Potential of Osmia cornuta as a carrier of antagonistic bacteria in biological control of fire blight: a comparison with Apis mellifera

Studies were carried out in 2002 and 2003 to evaluate the suitability of the highly efficient pollinator of pear Osmia cornuta (Hymenoptera Megachilidae) as a carrier of a rifampicin resistant mutant of the Bacillus subtilis strain BS-F4 (BS-F4rif) on pear flowers cultiver 'Abbé Fétel' for the biological control of fire blight, and its capacity was compared with that of Apis mellifera. A dispenser for the spread of bacteria by O. cornuta was successfully developed. Females exited and entered the dispenser through the proper pathways 81.4% and 97.7% of the attempts, respectively, and the number of BS-F4rif reisolated from the body of O. cornuta exiting the dispenser ranged from 104 to 107 CFU/insect. Individuals of A. mellifera carried an average population of 104 CFU/insect. In net screened tunnel conditions, the efficiency of O. cornuta to carry and deposit the BS-F4rif on pear flowers during a single visit was higher than that of A. mellifera. Both in 2002 and 2003, the dispersal ability of bees was assessed in a 7 ha pear orchard. Trees were labelled at 2, 10, 50, 110 m from the nesting shelter, in the direction of the four cardinal points. Flowers on branches covered with fine netting acted as controls. Mason bees distributed 30 g of the biocontrol agent (BCA). Fifty flowers from both exposed and netted branches were sampled at 3, 7 and 10 days (the latter only in 2003) after the beginning of distribution. BS-F4rif was not recovered from any of the flowers on net-covered branches, whereas the bacterium was consistently recovered on the flowers of non-screened plants. Variation in distribution and population density of the BCA among flowers were observed, with greater colonisation in 2002 than 2003. We conclude that O. cornuta may serve as a carrier for the dispersal of powdered BCA formulations to pear flowers.