Contributing to fire blight risk prognosis in orchards with honey bee colonies as a gateway for detection of Erwinia amylovora

Abstract: Fire blight infections can originate from previous outbreaks within orchards but also from inoculum sources of surrounding host plants. Forecasting models based on environmental conditions may indicate a timing for spraying plant protection products to prevent Erwinia amylovora infections. However, hardly any empirical data on inoculum levels of E. amylovora in individual orchards or their vicinity is included. It is well known that bees and other pollinating insects can play an important role in the secondary spread of the bacteria during bloom of host plants. As nectar and pollen collecting honeybees come into contact with the pathogen during their flower visits, they can be used as part of a vector based system to monitor the incidence of E. amylovora in blooming host plants within the flight range of the bee colony. An E. amylovora detection system has been developed using tube collectors mounted at the flight entrances of beehives. This system provides valuable information on the presence of the bacteria within the range of their flights, is less time consuming and the number of sampling points is dramatically higher compared to flower sampling by hand. Qualitative and quantitative data about the occurrence of epiphytic E. amylovora within the flight range of honeybees could be gained. In 2012 to 2016 monitoring of flowers and inlays of tube collectors were carried out in commercial and experimental orchards in Switzerland and Austria during bloom of apple and pear. Sample preparation and confirmation by specific qPCR of daily collected samples gave an estimation of the incidence of flowers colonized by E. amylovora. Orchards with no or low findings of E. amylovora on inlays did not show any fire blight symptoms at the end of the incubation period. High numbers of the pathogen on inlays were correlated with fire blight outbreaks. Still, inlays of different beehives in one orchard or even collectors mounted at the same hive could bear divergent amounts of the pathogen. This was likely due to different groups and numbers of forager bees using different food sources in varying numbers, frequencies and distances. Therefore, the simultaneous operation of 3 honeybee colonies with collectors in the same monitoring place is recommended.