DISPERSAL OF ERWINIA-AMYLOVORA AND PSEUDOMONAS-FLUORESCENS BY HONEY-BEES FROM HIVES TO APPLE AND PEAR BLOSSOMS

Ability of honey bees (Apis mellifera) to disperse the fire blight pathogen, Erwinia amylovora strain 153nal(R), and Pseudomonas fluorescens strain PfA506, a biocontrol agent for fire blight, from their hives to pistilate surfaces of pome fruit blossoms was investigated. Two hives, one to disperse Ea153nal(R) and the other to disperse PfA506, were placed in an apple orchard of cultivar Rome (1990 and 1991) or Golden Delicious (1992) during bloom. Pollen inserts were attached to each hive, which forced the bees to walk through a freeze-dried preparation of either bacterium as they exited. PfA506 inoculum contained 10(10) to 10(11) colony-forming units per gram (cfu/g) and was dispersed at 10(5) to 10(6) cfu/bee. Inoculum of Ea153nal(R) contained 10(9) to 10(11) cfu/g and was dispersed at 10(4) to 10(6) cfu per bee. In several experiments, a lognormal distribution described the variation in density of bacterial cfu/g bee. Each year, apple blossoms were sampled at various distances from the hives. Maximum frequency of recovery of PfA506 in sampled blossoms ranged from 23 (1990) to 81% (1991). Detection of Ea153nal(R) in blossoms was more variable, ranging from < 1 (1990) to 72% (1992). In experiments on pear, a single beehive was placed in a planting of 40 16-yr-old pear trees of cultivar Bartlett enclosed within a cage constructed of 30% shade cloth. Bees within the enclosed area were inoculated with freeze-dried Ea153nal(R) as they exited the hive through a pollen insert. Maximum detection of Ea153nal(R) in blossoms was 41% in 1991 and 27% in 1992. Incidence of fire blight was 9 and 41% of blossom clusters in 1991 and 1992, respectively. Among all experiments, the estimated efficiency of bees as bacterial vectors averaged 20 blossoms per hour of foraging activity. Use of honey bees to disperse bacteria should prove valuable as a method to inoculate flowers of pome fruits with F. amylovora, and to study ecology of bacterial epiphytes within conventionally managed orchards.