Phenology of the spotted-wing drosophila, Drosophila suzukii, in the lower San Joaquin Valley of California

The spotted wing drosophila (SWD), Drosophila suzukii, is a significant new pest of cherries and other small fruit throughout the western United States.  Unlike other species of Drosophila it is capable of attacking fruit before it is ripe, thus rendering it unmarketable.   Research on the seasonal biology of SWD is still in a stage of infancy, especially with regards to overwintering biology and movement among crops.  Kern County, CA, offers a unique opportunity to gather this type of information due to the presence of high numbers of SWD throughout the winter on damaged citrus fruit that has dropped to the ground as well as the variety of host plants SWD can move from throughout the growing season. In 2009-2012 traps were placed to monitor SWD populations. The results illustrated that SWD moved from citrus in the winter, to cherries early spring, “disappeared” in the summer, and then appeared in the citrus in the fall. Therefore, in 2011 we designed a transect study to evaluate SWD biology and interactions between adjacent citrus and cherry orchards.  Results showed that SWD overwinters almost exclusively in citrus and begins to migrate into cherries approximately 5 weeks before harvest.  Data also showed a gender bias whereby initial movement into cherries was almost exclusively by female flies.  This detail helps explain why early-warning monitoring programs based on counts of males in bait traps in cherries have been relatively unsuccessful.