721 to the nonrecombinant recipient P. cepacia strain was not observed. (Copyright (c) Springer-Verlag New York Inc. 1987.)